Kurzgesagt – In a Nutshell
Kurzgesagt – In a Nutshell
Embedded Files
Sources – Trees Are Dead
Sources – Trees Are Dead
We thank the following experts for their critical reading, feedback and corrections:
– Prof. Kathy Steppe
Ghent University, Belgium
– Prof. Rupert Seidl
Technical University of Munich, Germany
– Dr. Sofia van Moorsel
University of Zurich, Switzerland
Correction: In our newsletter, we incorrectly stated that “2% of a Tree is Alive” and included a visual that states “98% Dead”. The correct numbers are 5% and 95%, respectively. We apologize for the mistake. Please see below (Section “– The vast majority of the biomass of a tree is dead.”) for more details on these numbers.
– For over a billion years the ancestors of plants only inhabited the sun drenched surfaces of the oceans. Their bodies were thin and delicate and absorbed water straight through their surfaces, getting their energy through photosynthesis. Forging sunlight, carbon dioxide and water into sugar.
Land plants are also called “embryophytes” and are thought to have evolved from ocean-dwelling green algae. The typical “lifestyle” of algae includes photosynthesis at the surface of the water.
#McCourt RM, Lewis LA, Strother PK, Delwiche CF, et al. Green land: Multiple perspectives on green algal evolution and the earliest land plants. Am J Bot. 2023
https://pubmed.ncbi.nlm.nih.gov/37247371/
Quote: “The origin of the embryophytes, which dwarf other land plants in terms of diversity and biomass (Bar-On et al., 2018), appears as a unique evolutionary event that involved the de novo origin of complex multicellularity from less structurally complex, fully aquatic green algae.”
#Plackett AR, Coates JC. Life's a beach - the colonization of the terrestrial environment. New Phytol. 2016
https://pubmed.ncbi.nlm.nih.gov/27874985/
Quote: “Phylogenetic analysis indicates that all extant terrestrial plants (embryophytes) have a common ancestor (Qiu et al., 2006), arising from within the ‘Green Plant’ lineage of algae (Viridiplantae) c. 470 million years ago (Ma) (Fig. 2a).”
#Raven, John A. et al. Algae. Current Biology. 2014
https://doi.org/10.1016/j.cub.2014.05.039
Quote: “The majority of algae are photosynthetic and live in aquatic habitats, although some grow on soil, including desert crusts. In all cases, algae that depend on photosynthesis live where there is sufficient light to permit photosynthetic growth; in aquatic environments the limit for photosynthetic life is within, at most, 300 m of the water surface where the photon flux density (400–700 nm) is not more than 10−5 that at the surface.”
The animal we show floating below the surface is Aegirocassis, an extinct giant arthropod that lived during the early Ordovician, roughly 480 million years ago.
#Lerosey-Aubril, R., Pates, S. New suspension-feeding radiodont suggests evolution of microplanktivory in Cambrian macronekton. Nat Commun 9, 3774 (2018).
Fig. 5. Diversity of sizes and feeding habits in radiodonts. As exemplified by Aegirocassis and Pahvantia, there is no obvious relationships between size and feeding strategy in this group. Raptorial predators are represented in red, sediment sifters in purple, and suspension feeders in blue.”
– But about 470 million years ago they decided to conquer a hostile alien planet: The land.
There are different estimates of when land plants first emerged. We are using a conservative estimate here. It is therefore possible that land plants are even older than this.
#Kapoor B, Kumar P, Verma V, Irfan M, et al. How plants conquered land: evolution of terrestrial adaptation. J Evol Biol. 2023
https://pubmed.ncbi.nlm.nih.gov/36083189/
Quote: “Life on Earth originated approximately 3.7 billion years ago (Nutman et al., 2016; Rosing, 1999). All life forms of the past, present and future on the Earth are historically connected through evolution (Dobzhansky, 1973), the driving force for life's persistence and diversification. Around 600 Ma, a single freshwater algal lineage evolved to make the transition to land, which led to the advent of land plants about 450 Ma (Morris et al., 2018). This was one of the most significant events in Earth's history, which not only altered the evolutionary process of these newly evolved terrestrial creatures, but also the Earth's ecosystems, including the land, oceans and atmosphere (Plackett & Coates, 2016).
[...]
Phylogenetic and fossil pieces of evidence suggest that approximately 470 million years ago, land plants (embryophytes) made the progression from freshwater ecosystem to land, occupying different ecological niches of bacteria, algae, lichens and fungi (Edwards et al., 2015; Harholt et al., 2016; Wellman & Strother, 2015). However, a phylogenetic study coupled with a Bayesian relaxed molecular clock established that embryophytes originated on land in the middle of the Cambrian‐early Ordovician period around 515.2–473.5 Ma (Morris et al., 2018). Recently, a discovery by Strother and Foster (2021) using a molecular time tree suggested that embryophytes emerged around 500 Ma during the Cambrian Era while the chronological gap of around 80 Ma was attributed to a missing fossil record.”
– Like green rugs with ambition these plant ancestors began clinging to the ground wherever it was wet and damp.
The first land plants likely evolved from sea-dwelling algae. Because their soft bodies do not fossilize very well, it is hard to tell what exactly they looked like. What we do know is that the transition from water to land involved a multitude of changes to their body plan to cope with the stressors of a land-dwelling life. The following paper is a review on this topic.
#de Vries J, Archibald JM. Plant evolution: landmarks on the path to terrestrial life. New Phytol. 2018
https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.14975
Quote: “Molecular phylogenetic data show that land plants evolved from streptophyte algae most closely related to extant Zygnematophyceae, and one of the principal aims of plant evolutionary biology is to uncover the key features of such algae that enabled this important transition. At the present time, however, mosaic and reductive evolution blur our picture of the closest algal ancestors of plants. Here we discuss recent progress and problems in inferring the biology of the algal progenitor of the terrestrial photosynthetic macrobiome.”
#Rensing SA. Great moments in evolution: the conquest of land by plants. Curr Opin Plant Biol. 2018
https://pubmed.ncbi.nlm.nih.gov/29525128/
Quote: “500 Ma ago the terrestrial habitat was a barren, unwelcoming place for species other than, for example, bacteria or fungi. Most probably, filamentous freshwater algae adapted to aerial conditions and eventually conquered land. Adaptation to a severely different habitat apparently included sturdy cell walls enabling an erect body plan as well as protection against abiotic stresses such as ultraviolet radiation, drought and varying temperature. To thrive on land, plants probably required more elaborate signaling pathways to react to diverse environmental conditions, and phytohormones to control developmental programs. Many such plant-typical features have been studied in flowering plants, but their evolutionary origins were long clouded. With the sequencing of a moss genome a decade ago, inference of ancestral land plant states using comparative genomics, phylogenomics and evolutionary developmental approaches began in earnest. In the past few years, the ever increasing availability of genomic and transcriptomic data of organisms representing the earliest common ancestors of the plant tree of life has much informed our understanding of the conquest of land by plants.”
– But now, with solid ground beneath and no ability to float around a new dimension became a place of intense warfare: Up.
The higher they grew, the more sun they would get, while starving their competition below. Height became a deadly weapon. The battle for the sky had begun.
During the evolution of early land plants, several factors likely drove the evolution of increased plant height. One of them was competition for light, which we focus on here. In areas where plants grow close to each other, such as forests, trees and other plants compete over access to light. During this so-called “light competition”, plants that grow faster and/or taller have an advantage over other plants because of preferential access to light. Additionally, the taller plants cast shadows on the lower plants, further hindering their growth and survival. These effects are especially pronounced when a forest is developing in a specific area for the first time, or when trees regrow after a major disturbance such as a forest fire or other natural disaster (so-called “secondary forest”).
#Matsuo T, Martínez-Ramos M, Onoda Y, Bongers F, et al. Light competition drives species replacement during secondary tropical forest succession. Oecologia. 2024
https://pubmed.ncbi.nlm.nih.gov/38727828/
Quote: “During tropical secondary forest succession, different plant species attain their maximum biomass at different moments in time, and hence there is a gradual species replacement (Bryan 1996; Peña-Claros 2003). In tropical rainforest, species replacement is driven by light competition as light is the most limiting resource in vertically developed tropical forests (Fauset et al. 2017; Rozendaal et al. 2020). During tropical forest succession, there is a rapid build-up of the forest canopy, resulting in a marked vertical light gradient with less light in the forest understory (i) Investing in height or crown growth to increase light interception (LIE, light interception per unit aboveground biomass) (Hikosaka et al. 1999; Falster and Westoby 2003), and/or (ii) Utilizing the intercepted light more efficiently for their growth (i.e., light use efficiency, LUE) (Valladares and Niinemets 2008; Onoda et al. 2014). Tree species differ in their light competition strategies by having different whole-tree, stem, and leaf trait values (Falster et al. 2017; Maharjan et al. 2021).”
#Rozendaal DMA, Phillips OL et al. Competition influences tree growth, but not mortality, across environmental gradients in Amazonia and tropical Africa. Ecology. 2020
https://pmc.ncbi.nlm.nih.gov/articles/PMC7379300/
Quote: “The response of any given focal tree to competition will likely depend not only on the degree of crowding in its local neighborhood, but also on its size and functional traits. Smaller trees are more strongly affected by competition (Uriarte et al. 2004) because they are more heavily shaded by taller neighbors, and likely suffer from greater belowground competition. Shade‐intolerant tree species, which typically have low wood density (WD; van Gelder et al. 2006), respond more strongly to changes in light availability than shade‐tolerant species (Bazzaz 1979), and thus are likely to be more strongly affected by competition. Indeed, shade‐intolerant (Hubbell et al. 2001, Canham et al. 2006, Kunstler et al. 2011) and low WD tree species (Kunstler et al. 2016) often show greater growth decreases in response to neighborhood crowding. Hence, variation in the plot‐level strength of competition (i.e., the extent to which growth is reduced, or mortality is increased, by competition across all individual trees in a plot) across environmental gradients may not only depend on forest basal area, but also on tree size distributions and mean wood density.”
#Beauchamp N, Kunstler G, et al. Light competition affects how tree growth and survival respond to climate. Journal of Ecology. 2025
https://doi.org/10.1111/1365-2745.14489
Quote: “Light competition is known to be a key process in forests and is an important determinant of both forest structure and tree dynamics (Pacala et al., 1996), with tree species strongly varying in their level of shade tolerance (Valladares & Niinemets, 2008) and their sensitivity to light competition (Kunstler et al., 2011).”
– Until now, plants were built mostly from cellulose, which was great for shape, but not for strength, which limited how tall they could grow. Over dozens of millions of years of evolutionary warfare, a group of plants developed one of life's greatest breakthroughs: lignin.
There is some evidence that lignin-like materials already existed in algal lineages before they evolved into plants. As time and evolution went on, lignin was then structurally incorporated into cell walls, which gave early plants much better biomechanical support compared to their algal ancestors. This allowed them to evolve rigid structures such as stems, and grow upright.
Emonet A, Hay A. Development and diversity of lignin patterns. Plant Physiol. 2022
https://pubmed.ncbi.nlm.nih.gov/35642915/
Quote: “Although there is evidence of lignin-like material in algal lineages, it is the presence of a lignified xylem tissue that defines vascular plants (Martone et al., 2009; Sørensen et al., 2011).”
#Weng JK, Chapple C. The origin and evolution of lignin biosynthesis. New Phytol. 2010
https://pubmed.ncbi.nlm.nih.gov/20642725/
Quote: “Although the occurrence of phenylpropanoid metabolism in early land plants facilitated their initial move onto land, for tens of millions of years their body plans remained small because of a lack of mechanical reinforcement (Bateman et al., 1998). It was not until the rise of tracheophytes, which had developed the ability to deposit the phenylpropanoid polymer lignin in their cell wall, that land plants truly flourished and began their dominance of the terrestrial ecosystem. Lignin bestowed the early tracheophytes with the physical rigidity to stand upright, strengthened the water-conducting cells for long-distance water transport, and allowed plants to expand significantly in body size compared with their sister group, the bryophytes. The nature and randomness of the linkages in lignin also made lignin one of the hardest biopolymers to degrade, an ideal characteristic for a defensive barrier against the pathogens and herbivores that would soon co-evolve with vascular plants.”
– Lignin is a macromolecule made of ring-shaped structures. It is rigid, tough, waterproof, and incredibly hard to break down. Concrete in a world made of jelly. It filled gaps between cellulose threads, stiffening and locking everything into place. Lignin gave plants the strength to grow taller and claim the sun for themselves.
In the evolution of early land plants, several factors likely drove the evolution of lignin, and later wood. One of them is light competition: woody tissues allow for greater mechanical strength and are thus able to support greater plant heights, giving taller plants an edge over their competition. However, evidence in early land plants that were very small but still contained wood-like tissues suggests that wood formation might have also evolved to support plant functions such as water transport and drought resistance (independent of height).
#Wang Y, Hou Y, Li H, Wu W, et al. A New Structural Model of Enzymatic Lignin with Multiring Aromatic Clusters. ACS Omega. 2022
https://pmc.ncbi.nlm.nih.gov/articles/PMC9178751/
Quote: “Lignin is a natural aromatic compound in plants. Several lignin structural models have been proposed in the past years, but all the models cannot be converted to benzene carboxylic acids (BCAs) for all aromatic rings connected to oxygen. This inspired us to explore the structures of lignin. Based on the yields of BCAs, the results of 13C NMR and ethanolysis residues, and gas chromatography–mass spectrometry and electrospray ionization mass spectrometry of ethanolysis of lignin, we have constructed a structural model of lignin with a formula C6407H6736O2590N147S3. The model not only satisfies the results of analyses, but also explains the generation of BCAs from lignin oxidation and the ethanolysis products. Importantly, double-ring and triple-ring aromatic clusters are found in lignin, and some of them are connected by alkyl bridges, which results in conventional low conversions of lignin. Our findings in the structures of lignin may significantly influence the structures and applications of lignin.”
#Ponnusamy VK, Nguyen DD, Dharmaraja J, Shobana S, et al. A review on lignin structure, pretreatments, fermentation reactions and biorefinery potential. Bioresour Technol. 2019
https://pubmed.ncbi.nlm.nih.gov/30270050/
Quote: “Plant cell walls are composed of lignin which is a phenylpropanoid biopolymer, giving mechanical strength to the plant structure. Cellulose and hemicellulose make up the entire biomass and are firmly connected to the lignin molecules via covalent and hydrogenic linkages which make the structure extremely strong and recalcitrant to pretreatment techniques. Lignocellulosic biomass is primarily comprised of cellulose (38–50%), hemicellulose (23–32%) and lignin (12–25%) components (Sun and Cheng, 2002, Gonzalo et al., 2016). The lignin component is familiar as the second greatest inexhaustible natural organic polymer which conveys the greatest energetic substance of all the above, empowers plants to build rigid chemical structures and also provides a probability against hydrolysis of cellulose and hemicellulose in the lignocellulosic biomass (Saake and Lehnen, 2012, Zakzeski et al., 2010).”
#Lisý, A., Ház, A., Nadányi, R., et al. About Hydrophobicity of Lignin: A Review of Selected Chemical Methods for Lignin Valorisation in Biopolymer Production. Energies 2022
https://doi.org/10.3390/en15176213
Quote: “The formation of lignin occurs inside cell walls in plants, where it functions as a structural component, increasing structural integrity and strength, while serving as a protection against pathogens and insects, having antifungal and antimicrobial functions, thanks to its hydrophobic character, allowing for the transport of water and nutrients through the plant, absorbing UV-radiation and having fire-retardant properties [28,30].”
#Wu W, Li P, Huang L, Wei Y, et al. The Role of Lignin Structure on Cellulase Adsorption and Enzymatic Hydrolysis. Biomass. 2023
#Philippe Gerrienne et al., A Simple Type of Wood in Two Early Devonian Plants. Science 333,837-837(2011).
https://www.science.org/doi/abs/10.1126/science.1208882
Quote: “The advent of wood (secondary xylem) is a major event of the Paleozoic Era, facilitating the evolution of large perennial plants. The first steps of wood evolution are unknown. We describe two small Early Devonian (407 to 397 million years ago) plants with secondary xylem including simple rays. Their wood currently represents the earliest evidence of secondary growth in plants. The small size of the plants and the presence of thick-walled cortical cells confirm that wood early evolution was driven by hydraulic constraints rather than by the necessity of mechanical support for increasing height. The plants described here are most probably precursors of lignophytes.”
#Hoffman LA, Tomescu AM. An early origin of secondary growth: Franhueberia gerriennei gen. et sp. nov. from the Lower Devonian of Gaspé (Quebec, Canada). Am J Bot. 2013
https://pubmed.ncbi.nlm.nih.gov/23535772/
Quote: “Premise of the study: Secondary xylem (wood) produced by a vascular cambium supports increased plant size and underpins the most successful model of arborescence among tracheophytes. Woody plants established the extensive forest ecosystems that dramatically changed the Earth's biosphere. Secondary growth evolved in several lineages in the Devonian, but only two occurrences have been reported previously from the Early Devonian. The evolutionary history and phylogeny of wood production are poorly understood, and Early Devonian plants are key to illuminating them.
Methods: A fossil plant preserved anatomically by cellular permineralization in the Lower Devonian (Emsian, ca. 400-395 million years old) Battery Point Formation of Gaspé Bay (Quebec, Canada) is described using the cellulose acetate peel technique.
Key results: The plant, Franhueberia gerriennei Hoffman et Tomescu gen. et sp. nov., is a basal euphyllophyte with a centrarch protostele and metaxylem tracheids with circular and oval to scalariform bordered multiaperturate pits (P-type tracheids). The outer layers of xylem, consisting of larger-diameter P-type tracheids, exhibit the features diagnostic of secondary xylem: radial files of tracheids, multiplicative divisions, and a combination of axial and radial components.
Conclusions: Franhueberia is one of the three oldest euphyllophytes exhibiting secondary growth documented in the Early Devonian. Within the euphyllophyte clade, these plants represent basal lineages that predate the evolution of stem-leaf-root organography and indicate that underlying mechanisms for secondary growth became part of the euphyllophyte developmental toolkit very early in the clade's evolution.”
To illustrate some of the first land plants considered to have contained early versions of lignin-like structures, we used one of the oldest vascular plants: Cooksonia.
#Edwards, D. (2003), Xylem in early tracheophytes. Plant, Cell & Environment, 26: 57-72.
https://doi.org/10.1046/j.1365-3040.2003.00878.x
Quote: “Finally, in that Tracheophyta, now generally concluded as monophyletic on the basis of molecular (e.g. Qiu et al. 1998), and morphological (Kenrick 2000) data, are actually defined by the presence of water-conducting cells with secondary walls reinforced by lignin (Kenrick & Crane 1997), it seems appropriate to mention, albeit briefly, phylogenetic inferences from the tracheids themselves. Figure 6 shows an overview of the phylogenetic relationships of land plants based on Friedman & Cook (2000) and Kenrick & Crane (1997) with additions including incorporation of tracheid types. The position of Aglaophyton relies on the apparent absence of tracheidal secondary thickenings (Friedman & Cook 2000). Cooksonia pertoni has been included as part of a plexus of plants considered as sister groups to the Lycophytina.
Figure 6. Overview of phylogenetic relationships of embryophytes based on Friedman & Cook (2000) (P, G, C, etc. refer to tracheid types).”
– So as more millions of years passed, some plants just went all in on lignin and produced more and more of it, becoming stiffer, harder and stronger.
As plants evolved, many species developed lignin-like structures that gradually started to resemble the lignin we see today. However, these early versions were still very different from the “true” lignin used by modern plants today. Below, we list three Devonian genera with evidence supporting them containing lignin-like compounds. All three genera overlapped in time, i.e. they partially co-existed in the Devonian and they are not ancestors of each other.
Zosterophyllum:
#Huang P, Wang JS, Wang YL, Liu L, Zhao JY, Xue JZ. The smallest Zosterophyllum plant from the Lower Devonian of South China and the divergent life-history strategies in zosterophyllopsids. Proc Biol Sci. 2025
https://pmc.ncbi.nlm.nih.gov/articles/PMC11732410/
Quote: “Plants have evolved different life-history strategies to overcome limited amounts of available resources; however, when and how divergent strategies of sexual reproduction evolved in early land plants are not well understood. As one of the notable and vital components of early terrestrial vegetation, the Zosterophyllopsida and its type genus Zosterophyllum reached maximum species diversity during the Pragian (Early Devonian; ca 410.8–407.6 million years ago). Here we describe a new species, Zosterophyllum baoyangense sp. nov., based on well-preserved specimens from the Pragian-aged Mangshan Group of Duyun, Guizhou Province, China. The new plant is characterized by its small size, K-shaped branching and tiny spikes with 5–10 sporangia. This plant is most likely r-selected, completing its whole lifespan in a short time, and such a strategy contributes to reproduction in a suitable window time. In contrast, most other species of Zosterophyllum and the zosterophyllopsids on a broader scale are larger in body size and have greater investments in fertile tissues, reflected in the size and total number of sporangia. We argue that the zosterophyllopsids probably benefited from the divergence of various life-history strategies and thus constituted a major part of the Early Devonian floras.
[...]
Morphological evolution of members of the Lycophytina sensu Kenrick & Crane and the Zosterophyllopsida sensu Hao & Xue through the late Silurian to Early Devonian. (a–f) Lycophytina sensu Kenrick & Crane, indicated by §; (g–l) Zosterophyllopsida sensu Hao & Xue, indicated by ‡; for each group, the figures show the maximum preserved length of axes (a,g), width of axes (b,h), TSA (c,i), crossplot of sporangial width and height of different time bins (d,j), crossplot of maximum value of TSA and maximum axial width for all sample taxa (e,k), and crossplot of minimum value of TSA and minimal axial width for all sample taxa (f,l). For boxplots in (b,c,h,i), lines in the boxes are median values, boxes are 25%−75% quartiles and upper and lower values are range. Abbreviations same as in figure 2.”
#Ewbank, G., Edwards, D., and Abbott, G. D. Chemical characterization of Lower Devonian vascular plants. Organic Geochemistry, 1996
https://www.sciencedirect.com/science/article/abs/pii/S0146638096001404
Quote: “Vegetative remains of three coalified Lower Devonian vascular plants (Zosterophyllum, Psilophyton, Renalia) were analyzed using flash pyrolysis-gas chromatography-mass spectrometry. The distributions of pyrolysis products are compared with those from younger vascular plant fossil xylem (Cordaixylon, Callixylon) and cuticle (Pachypteris). The likelihood of the chemical preservation of characteristic higher plant macromolecules (e.g., lignin and cutan) in the Lower Devonian plant fossils is considered in light of this comparison and associated thermal maturity assessments. Reflectance values from vitrinite-like macerals, which may not be vitrinite sensu stricto in the Lower Devonian host rocks for the fossils selected for this study, are shown to provide a reasonable assessment of the thermal maturity of these early vascular plant fossils. Although lignin altered through burial maturation is the most likely source of the prominent alkylphenols and aromatic hydrocarbons in the Lower Devonian tracheophyte flash pyrolysates, a contribution from thermally modified tannins cannot be ruled out. Comparison of the highly aliphatic pyrolysates from the Zosterophyllum and Psilophyton axes with that of a thermally mature fossil gymnosperm leaf revealed that cutan was an important component in the Devonian plant remains. This is the earliest chemical evidence for the presence of cutan in vascular plants.”
Psilophyton:
#Doran, Jeff. (2011). A new species of Psilophyton from the Lower Devonian of northern New Brunswick, Canada. Canadian Journal of Botany.
Quote: “The morphological similarities between Psilophyton crenulaturn and certain ferns (e.g., Botryopteridaceae, Anachoropteridaceae) along with the multitude of intertwined axes (Fig. 2), the relatively small size of the vascular strand, the slender axes with profuse branching suggest that P. crenulaturn has a rhizomatous or stolon-like habit with occasional upright vegetative and reproductive shoots that were perhaps 30 cm tall (Fig. 57).”
#Ewbank, G., Edwards, D., and Abbott, G. D. Chemical characterization of Lower Devonian vascular plants. Organic Geochemistry, 1996
https://www.sciencedirect.com/science/article/abs/pii/S0146638096001404
Quote: “Vegetative remains of three coalified Lower Devonian vascular plants (Zosterophyllum, Psilophyton, Renalia) were analyzed using flash pyrolysis-gas chromatography-mass spectrometry. The distributions of pyrolysis products are compared with those from younger vascular plant fossil xylem (Cordaixylon, Callixylon) and cuticle (Pachypteris). The likelihood of the chemical preservation of characteristic higher plant macromolecules (e.g., lignin and cutan) in the Lower Devonian plant fossils is considered in light of this comparison and associated thermal maturity assessments. Reflectance values from vitrinite-like macerals, which may not be vitrinite sensu stricto in the Lower Devonian host rocks for the fossils selected for this study, are shown to provide a reasonable assessment of the thermal maturity of these early vascular plant fossils. Although lignin altered through burial maturation is the most likely source of the prominent alkylphenols and aromatic hydrocarbons in the Lower Devonian tracheophyte flash pyrolysates, a contribution from thermally modified tannins cannot be ruled out. Comparison of the highly aliphatic pyrolysates from the Zosterophyllum and Psilophyton axes with that of a thermally mature fossil gymnosperm leaf revealed that cutan was an important component in the Devonian plant remains. This is the earliest chemical evidence for the presence of cutan in vascular plants.”
Trimerophytes (e.g. Pertica quadrifaria):
#Kasper, A.E., Jr. and Andrews, H.N., Jr. (1972), Pertica, A New Genus Of Devonian Plants From Northern Maine. American Journal of Botany.
https://bsapubs.onlinelibrary.wiley.com/doi/abs/10.1002/j.1537-2197.1972.tb10165.x
Quote: “A new genus of Devonian age fossil plants is described from the Trout Valley Formation of northern Maine. Abundant compression material permits a rather complete understanding of its morphology. Pertica quadrifaria Kasper and Andrews, gen. et sp. nov., was an erect plant, perhaps a meter tall, with a pseudomonopodial main axis and dichotomous side branches. The side branches were arranged in a clockwise spiral (from base to apex) and were tetrastichous. They dichotomized numerous times, with the intervals between dichotomies decreasing distally. The ultimate branchlets bore numerous sporangia in dense clusters. Other side branches were completely sterile. Pertica quadrifaria is classified in the Subdivision Trimerophytina of Banks. Its evolutionary significance rests in the fact that it is a link in the chain of increasingly complex early vascular land plants.”
Although the exact structure and function of lignin-like compounds in Pertica are not yet well described, evolutionary studies suggest that the full lignin biosynthetic pathway likely emerged around the time of early tracheophytes, which includes trimerophytes such as Pertica:
#Weng JK, Chapple C. The origin and evolution of lignin biosynthesis. New Phytol. 2010
Figure 2. A plant phylogenetic tree marked with the major milestones of evolution of lignin biosynthesis. The distribution of lignin and its monomeric composition across major plant lineages are denoted by a circle at each branch. Open circle, no lignin; orange circle, presence of H and G lignin; red circle, presence of S lignin in addition to H and G lignin; circle with question mark, unknown. Note that, within several groups with G lignin, S lignin-containing exceptions are known. †Extinct lineage.”
– Until one day, around 385 million years ago they got the biological equivalent of steel reinforced concrete: Wood.
There is not a single point in evolution when wood and trees emerged, but rather it was a gradual process, and tree-like features evolved in many species independently. We are using a conservative estimate of 385 Mya (million years ago) here.
#Xu HH, Berry CM, Stein WE, Wang Y, et al. Unique growth strategy in the Earth's first trees revealed in silicified fossil trunks from China. Proc Natl Acad Sci U S A. 2017
https://pubmed.ncbi.nlm.nih.gov/29078324/
Quote: “The fossil record strongly indicates that the origin of trees was not a solitary evolutionary innovation confined to a single clade. Cladoxylopsid trees appeared first (4) [early Mid-Devonian (Eifelian), 393–388 Ma], followed by archaeopteridalean and lycopsid trees [late Mid-Devonian (Givetian), 388–383 Ma] (5, 6). Archaeopteris, an early lignophyte, has the most familiar tree growth strategy, exhibiting extensive secondary development by producing secondary xylem (wood) laid down in concentric growth increments by a cylindrical bifacial vascular cambium (7, 8).”
To illustrate the transition from earlier and smaller land plants to the first “modern” trees, we use a series of proto-trees (pre-trees), which already share some tree-like features with modern trees, such as a thick central trunk and wood-like tissue.
One of the earliest examples of this is Protolepidodendropsis:
#Christopher M. Berry, John E.A. Marshall; Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard. Geology 2015.
https://doi.org/10.1130/G37000.1
Quote: “The Middle to early Late Devonian transition from diminutive plants to the first forests is a key episode in terrestrialization. The two major plant groups currently recognized in such “transitional forests” are pseudosporochnaleans (small to medium trees showing some morphological similarity to living tree ferns and palms) and archaeopteridaleans (trees with woody trunks and leafy branches probably related to living conifers). Here we report a new type of “transitional” in-situ Devonian forest based on lycopsid fossils from the Plantekløfta Formation, Munindalen, Svalbard. Previously regarded as very latest Devonian (latest Famennian, 360 Ma), their age, based on palynology, is early Frasnian (ca. 380 Ma). In-situ trees are represented by internal casts of arborescent lycopsids with cormose bases and small ribbon-like roots occurring in dense stands spaced ∼15–20 cm apart, here identified as Protolepidodendropsis pulchra Høeg. This plant also occurs as compression fossils throughout most of the late Givetian–early Frasnian Mimerdalen Subgroup. The lycopsids grew in wet soils in a localized, rapidly subsiding, short-lived basin. Importantly, this new type of Middle to early Late Devonian forest is paleoequatorial and hence tropical. This high-tree-density tropical vegetation may have promoted rapid weathering of soils, and hence enhanced carbon dioxide drawdown, when compared with other contemporary and more high-latitude forests.
[...]
Re-dating the horizons to an early Frasnian age and accurate description and identification of the fossils allow a new perspective of these, the oldest known in-situ lycopsid forests, from the paleoequatorial tropical zone (Item DR7). Protolepidodendropsis pulchra had an enlarged base and narrow roots, also known from a drifted lycopsid of this age (the mid-Frasnian “Naples tree” from New York; White, 1907), and grew in dense stands in wet soils, reaching basal diameters of 20 cm and trunk diameters of typically 8–10 cm. The original dimensions of the Naples tree (5 m high, 38.5 cm at base, 7 cm at broken top), combined with the material outlined above, suggest that a height for the unbranched trunk would have been from 2 to 4 m (upper part reconstruction as in Schweitzer, 1965).”
The inner structure of the trunk in Protolepidodendropis is not known. But in closely related arborescent lycopsids, the stem contains a siphonostele—a central vascular cylinder with a pith surrounded by xylem and phloem. We have modeled our cross section after this.
#Wang, Q., Hao, S.-G., Wang, D.-M. and Dilcher, D.L. (2002), An anatomically preserved arborescent lycopsid, Sublepidodendron songziense (Sublepidodendraceae), from the Late Devonian of Hubei, China†. Am. J. Bot., 89: 1468-1477.
https://doi.org/10.3732/ajb.89.9.1468
Quote: “Sublepidodendron is a common megafossil plant in the Late Devonian of China, but historically the generic delimitation based on leaf bases masked its true systematic position. A reinvestigation of S. songziense from the Late Devonian Hsiehchingssu Formation, Hubei, China, provides new insights into its internal anatomy and reproductive morphology. This arborescent lycopsid is characterized by small, vertically elongated leaf bases arranged in spirals, presence of false leaf scars, possibly bearing separate cones, and association with a stigmarian rhizomorph. The potential for organic connections of these detached organ genera has been noted for other Sublepidodendron species. The anatomy of S. songziense axes from two levels reveals that the thinner axis may bear an ectophloic siphonostele with a filamentous pith and an outer cortex. The thicker axis has a siphonostele with a branch gap, two-zoned pith with secondary thickenings, multiseriate rays across secondary xylem, a thick periderm, and primary and secondary tracheid walls characterized by “Williamson's striations.” Similarities to synapomorphies of Diaphorodendraceae and Lepidodendraceae suggest that S. songziense bears a closer affinity to Lepidodendrales rather than Protolepidodendrales, as formerly thought. Widespread occurrence of Sublepidodendron implies that phylogenetically advanced arborescent lycopsids must have diverged by the Late Devonian.”
#Stem types. Encyclopaedia Britannica. Retrieved September 2025
https://cdn.britannica.com/30/55430-050-F996A31E/Stem-structures-ferns.jpg
– More on it later, but with this magic material the first trees emerged. Almost immediately they became the largest living beings alive.
Here, we list (in overlapping chronological order) some of the earliest land plants with a tree-like anatomy, i.e. a large prominent trunk and wood-like tissues supporting great mechanical stability.
Protolepidodendropsis (~390–370 Mya; Devonian):
#Christopher M. Berry, John E.A. Marshall; Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard. Geology 2015.
https://doi.org/10.1130/G37000.1
Quote: “The Middle to early Late Devonian transition from diminutive plants to the first forests is a key episode in terrestrialization. The two major plant groups currently recognized in such “transitional forests” are pseudosporochnaleans (small to medium trees showing some morphological similarity to living tree ferns and palms) and archaeopteridaleans (trees with woody trunks and leafy branches probably related to living conifers). Here we report a new type of “transitional” in-situ Devonian forest based on lycopsid fossils from the Plantekløfta Formation, Munindalen, Svalbard. Previously regarded as very latest Devonian (latest Famennian, 360 Ma), their age, based on palynology, is early Frasnian (ca. 380 Ma). In-situ trees are represented by internal casts of arborescent lycopsids with cormose bases and small ribbon-like roots occurring in dense stands spaced ∼15–20 cm apart, here identified as Protolepidodendropsis pulchra Høeg. This plant also occurs as compression fossils throughout most of the late Givetian–early Frasnian Mimerdalen Subgroup. The lycopsids grew in wet soils in a localized, rapidly subsiding, short-lived basin. Importantly, this new type of Middle to early Late Devonian forest is paleoequatorial and hence tropical. This high-tree-density tropical vegetation may have promoted rapid weathering of soils, and hence enhanced carbon dioxide drawdown, when compared with other contemporary and more high-latitude forests.
[...]
Re-dating the horizons to an early Frasnian age and accurate description and identification of the fossils allow a new perspective of these, the oldest known in-situ lycopsid forests, from the paleoequatorial tropical zone (Item DR7). Protolepidodendropsis pulchra had an enlarged base and narrow roots, also known from a drifted lycopsid of this age (the mid-Frasnian “Naples tree” from New York; White, 1907), and grew in dense stands in wet soils, reaching basal diameters of 20 cm and trunk diameters of typically 8–10 cm. The original dimensions of the Naples tree (5 m high, 38.5 cm at base, 7 cm at broken top), combined with the material outlined above, suggest that a height for the unbranched trunk would have been from 2 to 4 m (upper part reconstruction as in Schweitzer, 1965).”
Pseudosporochnus (~390–370 Mya; Devonian):
#Dambreville, Anaelle & Meyer-Berthaud, Brigitte & Jean-Francois, Barczi & Decombeix, Anne-Laure & Sébastien, Griffon & Rey, Hervé. Using architecture modeling of the Devonian tree Pseudosporochnus to compute its biomass. Transformative Paleobotany. 2018
Wattieza (~385 Mya; Middle Devonian):
Wattieza is a genus of early trees belonging to the class of Cladoxylopsida, and they are among the oldest known vascular plants growing as a tree-like structure, including wood-like tissues and a prominent trunk.
#Stein, W., Mannolini, F., Hernick, L. et al. Giant cladoxylopsid trees resolve the enigma of the Earth’s earliest forest stumps at Gilboa. Nature 446, 904–907 (2007).
Quote: “The evolution of trees of modern size growing together in forests fundamentally changed terrestrial ecosystems1,2,3. The oldest trees are often thought to be of latest Devonian age (about 380–360 Myr old) as indicated by the widespread occurrence of Archaeopteris (Progymnospermopsida)4. Late Middle Devonian fossil tree stumps, rooted and still in life position, discovered in the 1870s from Gilboa, New York5, and later named Eospermatopteris, are widely cited as evidence of the Earth’s ‘oldest forest’6,7. However, their affinities and significance have proved to be elusive because the aerial portion of the plant has been unknown until now. Here we report spectacular specimens from Schoharie County, New York, showing an intact crown belonging to the cladoxylopsid Wattieza (Pseudosporochnales)8 and its attachment to Eospermatopteris trunk and base. This evidence allows the reconstruction of a tall (at least 8 m), tree-fern-like plant with a trunk bearing large branches in longitudinal ranks. The branches were probably abscised as frond-like modules. Lower portions of the trunk show longitudinal carbonaceous strands typical of Eospermatopteris, and a flat bottom with many small anchoring roots. These specimens provide new insight into Earth’s earliest trees and forest ecosystems. The tree-fern-like morphology described here is the oldest example so far of an evolutionarily recurrent arborescent body plan within vascular plants. Given their modular construction, these plants probably produced abundant litter, indicating the potential for significant terrestrial carbon accumulation and a detritus-based arthropod fauna by the Middle Devonian period.”
#Berry, Christopher & Casas, Jhonny. (2024). Wattieza - The World's Oldest Giant Cladoxylopsid Tree And The First Forests During Devonian Times.
https://acfiman.org/wp-content/uploads/2024/12/bacfiman84.2.13.pdf
Quote: “Evidence from the specimens recovered from the new quarry in New York, indicates that the height of these trees may well have exceeded eight meters. The basal diameter falls within the range of the stump sizes observed at Gilboa. However, the largest stumps at Gilboa are twice the diameter of those studied by [1], suggesting even greater heights for Wattieza, maybe in the ten-twelve meters range [1] have traced the age of Wattieza species from the latest Givetian to earliest Frasnian age, for New York’s material, but the Wattieza material described by [2, 3] in Sierra de Perijá was dated a little bit earlier: Middle Givetian (380-385 Ma), a time in which the world’s primitive land plants, developed the first characteristics associated with modern-day trees, such as taller and wider trunks, the first signs of leaf development and more diverse reproductive methods. This was also the period when the first seed-bearing plants spread across to form forests, conquering the continents.”
#Xu HH, Berry CM, Stein WE, Wang Y, et al. Unique growth strategy in the Earth's first trees revealed in silicified fossil trunks from China. Proc Natl Acad Sci U S A. 2017
https://pubmed.ncbi.nlm.nih.gov/29078324/
Quote: “The fossil record strongly indicates that the origin of trees was not a solitary evolutionary innovation confined to a single clade. Cladoxylopsid trees appeared first (4) [early Mid-Devonian (Eifelian), 393–388 Ma], followed by archaeopteridalean and lycopsid trees [late Mid-Devonian (Givetian), 388–383 Ma] (5, 6). Archaeopteris, an early lignophyte, has the most familiar tree growth strategy, exhibiting extensive secondary development by producing secondary xylem (wood) laid down in concentric growth increments by a cylindrical bifacial vascular cambium (7, 8).”
– Shooting up to 20 meters high into the sky – and they only got bigger from here.
Considered one of the first “modern” trees, Archaeopteris (not to be confused with the dinosaur Archaeopteryx), is estimated to have grown up to a height of 20 m.
#Meyer-Berthaud, B., Scheckler, S. & Wendt, J. Archaeopteris is the earliest known modern tree. Nature 398, 700–701 (1999).
Quote: “Archaeopteris is an extinct plant which is of botanical interest for two reasons. It was the main component of the earliest forests until its extinction around the Devonian/Carboniferous boundary1,3, and phylogenetically, it is the free-sporing taxon that shares the most characteristics with the seed plants4,5. Here we describe the largest group of anatomically preserved Archaeopteris remains ever found, from the Famennian marine beds of south-eastern Morocco6, and provide the first evidence that, in terms of development and branching strategies, these 370-million-year-old plants were the earliest known modern trees. This modernization involved the evolution of four characteristics: a lateral branching syndrome similar to the axillary branching of early seed plants; adventitious latent primordia similar to those produced by living trees, which eventually develop into roots on stem cuttings; nodal zones as important sites for the subsequent development of lateral organs; and wood anatomy strategies that minimize the mechanical stresses caused by perennial branch growth.”
#Gess, Robert & Berry, Christopher. (2024). Archaeopteris trees at high southern latitudes in the late Devonian. Review of Palaeobotany and Palynology.
https://orca.cardiff.ac.uk/id/eprint/173763/1/1-s2.0-S0034666724001635-main.pdf
Quote: “During the Devonian Period plants first reached forest stature, impacting chemical weathering of rocks, fluvial systems, atmospheric composition and possibly aquatic eutrophication. Hypothetically these factors contributed to increasing climatic instability culminating in the End Devonian Mass Extinction Event. Understanding the timing of the spread of forests is however a prerequisite to correlation with its proposed consequences. Though evidence for forests at low palaeolatitudes demonstrates their emergence by the mid Devonian, sparse high-palaeolatitude records almost entirely comprise herbaceous lycopods. By the Famennian forest ecosystems are widely evidenced at low palaeolatitudes, however high latitude palaeofloras are almost exclusively represented by a single locality, the Waterloo Farm lagerstätte from South Africa (approximate palaeolatitude, 70°S). Understanding climatic and ecological conditions at this locality is doubly important as it also hosts diverse vertebrate taxa, including the only high latitude Devonian tetrapods. Archaeopteris, the quintessential Late Devonian woody tree, has previously been identified at this locality on the basis of leafy branch system fragments, though some uncertainty has remained as to whether these represent tree sized organisms. Here we present co-occurring large axes, including a trunk base, attributable to Archaeopteris trees inferred to be in excess of 20 m height, the first demonstration of forest stature at high latitudes in the Devonian. This possibly reflects high latitude climatic amelioration, resultant from warm ocean currents circulating southwards in response to progressive closure of the Iapetus Sea. As such, changing continental configurations may have indirectly facilitated the spread of forest ecosystems and helped to drive climatic instability and ultimately extinctions towards the end of the Devonian.”
Examples of modern trees and their heights which we show in the video:
Acer saccharum (Sugar maple)
#Missouri Botanical Garden. Acer saccharum 'Green Mountain'. Retrieved October 2025
https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=d363
Quote: “Acer saccharum commonly known as sugar maple is a deciduous, Missouri native tree which will typically grow 40' to 80' tall (sometimes to 100') with a dense, rounded crown. This tree is a main component of the Eastern U.S. hardwood forest and is one of the trees which is most responsible for giving New England its reputation for spectacular fall color.”
Eucalyptus regnans (Mountain ash) and Sequoia sempervirens (Coast redwood)
#Mifsud B, Prior LD, Williamson GJ, Corigliano J, Hansen C, Van Pelt R, Pearce S, Greenwood T, Bowman DMJS. Tasmania’s giant eucalypts: discovery, documentation, macroecology and conservation status of the world’s largest angiosperms. Australian Journal of Botany (2025)
– On the scale of a cell a distance of a few meters is like you’re working in Britain while your lunch box is in Egypt and your drink in New York.
For this calculation, we assume a tree height of 40 m (and illustrate it using the Scots pine Pinus sylvestris, which can reach heights of up to 40 m).
Range of plant cell diameters: 10-100 μm (let’s assume 10 μm)
https://bionumbers.hms.harvard.edu/bionumber.aspx?s=n&v=2&id=108685
40 m = 4*10^7 μm = 4*10^6 cell lengths
For simplicity, let’s assume humans are 2m tall.
2 m*4*10^6 = 8*10^3 km = 8000 km
Distance from London to Kairo by air: ca. 3535 km.
Distance from London to New York City: ca. 5564 km.
#Air Miles Calculator. London to Cairo distance (LHR to CAI). Retrieved September 2025
https://www.airmilescalculator.com/distance/lhr-to-cai/
Quote: “The air distance between London (London Heathrow Airport – LHR) and Cairo (Cairo International Airport – CAI) is 2196.9 miles (3535.5 kilometers or 1909 nautical miles).”
#Air Miles Calculator. London to New York distance (LHR to JFK). Retrieved September 2025
https://www.airmilescalculator.com/distance/lhr-to-jfk/
Quote: “The air distance between London (London Heathrow Airport – LHR) and New York (John F. Kennedy International Airport – JFK) is 3451.4 miles (5554.5 kilometers or 2999.2 nautical miles).”
#Monumental Trees. The thickest, tallest, and oldest Scots Pines (Pinus sylvestris). Retrieved October 2025.
– Let us slice an ancient tree in half and get to the heart running it all: the cambium, a razor-thin, circular zone, just a few stem cells wide.
#Wybouw B, Zhang X, Mähönen AP. Vascular cambium stem cells: past, present and future. New Phytol. 2024
https://pubmed.ncbi.nlm.nih.gov/38890801/
Quote: “In plants, growth that originates from the shoot and root apical meristems (SAM and RAM) is called primary or longitudinal growth. Later, secondary or radial growth, takes place in lateral meristems referred to as vascular and cork cambium. The vascular cambium, the focus of this review, provides new cells towards both secondary xylem and secondary phloem, which are positioned on opposite sides of the vascular cambium (Box 1) (Fig. 1). The stem cell concept (Box 1) is not well established in vascular cambium research, and thus the words cambium and stem cell have often been used interchangeably in the literature. Vascular cambium or cambial zone refers to the anatomically recognized meristematic cells in the secondary tissue (Evert, 2006), and the number of these cells in each radial file in vascular cambia can vary from one to several dozen, depending on the species. Lineage tracing studies (Box 1) both in Arabidopsis and poplar show that there is a single bifacial stem cell in each radial cell file producing both secondary xylem and phloem (Fig. 2a,b; Bossinger & Spokevicius, 2018; Shi et al., 2019; Smetana et al., 2019).”
– These stem cells grow inward and outwards, turning into two groups of specialists.
The inward specialists are on a conveyor belt of death, the xylem. With each new division it is pushing the cambium outwards, making the tree thicker the older it gets. As the xylem cells mature their lignin production goes into overdrive, and they become hard, like a muscle slowly transforming into bone. They begin to hollow themselves out, shedding everything that once made them alive. And then they die.
What’s left is a corpse, a hard, empty tube. As the tree grows year after year, new corpses are layered on old corpses, forming rings of hardened, dead tissue. A graveyard of trillions of plant bones. This is what we call wood. Stacked together they form a giant network of pipes that stretches the whole length of the tree.
Xylem is a complex tissue made of many cell types with different functions. Some cell types die as they mature (vessels, tracheids, fibers), as we describe here, but some others remain alive and play various roles in the physiology of the tree. Parenchyma cells, for example, are part of the xylem and remain alive. Among other things, they play an important role in nutrient storage. How many of each (dead or alive) cell type there are, depends on the species of tree and its age. In the wood of many species, the bulk of the xylem biomass is composed of dead cells. We call these dead cells “corpses” here as a metaphor. But “corpse” technically refers to a whole dead organism, whereas here we are referring to the death of individual cells which are part of an organism (the tree).
#Rathgeber CB, Cuny HE, Fonti P. Biological Basis of Tree-Ring Formation: A Crash Course. Front Plant Sci. 2016
https://pmc.ncbi.nlm.nih.gov/articles/PMC4880555/
Quote: “Xylogenesis consists in the production and differentiation of new xylem cells into mature functional wood cells. During their differentiation, xylem cells undergo profound morphological and physiological transformations, which will craft them according to their future functions (Wilson, 1970). The formation of a xylem tracheary element can be divided in five major steps: (1) the periclinal division of a cambial mother cell that creates a new daughter cell; (2) the enlargement of the newly formed xylem cell; (3) the deposition of cellulose and hemi-cellulose to build the secondary cell wall; (4) the impregnation of the cell walls with lignin; and finally, (5) the programmed cell death (Figure 1). This sequence is common to both angiosperms and gymnosperms but variations in duration and intensity of the differentiation phases, as well as in molecular components involved, finally result in different cell types and tree-ring structures. [...] Programmed cell death (also called apoptosis) marks the end of xylem cell differentiation and the advent of mature, fully functional, xylem elements (tracheids for gymnosperms, vessels and fibers for angiosperms). It is a highly coordinated and active process of cellular “suicide,” which is widespread in multicellular organisms. However, while most cells perform a specific function until their death, xylem tracheary elements die to become functional. In xylem, only parenchyma cells escape programmed cell death and remain alive for several years. [...] Programmed cell death is an essential step of xylem cell differentiation, allowing mature xylem cells to perform their specific functions in trees. The cell walls (in particular in tracheids and fibers) endow the function of mechanical support to the wood, while the empty cell lumens and the pits (in particular in tracheids and vessels) offer the necessary pathway for water transport into the plant.”
#Morris H, Plavcová L et al. A global analysis of parenchyma tissue fractions in secondary xylem of seed plants. New Phytol. 2016
https://pubmed.ncbi.nlm.nih.gov/26551018/
Quote: “Parenchyma is an important tissue in secondary xylem of seed plants, with functions ranging from storage to defence and with effects on the physical and mechanical properties of wood. Currently, we lack a large-scale quantitative analysis of ray parenchyma (RP) and axial parenchyma (AP) tissue fractions.
Here, we use data from the literature on AP and RP fractions to investigate the potential relationships of climate and growth form with total ray and axial parenchyma fractions (RAP).
We found a 29-fold variation in RAP fraction, which was more strongly related to temperature than with precipitation. Stem succulents had the highest RAP values (mean ± SD: 70.2 ± 22.0%), followed by lianas (50.1 ± 16.3%), angiosperm trees and shrubs (26.3 ± 12.4%), and conifers (7.6 ± 2.6%). Differences in RAP fraction between temperate and tropical angiosperm trees (21.1 ± 7.9% vs 36.2 ± 13.4%, respectively) are due to differences in the AP fraction, which is typically three times higher in tropical than in temperate trees, but not in RP fraction.
[...] Besides the occurrence of so-called living fibres (Wolkinger, 1970, 1971), ray and axial parenchyma (RAP) tissue represents the bulk of living cells in wood.
[...] RAP amount responds to both phylogenetic and environmental factors (Fig. 2). Also, RAP, with the exclusion of rayless species (Carlquist, 2001), is typically higher in angiosperm than in conifer wood for both RP (15–20% compared to 4–8%) and AP (≤ 1% to ≥ 30% compared to ≤ 1%), respectively (Koch, 1985; Spicer, 2014). The level of RAP also depends on growth forms, with a surprisingly high level occurring in woody succulents and lianas (Hearn, 2009).”
#Pettruzzello M. Xylem. Encyclopaedia Britannica. Retrieved June 2025
Phloem and xylem cells. Cells of the (left) phloem and (right) xylem.
#Graf I, Ceseri M, Stockie JM. Multiscale model of a freeze-thaw process for tree sap exudation. J R Soc Interface. 2015
Xylem microstructure. (a) Cross-sectional view of hardwood xylem, showing tracheids connected hydraulically to vessels and other tracheids via paired pits. Fibers appear similar to tracheids except that they have fewer pits, most of which are blind or unpaired. The parenchyma are living cells whose main role is carbohydrate storage and so they are ignored here. (b) A fiber-vessel pair approximated as circular cylinders, showing typical dimensions of the fiber (length L f = 1.0 × 10 −3 m and radius R f = 3.5 × 10 −6) and vessel (L v = 5.0 × 10 −4 m and R v = 2.0 × 10 −5 m). The model domain corresponds to the horizontal cross-section through the middle of the diagram.
#Ha M, Morrow M, Algiers K. Botany - Plant Structure - Secondary Stem. LibreTexts Biology. Retrieved June 2025
Figure 3.3.3.2: Cross section of a basswood (Tilia) stem magnified at 400X. The periderm is the outermost layer and has thin, dark layers. Some appear to be peeling. Within this is the cortex. Next are clusters of primary phloem fibers, narrow, thick-walled sclerenchyma cells, which stained bright pink. Within this is the secondary phloem. It alternates with wedge-shaped phloem rays. The phloem rays consist of plump, thin-walled parenchyma cells. Within the secondary phloem is the thin vascular cambium, which divides to produce secondary phloem externally and secondary xylem internally. Next are several thick layers of secondary xylem, which contain wide, thick-walled vessel elements. There are five concentric layers in the secondary xylem, and each is annual ring. The inner portion of each annual ring has the widest vessel elements. This is the spring (early) wood. The outer portion of each annual ring has narrower conducting cells of the summer (late wood). Thin, xylem rays pass through the secondary xylem like spokes of a wheel. Some align with the phloem rays. Within the secondary xylem is the primary xylem. In the center is the pith, which contains plump, thin-walled, white parenchyma cells. Image by Melissa Ha (CC-BY-NC).
– This network uses the chemical properties of water and a few other tricks to move it with incredible efficiency. Water molecules are, for a lack of a better word, sticky, like tiny magnets, and naturally cling tightly to each other. When one moves, it pulls the next along with it, like pulling on a rope.
#U.S. Geological Survey's (USGS) Water Science School. The strong polar bond between water molecules creates water cohesion. Retrieved June 2025.
https://www.usgs.gov/media/images/strong-polar-bond-between-water-molecules-creates-water-cohesion
Quote: “Hydrogen bonds are attractions of electrostatic force caused by the difference in charge between slightly positive hydrogen ions and other, slightly negative ions. In the case of water, hydrogen bonds form between neighboring hydrogen and oxygen atoms of adjacent water molecules. The attraction between individual water molecules creates a bond known as a hydrogen bond.”
– In trees this rope starts in the roots and ends in the leaves that bathe in the warmth from the sun.
Here the heat from the sun evaporates 95% of the water that got sucked into the roots, from billions of tiny pores, releasing a constant invisible mist of water molecules around the crown of the tree. This process, called transpiration in plants, creates tension on the rope of water molecules, stretching and lifting the entire column upward, all the way from the roots to the leaves.
Estimates for the proportion of water taken up through the roots that is lost to transpiration vary. But it is known to be a huge fraction.
The scientific model about water molecules “sticking” together and being moved up the tree is called the “cohesion-tension theory”, but water transport in plants involves more than just this mechanism. There is adhesion, where water sticks to the xylem walls, capillary action, which helps water rise through narrow tubes, and water potential, which drives water from wetter to drier areas. All of these play a role in keeping the water column in trees stable and moving. There are many other processes involved too, like positive root pressure, water stored in tissues that helps buffer flow, and lateral movement between xylem and phloem through so-called ray cells. So the current scientific view is that water transport in trees likely relies on a combination of mechanisms, not only the cohesion-tension effect.
#Kochhar SL, Gujral SK. Transpiration. In: Plant Physiology: Theory and Applications. Cambridge University Press. 2020
Quote: “ Out of the total water absorbed by the plant during a growing season, about 99 per cent is lost to the atmosphere in the form of water vapour and only 1 per cent is retained by the plant. Out of this 1 per cent, roughly 0.9 per cent is retained as free water within the tissue and about 0.1 per cent enters into the plant's metabolism as a reactant in chemical reactions (e.g., photosynthesis). The process of water loss in the form of vapour from the leaf surfaces of the plant is called transpiration.”
#Brown HR, Sutton AP. Trees suck. Notes on the physics of transpiration in trees. Prog Biophys Mol Biol. 2025
https://pubmed.ncbi.nlm.nih.gov/39725358/
Quote: “According to the Cohesion-Tension (CT) theory, evaporation taking place in daylight hours at the menisci (water–air interfaces) of minute columns of water in the interstices or pores within mesophyll cell walls inside leaves of a tree, together with capillary action within these columns, serve to suck water upwards through the xylem system from the roots. This mechanism relies on the forces of cohesion (H-bonds) between water molecules which account for water’s ability to form continuous threads within the xylem, capable through their “tensile strength” of being pulled from above.1 For tall trees the water in this process is under tension (negative absolute pressure), so metastable and prone to cavitation. And yet, as Galileo might say, it moves.”
#Bentrup FW. Water ascent in trees and lianas: the cohesion-tension theory revisited in the wake of Otto Renner. Protoplasma. 2017
https://pmc.ncbi.nlm.nih.gov/articles/PMC5591614/
Quote: “The cohesion-tension theory of water ascent (C-T) has been challenged over the past decades by a large body of experimental evidence obtained by means of several minimum or non-invasive techniques. The evidence strongly suggests that land plants acquire water through interplay of several mechanisms covered by the multi-force theory of (U. Zimmermann et al. New Phytologist 162: 575–615, 2004). The diversity of mechanisms includes, for instance, water acquisition by inverse transpiration and thermodynamically uphill transmembrane water secretion by cation-chloride cotransporters (L.H. Wegner, Progress in Botany 76:109–141, 2014).”
#Couvreur V, Ledder G, Manzoni S, Way DA, et al. Water transport through tall trees: A vertically explicit, analytical model of xylem hydraulic conductance in stems. Plant Cell Environ. 2018
https://pubmed.ncbi.nlm.nih.gov/29739034/
Quote: “Trees grow by vertically extending their stems, so accurate stem hydraulic models are fundamental to understanding the hydraulic challenges faced by tall trees. Using a literature survey, we showed that many tree species exhibit continuous vertical variation in hydraulic traits. To examine the effects of this variation on hydraulic function, we developed a spatially explicit, analytical water transport model for stems. Our model allows Huber ratio, stem-saturated conductivity, pressure at 50% loss of conductivity, leaf area, and transpiration rate to vary continuously along the hydraulic path. Predictions from our model differ from a matric flux potential model parameterized with uniform traits. Analyses show that cavitation is a whole-stem emergent property resulting from non-linear pressure-conductivity feedbacks that, with gravity, cause impaired water transport to accumulate along the path. Because of the compounding effects of vertical trait variation on hydraulic function, growing proportionally more sapwood and building tapered xylem with height, as well as reducing xylem vulnerability only at branch tips while maintaining transport capacity at the stem base, can compensate for these effects. We therefore conclude that the adaptive significance of vertical variation in stem hydraulic traits is to allow trees to grow tall and tolerate operating near their hydraulic limits.”
– This pull is so insanely strong that it can lift water over 100 meters, which requires sucking forces equivalent to the pressure of dozens of atmospheres – as much as the crushing pressure hundreds of meters deep in the ocean.
(Illustration note: At this point in the video we show a leaf shape reminiscent of that of Ginkgo biloba, but this is purely for illustration purposes. Trees of the species Ginkgo biloba reach maximum heights of roughly 35 m).
Internal pressure in trees typically refers to the water potential, i.e. the potential energy of water unit per volume. It essentially describes the tendency of water in the tree to move from place to another, for example up the trunk towards the leaves, and is often expressed in negative pressure units (atm, bar or MPa). The strength of the water potential strongly varies between tree species, within one tree depending on the location, and also depending on the measurement method. Because measuring internal pressure in big, living trees is very challenging, some measurements are taken in a laboratory environment. Some studies predict a limit of negative pressure of about -100 bar (roughly -100 atmospheres) that trees can sustain over time, but some studies found even lower pressures in some species living in extreme (desert) environments. In many tree species, and in most of the trees’ body, the internal pressure will be much less extreme, but still around -10-20 bar (roughly -10-20 atmospheres).
#Stroock AD, Pagay VV et al. The Physicochemical Hydrodynamics of Vascular Plants. Annual Review of Fluid Mechanics. 2014.
#Larter M, Brodribb TJ, Pfautsch S, Burlett R, et al. Extreme Aridity Pushes Trees to Their Physical Limits. Plant Physiol. 2015
https://pubmed.ncbi.nlm.nih.gov/26034263/
Quote: “Drought-induced hydraulic failure is a leading cause of mortality of trees (McDowell et al., 2008; Anderegg et al., 2012) and has become a major concern in light of future climate predictions, with forests across the world showing signs of vulnerability to intense and prolonged drought events (Allen et al., 2010). We show here that Callitris tuberculata, a conifer species from extremely dry areas of Western Australia, is the most cavitation-resistant tree species in the world to date (mean xylem pressure leading to 50% loss of hydraulic function [P 50] = −18.8 MPa). Hydraulic conductance is maintained in these plants at pressures remarkably close to the practical limit of water metastability, suggesting that liquid water transport under the cohesion-tension theory has reached its operational boundary.”
#Ambrose AR, Sillett SC, Koch GW, Van Pelt R, et al. Effects of height on treetop transpiration and stomatal conductance in coast redwood (Sequoia sempervirens). Tree Physiol. 2010
https://pubmed.ncbi.nlm.nih.gov/20631010/
Quote: “Treetops become increasingly constrained by gravity-induced water stress as they approach maximum height. Here we examine the effects of height on seasonal and diurnal sap flow dynamics at the tops of 12 unsuppressed Sequoia sempervirens (D. Don) Endl. (coast redwood) trees 68-113 m tall during one growing season. Average treetop sap velocity (V(S)), transpiration per unit leaf area (E(L)) and stomatal conductance per unit leaf area (G(S)) significantly decreased with increasing height. These differences in sap flow were associated with an unexpected decrease in treetop sapwood area-to-leaf area ratios (A(S):A(L)) in the tallest trees. Both E(L) and G(S) declined as soil moisture decreased and vapor pressure deficit (D) increased throughout the growing season with a greater decline in shorter trees. Under high soil moisture and light conditions, reference G(S) (G(Sref); G(S) at D = 1 kPa) and sensitivity of G(S) to D (-δ; dG(S)/dlnD) significantly decreased with increasing height. The close relationship we observed between G(Sref) and -δ is consistent with the role of stomata in regulating E(L) and leaf water potential (Ψ(L)). Our results confirm that increasing tree height reduces gas exchange of treetop foliage and thereby contributes to lower carbon assimilation and height growth rates as S. sempervirens approaches maximum height.”
#Kanduč M, Schneck E, Loche P, Jansen S, et al. Cavitation in lipid bilayers poses strict negative pressure stability limit in biological liquids. Proc Natl Acad Sci U S A. 2020
https://pubmed.ncbi.nlm.nih.gov/32358185/
Quote: “Metastable water under negative pressures is encountered in various biological and technological processes. Examples include lithotripsy and sonoporation of cell membranes and other biological matter (1, 2), drying stresses in unsaturated porous materials (3–6), catapulting mechanisms of fern spores (7, 8), octopus suckers (9), and the most widespread example, the hydraulic system in plants (3, 10, 11). In the latter, negative pressures are generated through evaporation of water from leaf cell walls, with resistance in the hydraulic system (the xylem) causing negative pressure in the liquid (xylem sap), which serves to suck water out of the soil up to the leaves. Negative pressures in plants are typically around several −10 bar but can reach −80 bar in certain desert species (3). Under these conditions, the vascular system is vulnerable to cavitation (i.e., the spontaneous formation of rapidly expanding voids or gas bubbles), which can spread and result in fatal embolic crisis (12, 13).
Although pure bulk water is stable against cavitation at pressures less negative than −1 kbar over astronomically long times (14–17), the empirical limit that plants can sustain over the relevant timescales of hours to days is about −100 bar (3).”
#Pressure-to-Depth and Depth-to-Pressure Calculator. Rusty / Blue Robotics. 2015
https://bluerobotics.com/learn/pressure-depth-calculator/?input=100%20m
– Nothing humans have ever built comes even close to this power. Even our best machines can’t pull water higher than about 10 meters, because the negative pressure required to pull hard enough makes it boil.
The limit of 10 m (more specifically, ca. 10.3 m) applies when a pump acts by suction alone. An example would be a siphon where the water is initially forced into the tube via suction. At sea level, the required air pressure to “lift” water above 10.3 m via suction alone would require pressures so low that water at the top of the column begins to boil.
Modern pumping systems employ various engineering tricks to circumvent this ~10 m limit, e.g. by having the pump be under water and then “pushing” the water upwards (as opposed to applying “sucking forces” at the top).
#Boatwright A, Hughes S, Barry J. The height limit of a siphon. Sci Rep. 2015 https://pubmed.ncbi.nlm.nih.gov/26628323/
Quote: “The maximum height of a siphon is generally assumed to be dependent on barometric pressure—about 10 m at sea level. This limit arises because the pressure in a siphon above the upper reservoir level is below the ambient pressure and when the height of a siphon approaches 10 m, the pressure at the crown of the siphon falls below the vapour pressure of water causing water to boil breaking the column.”
– But the water pipes of trees are so tiny and narrow, almost perfectly airtight that despite the insane suction pressure inside a tree, water stays liquid and reaches the top.
#Sperry, J.S., Hacke, U.G. and Pittermann, J. (2006), Size and function in conifer tracheids and angiosperm vessels†. Am. J. Bot., 93: 1490-1500.
https://doi.org/10.3732/ajb.93.10.1490
Quote: “Figure 1. Scaling of conduit diameter with length for conifer tracheids and angiosperm vessels. Data points are mean values for different species or organs; vessel data from stems only. Regression slopes (reduced major axis) were 0.87 in conifers and 0.53 in angiosperms and not different from the 0.67 scaling predicted to maintain the observed proportionality between lumen and end-wall resistivity (P > 0.05 in conifers, P > 0.46 in angiosperms). Data from Pittermann et al. (2005) and Hacke et al. (2006)”
#Brown HR, Sutton AP. Trees suck. Notes on the physics of transpiration in trees. Prog Biophys Mol Biol. 2025
https://pubmed.ncbi.nlm.nih.gov/39725358/
Quote: “According to the Cohesion-Tension (CT) theory, evaporation taking place in daylight hours at the menisci (water–air interfaces) of minute columns of water in the interstices or pores within mesophyll cell walls inside leaves of a tree, together with capillary action within these columns, serve to suck water upwards through the xylem system from the roots. This mechanism relies on the forces of cohesion (H-bonds) between water molecules which account for water’s ability to form continuous threads within the xylem, capable through their “tensile strength” of being pulled from above.1 For tall trees the water in this process is under tension (negative absolute pressure), so metastable and prone to cavitation. And yet, as Galileo might say, it moves.”
– As the tree ages old xylem cells eventually stop working and fill up with resins and other protective substances. Slowly they turn into heartwood, a dense, chemically fortified core that is extremely resistant to decay. The core of a mighty tree.
#Yang S, Qin F, Wang S, Li X, et al. Advances in the Study of Heartwood Formation in Trees. Life (Basel). 2025
https://pmc.ncbi.nlm.nih.gov/articles/PMC11766836/
Quote: “Wood is comprised predominantly of secondary xylem, a vital tissue that fulfills the dual role of providing mechanical support and facilitating water conduction [1]. The production of secondary xylem is attributed to the activity of the cambial zone, a lateral meristem situated between the primary xylem and phloem. The cambial zone is responsible for the diameter growth of tree axes, both in the shoot and the root, through its continuous cell division and subsequent differentiation. Newly formed cells are attached to the outer layer of the primary xylem. This contributes to the radial expansion [2]. As the cambial zone expands outward, secondary xylem undergoes expansion and thickening, resulting in an augmentation of stem diameter [3]. Sapwood (SW) is adjacent to the inside of the cambial zone, while the heartwood (HW) is adjacent to the inside of the sapwood. And the intermediate wood is present between the heartwood and sapwood. They can usually be identified in tree disc by their respective colors [4,5] (Figure 1). The sapwood, characterized by its lighter color and greater porosity, contrasts with the heartwood, which is darker in color and more compact in structure [1,6]. The intermediate wood acts as a transition zone (TZ) between the middle sapwood and the heartwood. In some instances, the TZ can be distinguished by its white color. This phenomenon occurs when water loss occurs in the intermediate wood region during heartwood formation [7].
The location of heartwood, transition zone, sapwood, growth ring, cambial zone, and bark in the sandalwood disc is demonstrated, with brown heartwood in the centre and light sapwood in the periphery.
Sapwood (SW) represents the secondary xylem generated by the vascular cambium during the later life stages of a tree [8]. It is characterized by a relatively low accumulation of substances such as resins, gums, and tannins [9]. Heartwood is earlier-formed secondary xylem; both angiosperms and gymnosperms have heartwood [10]. The sapwood (SW) remains active for a finite duration, subsequently transitioning into heartwood (HW). At the micro level, vessels and tracheids are present in secondary xylem, which are responsible for conducting water and nutrients, as well as wood parenchyma cells and wood rays, which serve functions in nutrient storage and transport [11]. Gymnosperms characteristically lack vessels, except in Gnetopsidas, where some species exhibit vessel presence [12]. During the process of heartwood formation, there is an explosive increase in cellular metabolic activity in the transition zone [13,14], during which primary metabolism proceeds rapidly, resulting in substantial nutrient accumulation. This process leads to the accumulation of secondary metabolites by the parenchyma cells in the sapwood, which subsequently undergo programmed cell death (PCD) to form HW tissue [15,16,17,18]. Vessels and tracheids (or tracheids only) within HW lost their conductive capabilities due to the formation of tyloses from the dead parenchyma cells, which block the vessel pores as a result of metabolic processes [19]. Consequently, HW forms a hard cylinder composed of dead cells, with an accumulation of various secondary metabolites (woody known as extractives) [13]. These extractives usually exhibit distinctive coloration, resulting in a darker hue for HW that can be readily discerned from SW.”
– But water is only one half of the story. The sugar produced in the sky needs to be transported to nourish cells down below. And cells from the roots to the leaves need to coordinate and exchange information about damage and growth. This is the job of the cambium stem cells that grow outwards: The phloem.
#De Schepper V, De Swaef T, Bauweraerts I, Steppe K. Phloem transport: a review of mechanisms and controls. J Exp Bot. 2013
https://pubmed.ncbi.nlm.nih.gov/24106290/
Quote: “The evolutionary journey of plants onto land involved the differentiation of the plant body into decentralized organs, such as leaves, roots, stem, and branches. These organs are interconnected at the whole-plant level by long-distance transport. Besides water, sugars are one of the most important components involved in this transport. The phloem tissue is the principal sugar conductive tissue in plants. Over 80 years ago, Ernest Münch (1930) proposed the now widely accepted mechanism for phloem transport. According to his theory, the mass flow in the phloem is driven by an osmotically generated pressure gradient. As the sieve pores interconnect the protoplasts of the sieve tubes, the transport in the sieve tube itself is a mass flow driven by a pressure (or turgor) gradient. Because the sieve tubes are separated by a plasma membrane from the surrounding plant cells, a higher solute concentration indirectly implies a higher turgor pressure as water will enter the sieve tubes by osmosis (Gould et al., 2005). The pressure gradient in the sieve tubes is generated by the accumulation (loading) of sugars and other osmotic substances at the sources and by their release (unloading) at the sinks (Fig. 1). The sources are mainly leaves, whereas all energy-demanding or storage tissues are sinks (e.g. roots, fruits, and meristematic tissues).”
#Van Bel AJ, Helariutta Y, Thompson GA, Ton J, et al. Phloem: the integrative avenue for resource distribution, signaling, and defense. Front Plant Sci. 2013
https://pmc.ncbi.nlm.nih.gov/articles/PMC3838965/
Quote: “Research over the past 20 years has revealed new functions of the phloem beyond resource allocation to a system that combines distribution and messaging (Thompson and van Bel, 2013) analogous to the circulatory and nervous systems in animals. Apart from allocating resources for maintenance and growth, the phloem distributes hormonal signals and a broad spectrum of protein- and RNA-based messages throughout the plant to regulate a myriad of physiological and developmental processes. Resources and signals, collectively, coordinate development, and growth as well as integrate responses to both biotic and abiotic environmental challenges.”
– As the Cells of the phloem grow outwards they separate into three teams and make a brutal compromise.
The first group are Sieve cells and their grim fate is to become a living transport system. Although “living” is not exactly right. As they mature they start to destroy themselves, digest their organelles and even their nuclei that houses their genetic code. At the same time they hollow themselves out, connecting to the sieve cells above and below them. This goes on until they are a sad shadow of living beings. A drooling, living tool without a brain or arms, unable to support themselves.
Only the second group, their companion cells, keeps them alive. They connect with the crippled Sieve cells via tiny channels and start maintaining them. Sending over energy, instructions or repairing them if needed.
The anatomy of Sieve cells and whether they have companion cells depends on the species of tree. Here, we describe how it works in Angiosperms, a group of trees that includes oak, maple, and fruit trees such as apple trees. Many of the oldest living trees belong to the Gymnosperms, where sieve cells are instead surrounded by albuminous cells (also called Strasburger cells).
#Ham BK, Lucas WJ. The angiosperm phloem sieve tube system: a role in mediating traits important to modern agriculture. J Exp Bot. 2014
https://pubmed.ncbi.nlm.nih.gov/24368503/
Quote: “The second component of the vascular system, the phloem, develops also from the cambium (Fig. 1A). Here, the conducting cells, termed sieve cells in lower vascular plants and sieve elements (SEs) in the advanced angiosperms, are not dead at maturity. In the angiosperms, during SE maturation, each cell in a specific file undergoes a process in which its vacuole and nucleus are degraded, and many of the organelles are either degraded or become greatly reduced in number. At maturity, each SE retains its plasma membrane and cytoplasmic continuity between neighbouring SEs is established by structurally modifying plasmodesmata (PD) to form sieve plate pores (Fig. 1B). Such files of enucleate cells form an individual sieve tube (ST), and collectively, all sieve tubes within the plant establish the sieve tube system which functions in the delivery of nutrients (as fixed carbon, amino acids, etc.) to developing tissues (Fig. 1A and B).
Physiological functions of the mature enucleate SEs are maintained by specialized cells, termed companion cells (CCs). This support is achieved by the intercellular movement of macromolecules through the PD that connect CCs to their neighbouring SEs (Fig. 1C). Although it is generally considered that mature SEs lack the capacity for protein synthesis, recent studies on the sieve tube system of the cucurbits (Lin et al., 2009; Ma et al., 2010) identified many ribosomal and associated components involved in protein synthesis. In addition, a comparative analysis between the phloem proteome and the special messenger (m)RNA population identified from Cucurbita maxima (pumpkin) phloem sap indicated a match between a significant number of mRNA species and their cognate proteins. These findings are consistent with a model in which mRNA produced in the CCs is trafficked through the CC–SE PD into the SEs where protein is produced (Fig. 1C). Of interest here is the recent finding that an in vitro wheat germ lysate translation system was found to be inhibited by the presence of tRNA fragments (30–90 bases) isolated from pumpkin phloem sap (Zhang et al., 2009). Hence, it is possible that such tRNA fragments may serve to regulate translation in the enucleate sieve tube system. In any event, either mRNA import into SEs, followed by protein synthesis, or protein synthesis in CCs, followed by trafficking through the PD into the SEs, must occur for the continued functioning of mature SEs over their life span of from weeks to years, without their nuclei.”
#Nickrent DL. Plant Anatomy Lecture 13 - Phloem. Retrieved June 2025
https://nickrentlab.siu.edu/PlantAnatomyWeb/LecturesDLN/Lecture13_Phloem.html
Quote: “Companion cells are ontogenetically related to sieve elements, derived from the same precursor cell. Can be one or more companion cells on one or more sides of sieve element. In gymnosperms, the functional counterpart of companion cells are called albuminous cells (= Strasburger cells). These are not derived from the same precursor as sieve elements but originate from adjacent cells.”
– Running along the entire tree, these two teams form a tiny and very thin layer of living sugar pipelines and signal cables that stretch throughout the entire body. Providing food and information wherever it is needed in the tree.
#De Schepper V, De Swaef T, Bauweraerts I, Steppe K. Phloem transport: a review of mechanisms and controls. J Exp Bot. 2013
https://pubmed.ncbi.nlm.nih.gov/24106290/
Quote: “The evolutionary journey of plants onto land involved the differentiation of the plant body into decentralized organs, such as leaves, roots, stem, and branches. These organs are interconnected at the whole-plant level by long-distance transport. Besides water, sugars are one of the most important components involved in this transport. The phloem tissue is the principal sugar conductive tissue in plants. Over 80 years ago, Ernest Münch (1930) proposed the now widely accepted mechanism for phloem transport. According to his theory, the mass flow in the phloem is driven by an osmotically generated pressure gradient. As the sieve pores interconnect the protoplasts of the sieve tubes, the transport in the sieve tube itself is a mass flow driven by a pressure (or turgor) gradient. Because the sieve tubes are separated by a plasma membrane from the surrounding plant cells, a higher solute concentration indirectly implies a higher turgor pressure as water will enter the sieve tubes by osmosis (Gould et al., 2005). The pressure gradient in the sieve tubes is generated by the accumulation (loading) of sugars and other osmotic substances at the sources and by their release (unloading) at the sinks (Fig. 1). The sources are mainly leaves, whereas all energy-demanding or storage tissues are sinks (e.g. roots, fruits, and meristematic tissues).”
#Van Bel AJ, Helariutta Y, Thompson GA, Ton J, et al. Phloem: the integrative avenue for resource distribution, signaling, and defense. Front Plant Sci. 2013
https://pmc.ncbi.nlm.nih.gov/articles/PMC3838965/
Quote: “Research over the past 20 years has revealed new functions of the phloem beyond resource allocation to a system that combines distribution and messaging (Thompson and van Bel, 2013) analogous to the circulatory and nervous systems in animals. Apart from allocating resources for maintenance and growth, the phloem distributes hormonal signals and a broad spectrum of protein- and RNA-based messages throughout the plant to regulate a myriad of physiological and developmental processes. Resources and signals, collectively, coordinate development, and growth as well as integrate responses to both biotic and abiotic environmental challenges.”
– The third team are Parenchyma cells, the silent workers of the tree that are carrying out essential labor in the background. Some are like mini pantries that store nutrients, sugars or water that the tree uses to survive the winter, when it is unable to produce food. Others are like mini healers that can repair damage, while some go onto the offensive and create toxins and anti fungal bio weapons to kill intruders.
#Słupianek A, Dolzblasz A, Sokołowska K. Xylem Parenchyma-Role and Relevance in Wood Functioning in Trees. Plants (Basel). 2021
https://pmc.ncbi.nlm.nih.gov/articles/PMC8235782/
Quote: “One of the primary roles of xylem parenchyma cells is their function as storage tissue (e.g., [69,70]). The pool of stored compounds depends on stem anatomical features, and a key variable in their storage ability is the abundance of the xylem parenchyma, usually the only cells that remain metabolically active in mature secondary xylem [26,43]. Since living xylem parenchyma cells are an essential element of wood, molecules such as water, non-structural carbohydrates (NSCs), and lipids stored in these cells constitute a large portion of accumulated compounds.
[...]
Xylem sap transported within the interconnected system of dead tracheary elements under negative pressure (tension) is prone to cavitation [105], which can be induced by air seeding and leads to bubble formation when air passes through the pores of the pit membranes [106,107,108]. Cavitation can also be induced by air bubbles adhered to the cracks in the walls of dead elements [109,110,111]. Further expansion of the gas bubble causes embolism formation, which can spread within a single tracheary element and to other elements of the vascular system relatively quickly and easily. [...] Embolism repair in trees is a three-step process, including (1) sensing embolism formation signals by contact cells/VACs, (2) generating an osmotic gradient between embolised conduits and adjacent living cells, and consequently creating a driving force for (3) water movement into the tracheary elements.
[...]
Compartmentalisation of Decay in Trees (CODIT) Model.
The CODIT model was developed in the 1970s, based on numerous observations of the plant defence responses against infections caused by decaying fungal pathogens [161,162]. The model assumes that defence mechanisms in big woody organisms rely on anatomical compartments that exist in the secondary xylem or those that are created immediately after wounding to prevent the spread of a pathogen [161,162,163]. Subsequently, an expanded version of the CODIT model was proposed, according to which, compartmentalisation was used in a much broader sense, including the responses to other biotic (e.g., herbivores, insects) and abiotic (e.g., damages caused via wind and snow) factors [163,164,165,166]. The latest interpretation of the CODIT model applies to responses to any desiccation-inducing phenomenon [167]. Therefore, the postulate that “D” in the CODIT model should stand for “Damage” or “Dysfunction” rather than only for “Decay”, as in the original model, seems fully entitled [163,164,165,166].
According to the CODIT model, a plant’s reaction to a wounding factor can be viewed as a two-part process, which is based on the so-called four “walls” [161]. The xylem parenchyma cells play a fundamental role in limiting the spread of the damage, as they are involved in each of the four walls of the CODIT model [161,163]. The walls are a representation of the anatomical compartments, which (1) are already present at the time of wounding in the secondary xylem and form a reaction zone (Part I, Walls 1–3), and (2) are formed after wounding, creating a barrier zone (Part II, Wall 4). Notably, the walls in the CODIT model have a gradable barrier strength, with Wall 1 being the weakest and Wall 4 being the strongest [161,163].”
#Kotowska MM, Wright IJ, Westoby M. Parenchyma Abundance in Wood of Evergreen Trees Varies Independently of Nutrients. Front Plant Sci. 2020
https://pmc.ncbi.nlm.nih.gov/articles/PMC7045414/
Quote: “Various physiological functions have been linked to parenchyma including transport and storage of non-structural carbohydrates (Johnson et al., 2012; Plavcová et al., 2016), hydraulic capacitance and vessel refilling after embolism (e.g. De Boer and Volkov, 2003; Secchi et al., 2017), and accumulation of anti-microbial compounds as defense against pathogens and heartwood formation (Spicer, 2005; Morris et al., 2016). Functionality within parenchyma is likely to be influenced by the orientation and connectivity of parenchyma cells—radially as rays, or axially—and additionally by the arrangement type and proximity to tracheary elements—paratracheal or apotracheal—which are commonly used as anatomical classification criteria (Carlquist, 2001). While these distribution patterns have been confirmed for their taxonomic value (Baas, 1982; Wheeler and Baas, 1998), inferences on their functionality often remain indirect or based on morphological observation (Secchi et al., 2017).”
– On top of the phloem sits another layer of stem cells. They are producing a second conveyor belt of death, moving outside. Special cells grow from this layer and as they mature, just like the xylem cells in the center, they kill themselves for the team, turning into a hard guard wall – the bark. Just like your skin, it protects the tiny living layer from damage, parasites and invaders.
The additional layer of stem cells we refer to here is the “phellogen”, sometimes also called “cork cambium”. The phellogen produces the periderm, the outermost layer of the bark.
#Teixeira RT. Cork Development: What Lies Within. Plants (Basel). 2022
https://pubmed.ncbi.nlm.nih.gov/36297695/
Quote: “The cork layer present in all dicotyledonous plant species with radial growth is the result of the phellogen activity, a secondary meristem that produces phellem (cork) to the outside and phelloderm inwards. These three different tissues form the periderm, an efficient protective tissue working as a barrier against external factors such as environmental aggressions and pathogen attacks.”
#Alonso-Serra, J., Safronov, O., Lim, K.-J., et al. (2019), Tissue-specific study across the stem reveals the chemistry and transcriptome dynamics of birch bark. New Phytol, 222: 1816-1831.
https://doi.org/10.1111/nph.15725
Quote: “Tree bark displays a broad morphological diversity. Most angiosperm and gymnosperm tree species typically have thick and fissured barks (for example Quercus spp., Picea spp.), while smooth textures and various colors are found, for example, in cherry trees (Prunus spp.) and birches (Betula spp.). Anatomically, bark consists of tissues outwards of the vascular cambium: the phloem and periderm, the latter comprised of phelloderm, phellogen and phellem. Bark tissues originate from stem secondary development, which in woody plants takes place in two distinct lateral meristems. The first meristem, vascular cambium, produces the phloem and xylem tissues. The second meristem, phellogen (cork cambium), produces phellem towards outside of the meristem, and phelloderm towards the inside. Phellem constitutes the outermost barrier between the stem and the environment, while phelloderm is typically limited to a few parenchymatic cell layers.”
#Serra O, Mähönen AP, Hetherington AJ, Ragni L. The Making of Plant Armor: The Periderm. Annu Rev Plant Biol. 2022
https://pubmed.ncbi.nlm.nih.gov/34985930/
Quote: “As tissues mature, a new protective armor, the periderm, is formed. The periderm replaces the endodermis and epidermis when they break or die due to root or shoot thickening (secondary/ radial growth) (Figure 1a,b). In addition, another type of periderm, wound periderm, is produced in response to injuries and forms to repair and seal the wounded area during the healing process. Periderm formation is prevalent in trees but also occurs in many herbaceous plants that undergo secondary growth (42, 43). In many woody plants, when the first periderm is no longer functional, and thus cannot protect the growing tissuey, it is replaced by a new periderm that forms underneath, which in turn is replaced by (sub)sequent periderms forming over the years, leading to the formation of the rhytidome. Hence, the rhytidome comprises a succession of dead periderm layers alternating with layers of nonfunctional secondary phloem plus the last active periderm (4, 42, 135) [...]. By contrast, in a few species such as cork oak, it appears that the same periderm grows over the years, known as the persistent periderm. In both cases, the periderm and the rhytidome are also commonly referred to as outer bark [...].”
#Encyclopaedia Britannica. Cambium. Retrieved June 2025
– So what is a tree? The living part of the stem is really just this extremely thin and tiny layer, a few millimeters thick, sitting on a thick mountain of cell corpses, surrounded by another layer of cell corpses.
The thickness of the living phloem and xylem layer depends on the species of tree, its age, and to some extent also on environmental conditions. The cambium is sandwiched between the phloem and xylem layer but it is only a few cells wide, so it does not significantly affect the thickness of the living layer in a tree stem.
Phloem (also called “inner bark”) thickness has been reported for Aleppo pine (Pinus halapensis) as around 1 mm, and around 6.7 mm for sessile oak (Quercus petraea).
The living part of the xylem is also called “sapwood”, and its thickness typically ranges from 10-30 mm but varies strongly depending on tree species, age, and size. However, most cells in the sapwood are dead: depending on the tree species, the proportion of live cells in sapwood can be as low as 5%.
#Wybouw B, Zhang X, Mähönen AP. Vascular cambium stem cells: past, present and future. New Phytol. 2024
https://pubmed.ncbi.nlm.nih.gov/38890801/
Quote: “In plants, growth that originates from the shoot and root apical meristems (SAM and RAM) is called primary or longitudinal growth. Later, secondary or radial growth, takes place in lateral meristems referred to as vascular and cork cambium. The vascular cambium, the focus of this review, provides new cells towards both secondary xylem and secondary phloem, which are positioned on opposite sides of the vascular cambium (Box 1) (Fig. 1). The stem cell concept (Box 1) is not well established in vascular cambium research, and thus the words cambium and stem cell have often been used interchangeably in the literature. Vascular cambium or cambial zone refers to the anatomically recognized meristematic cells in the secondary tissue (Evert, 2006), and the number of these cells in each radial file in vascular cambia can vary from one to several dozen, depending on the species. Lineage tracing studies (Box 1) both in Arabidopsis and poplar show that there is a single bifacial stem cell in each radial cell file producing both secondary xylem and phloem (Fig. 2a,b; Bossinger & Spokevicius, 2018; Shi et al., 2019; Smetana et al., 2019).”
#Preisler, Y., Tatarinov, F., Grünzweig, J.M. and Yakir, D. (2022), CORRIGENDUM: Seeking the “point of no return” in the sequence of events leading to mortality of mature trees. Plant Cell Environ, 45: 1333-1333.
https://doi.org/10.1111/pce.14302
Quote: “Aleppo pine phloem is ~1 mm thick and the bark can reach 2–3 cm thick.”
#Gričar, J., Jagodic, Š., & Prislan, P. (2015). Structure and subsequent seasonal changes in the bark of sessile oak (Quercus petraea). Trees, 29, 747-757. https://doi.org/10.1007/s00468-015-1153-z
Quote: “Bark tissue in oak consists of non-collapsed and collapsed inner bark and of rhytidome (Fig. 2). The entire bark was 18.3 ± 4.8 mm wide, of which about 39 % (6.7 ± 1.1 mm) was inner bark and 61 % (11.7 ± 4.6 mm) rhytidome.”
#K.C. Yang and H.G. Murchison. 1992. Sapwood thickness in Pinuscontorta var. latifolia. Canadian Journal of Forest Research. 22(12): 2004-2006.
https://doi.org/10.1139/x92-262
Quote: “The vertical variation in the number of sapwood growth rings and sapwood thickness in Pinuscontorta Dougl. var. latifolia Engelm. was studied in relation to aspect, tree age, bole diameter, sapwood radial growth rate, and whole-xylem radial growth rate. Samples from 19 trees growing on the western slope of the Rocky Mountains near Kamloops, British Columbia, Canada, formed the data base. Sapwood width for individual trees ranged from 20 to 26 mm for both the east and west aspects, and was constant at various heights of the tree bole. Sapwood width for this species was found to be independent of age, diameter, sapwood radial growth rate, and whole-xylem radial growth rate. Sapwood consisted of 25 to 50 growth rings and decreased from the ground level upward to the tree crown. The number of sapwood growth rings was strongly correlated with age, diameter, and radial growth rates for both sapwood and the whole tree. No significant correlation existed between sapwood width and sapwood growth-ring counts.”
#Miranda, I., Gominho, J. & Pereira, H. Variation of heartwood and sapwood in 18-year-old Eucalyptus globulus trees grown with different spacings. Trees 23, 367–372 (2009). https://doi.org/10.1007/s00468-008-0285-9
Quote: “Heartwood and sapwood development was studied in 18-year-old Eucalyptus globulus trees from pulpwood plantations with different spacings (3 × 2, 3 × 3, 4 × 3, 4 × 4 and 4 × 5 m), on cross-sectional discs taken at breast height. The trees possessed a large proportion of heartwood, on average 60% of the wood cross-sectional surface. Spacing was a statistically significant source of variation of heartwood area, which ranged between 99 and 206 cm2 for the closer (3 × 2) and wider (4 × 5) spacings, respectively. There was a positive and high statistical significant correlation between heartwood diameter and tree diameter (heartwood diameter = −0.272 + 0.616 dbh; r 2 = 0.77; P < 0.001), and larger trees contained more heartwood regardless of spacing. Heartwood proportion in cross-section remained practically constant between spacings but increased with tree diameter class: 55.1, 62.2, 65.0 and 69.5% for diameter at breast height classes <15, 15–20, 20–25 and >25 cm, respectively. The sapwood width did not depend on tree diameter growth and remained practically constant at an average of 18 mm (range 15–21 mm), but sapwood area showed a good linear regression with tree diameter. Therefore, tree growth enhancement factors, such as wide spacings, will induce formation of larger heartwoods that can negatively impact raw-material quality for pulping. The increase in heartwood in relation with tree dimensions should therefore be taken into account when designing forest management guidelines.”
#Sofia Knapic, Fátima Tavares, Helena Pereira, Heartwood and sapwood variation in Acacia melanoxylon R. Br. trees in Portugal, Forestry: An International Journal of Forest Research. 2006
https://doi.org/10.1093/forestry/cpl010
Quote: “The development of heartwood and sapwood in blackwood (Acacia melanoxylon R. Br.) was studied in a total of 20 trees with a 40-cm-diameter class sampled over four stands in northern Portugal at harvest for timber production. Stem discs with 5-cm thickness were taken at different height levels (stem base and 5, 15, 35, 50, 65, 75, 85 and 90 per cent of total height). Cross-sectional area and heartwood area were measured by image analysis. Heartwood represented a substantial part of the trees and within the tree it attained on average 81 per cent of total height, and represented 69, 62, 58 and 44 per cent of the stem cross-sectional area, respectively, at 5, 35, 50 and 65 per cent of total tree height. The heartwood followed closely the stem wood profile both axially and radially. Estimation of heartwood dimensions from external wood diameters (either over or under bark) was possible using a linear model, which had a very high correlation coefficient (R2 = 0.97). The sapwood radial width showed a very small variation within and between trees and maintained a constant value of 31 mm up to ∼65 per cent of tree height. No site influence was found for the heartwood development and the between-tree variation was small. The species and the sampled individuals in Portugal showed potential for the diversification of forest production and increasing the industrial supply of a valuable timber hardwood.”
#Morris H, Plavcová L, et al. A global analysis of parenchyma tissue fractions in secondary xylem of seed plants. New Phytol. 2016
https://pubmed.ncbi.nlm.nih.gov/26551018/
Quote: “Parenchyma is an important tissue in secondary xylem of seed plants, with functions ranging from storage to defence and with effects on the physical and mechanical properties of wood. Currently, we lack a large-scale quantitative analysis of ray parenchyma (RP) and axial parenchyma (AP) tissue fractions.
Here, we use data from the literature on AP and RP fractions to investigate the potential relationships of climate and growth form with total ray and axial parenchyma fractions (RAP).
We found a 29-fold variation in RAP fraction, which was more strongly related to temperature than with precipitation. Stem succulents had the highest RAP values (mean ± SD: 70.2 ± 22.0%), followed by lianas (50.1 ± 16.3%), angiosperm trees and shrubs (26.3 ± 12.4%), and conifers (7.6 ± 2.6%). Differences in RAP fraction between temperate and tropical angiosperm trees (21.1 ± 7.9% vs 36.2 ± 13.4%, respectively) are due to differences in the AP fraction, which is typically three times higher in tropical than in temperate trees, but not in RP fraction.
[...] Besides the occurrence of so-called living fibres (Wolkinger, 1970, 1971), ray and axial parenchyma (RAP) tissue represents the bulk of living cells in wood.
[...] RAP amount responds to both phylogenetic and environmental factors (Fig. 2). Also, RAP, with the exclusion of rayless species (Carlquist, 2001), is typically higher in angiosperm than in conifer wood for both RP (15–20% compared to 4–8%) and AP (≤ 1% to ≥ 30% compared to ≤ 1%), respectively (Koch, 1985; Spicer, 2014). The level of RAP also depends on growth forms, with a surprisingly high level occurring in woody succulents and lianas (Hearn, 2009).”
– The vast majority of the biomass of a tree is dead.
When we refer to the “biomass of the tree” here, we are referring to the visible, above-ground structure of the tree trunk, which is mostly made of wood. Wood is composed of different cell types, some of them alive and some not. The structure of wood, and thus the relative proportion of these cell types, varies a lot between tree species: some species are much more “woody” than others. In conifers, the fraction of parenchyma in the wood - which represents the bulk of live wood cells - can be less than 5%.
The proportion of dead tissue also changes substantially as the tree grows older. Typically, the older the tree, the higher the proportion of dead tissue in the wood. The non-wood structures of the tree (e.g. the leaves, roots and shoots) are mostly composed of live tissue.
#Morris H, Plavcová L, et al. A global analysis of parenchyma tissue fractions in secondary xylem of seed plants. New Phytol. 2016
https://pubmed.ncbi.nlm.nih.gov/26551018/
Quote: “Parenchyma is an important tissue in secondary xylem of seed plants, with functions ranging from storage to defence and with effects on the physical and mechanical properties of wood. Currently, we lack a large-scale quantitative analysis of ray parenchyma (RP) and axial parenchyma (AP) tissue fractions.
Here, we use data from the literature on AP and RP fractions to investigate the potential relationships of climate and growth form with total ray and axial parenchyma fractions (RAP).
We found a 29-fold variation in RAP fraction, which was more strongly related to temperature than with precipitation. Stem succulents had the highest RAP values (mean ± SD: 70.2 ± 22.0%), followed by lianas (50.1 ± 16.3%), angiosperm trees and shrubs (26.3 ± 12.4%), and conifers (7.6 ± 2.6%). Differences in RAP fraction between temperate and tropical angiosperm trees (21.1 ± 7.9% vs 36.2 ± 13.4%, respectively) are due to differences in the AP fraction, which is typically three times higher in tropical than in temperate trees, but not in RP fraction.
[...] Besides the occurrence of so-called living fibres (Wolkinger, 1970, 1971), ray and axial parenchyma (RAP) tissue represents the bulk of living cells in wood.
[...] RAP amount responds to both phylogenetic and environmental factors (Fig. 2). Also, RAP, with the exclusion of rayless species (Carlquist, 2001), is typically higher in angiosperm than in conifer wood for both RP (15–20% compared to 4–8%) and AP (≤ 1% to ≥ 30% compared to ≤ 1%), respectively (Koch, 1985; Spicer, 2014). The level of RAP also depends on growth forms, with a surprisingly high level occurring in woody succulents and lianas (Hearn, 2009).”
– This is also why you really should not damage the bark of trees because while it seems you are only doing a little damage, you are actually killing the living part of the tree.
Trees may be able to heal small wounds to their bark, but larger wounds can be lethal to them. “Bark girdling”, where a thin strip of bark is removed from the entire circumference of the lower section of a tree, is a deliberately applied forestry technique to kill trees without cutting them down.
#Karunarathne SI, Spokevicius AV, Bossinger G, Golz JF. Trees need closure too: Wound-induced secondary vascular tissue regeneration. Plant Sci. 2024
https://pubmed.ncbi.nlm.nih.gov/38070652/
Quote: “Long-lived trees are exposed to a myriad of biological and environmental stresses that may result in wounding, leading to a loss of bark and the underlying vascular cambium. This affects both wood formation and the quality of timber arising from the tree. In addition, the exposed wound site is a potential entry point for pathogens that cause disease. In response to wounding, trees have the capacity to regenerate lost or damaged tissues at this site.”
#Methods for Managing Weeds in Wildlands: Non-chemical Control - Girdling. Weed Control User Tool (WeedCUT). Retrieved June 2025
https://weedcut.ipm.ucanr.edu/management-practices/girdling/#gsc.tab=0
Quote: “Girdling is a technique that kills woody plants in place without cutting them down. It uses a sharp tool to cut through the bark of a woody plant in a strip all the way around the stem down to the wood. This severs the vascular cambium of the woody plant and cuts nutrient flow between the foliage and the roots. As a result, roots are starved of nutrients and the plant cannot grow more stems and foliage. Unless the plant can heal over the wound it will die unless it has reproductive capacity to send up resprouts.
Girdling is often used to control or eradicate large woody species where physical removal of woody biomass would be undesirable, unpopular, or cost prohibitive.”
– But unless a tree is stopped by droughts, diseases, storms or a human axe, this system of being mostly dead kind of makes trees potentially immortal. They don’t age like we do, in principle they could grow this way nearly forever.
Aging is also called “senescence” in biology. Most tree species live between 100 to 300-400 years, and while they are highly likely to die from external causes, they still show signs of senescence eventually. But some species are known to live to several thousand years of age, calling into question whether (or how) they senesce at all, and what their “true” lifespan limits are in the absence of external factors such as diseases, storms etc. This is an active field of scientific research. While in theory trees could live for an extremely long time, researchers have found hints that even in the most ancient trees there are very subtle signs of age-related shifts in physiology over time. But whether this is “true aging” in the same way that humans and other animals age is very hard to answer, and maybe we will never be able to. Because trees live far beyond our own lifespans, their full story is always just beyond our reach.
#Munné-Bosch S. Limits to Tree Growth and Longevity. Trends Plant Sci. 2018
https://pubmed.ncbi.nlm.nih.gov/30166058/
Quote: “Tree growth and longevity are key features to understand fundamental issues of plant biology, environmental sciences, and current forest management plans. Here I discuss current evidence on the limits of tree growth and longevity and present a new conceptual framework to understand how and why they are closely interconnected. Despite the tremendous plasticity of trees, growth and longevity are limited not only by biotic and abiotic stresses, but also by age-related structural constraints such as height-related hydraulic limitations and vascular discontinuities, which are strongly species specific. Continuous growth and plastic branching may serve as a means to reach extreme longevities in some nonclonal trees, but even in these millennial organisms immortality can be attained only through the germ line.”
#Piovesan G, Biondi F. On tree longevity. New Phytol. 2021
https://pubmed.ncbi.nlm.nih.gov/33305422/
Quote: “Published evidence suggests that trees do not die because of genetically programmed senescence in their meristems, but rather are killed by an external agent or a disturbance event. Long tree lifespans are therefore allowed by specific combinations of life history traits within realized niches that support resistance to, or avoidance of, extrinsic mortality.”
#Munné-Bosch S. Long-Lived Trees Are Not Immortal. Trends Plant Sci. 2020
https://pubmed.ncbi.nlm.nih.gov/32732116/
Quote: “Separating out the different effects of ageing on long-lived trees remains challenging. Herein current approaches used to explore senescence in millennial trees are highlighted. Molecular and biochemical analyses of the vascular cambium provide novel insight into the extent to which millennial trees can withstand the wear and tear of ageing.”
#Zuleta D, Arellano G, Muller-Landau HC, McMahon SM, et al. Individual tree damage dominates mortality risk factors across six tropical forests. New Phytol. 2022
https://pubmed.ncbi.nlm.nih.gov/34716605/
Quote: “The relative importance of tree mortality risk factors remains unknown, especially in diverse tropical forests where species may vary widely in their responses to particular conditions. We present a new framework for quantifying the importance of mortality risk factors and apply it to compare 19 risks on 31 203 trees (1977 species) in 14 one-year periods in six tropical forests. We defined a condition as a risk factor for a species if it was associated with at least a doubling of mortality rate in univariate analyses. For each risk, we estimated prevalence (frequency), lethality (difference in mortality between trees with and without the risk) and impact ('excess mortality' associated with the risk, relative to stand-level mortality). The most impactful risk factors were light limitation and crown/trunk loss; the most prevalent were light limitation and small size; the most lethal were leaf damage and wounds. Modes of death (standing, broken and uprooted) had limited links with previous conditions and mortality risk factors.”
#Esquivel-Muelbert A et al. Tree mode of death and mortality risk factors across Amazon forests. Nat Commun. 2020
https://pubmed.ncbi.nlm.nih.gov/33168823/
Quote: “While tree mortality rates vary greatly Amazon-wide, on average trees are as likely to die standing as they are broken or uprooted—modes of death with different ecological consequences. Species-level growth rate is the single most important predictor of tree death in Amazonia, with faster-growing species being at higher risk. Within species, however, the slowest-growing trees are at greatest risk while the effect of tree size varies across the basin. In the driest Amazonian region species-level bioclimatic distributional patterns also predict the risk of death, suggesting that these forests are experiencing climatic conditions beyond their adaptative limits.”
#Rocky Mountain Tree-Ring Research. OLDLIST, a database of old trees. Retrieved June 2025
https://www.rmtrr.org/oldlist.htm
Quote: “OldList is a database of ancient trees. Its purpose is to identify maximum ages that different species in different localities may attain such that exceptionally old age individuals are recognized. [...] * Tree is still living as of late 2010s; age given is additional years since it was first sampled when this is known. [...] ** Crossdated ages are derived through recognized dendrochronological procedures (e.g., Stokes and Smiley 1968; Swetnam, Thompson, and Sutherland 1985; Schweingruber 1987; Speer 2010). [...] Species: Pinus longaeva, Great Basin bristlecone pine. Age: 4850*. Type: XD (crossdated**). ID: Methuselah. Location: White Mountains, California, USA. Collector(s), Dater(s): Ed Schulman, Tom Harlan.”
#Wang L, Cui J, Jin B, Zhao J, et al. Multifeature analyses of vascular cambial cells reveal longevity mechanisms in old Ginkgo biloba trees. Proc Natl Acad Sci U S A. 2020
https://pmc.ncbi.nlm.nih.gov/articles/PMC6995005/
Quote: “There is considerable interest in how ancient trees maintain their longevity. Ginkgo biloba is the only living species in the division Ginkgophyta, and specimens can live for over 1,000 y. Here, we show that trees up to 600 y of age display similar leaf areas, leaf photosynthetic efficiencies, and seed germination rates. Transcriptomic analysis indicates that the vascular cambium of the oldest trees, although undergoing less xylem generation, exhibits no evidence of senescence; rather, extensive expression of genes associated with preformed and inducible defenses likely contributes to the remarkable longevity of this species.”
– Which is why we still have trees around that were born when the Egyptians started building their first pyramid, 5000 years ago. Really what kills a tree is the world around it.
Most trees “only” live 100 to 300-400 years on average, but there are some species of conifers who can reach ages of several thousand years. The “OLDLIST” linked below contains an overview of some of the oldest trees on the planet, including information on how their age was determined.
#Rocky Mountain Tree-Ring Research. OLDLIST, a database of old trees. Retrieved June 2025
https://www.rmtrr.org/oldlist.htm
Quote: “OldList is a database of ancient trees. Its purpose is to identify maximum ages that different species in different localities may attain such that exceptionally old age individuals are recognized. [...] * Tree is still living as of late 2010s; age given is additional years since it was first sampled when this is known. [...] ** Crossdated ages are derived through recognized dendrochronological procedures (e.g., Stokes and Smiley 1968; Swetnam, Thompson, and Sutherland 1985; Schweingruber 1987; Speer 2010). [...] Species: Pinus longaeva, Great Basin bristlecone pine. Age: 4850*. Type: XD (crossdated**). ID: Methuselah. Location: White Mountains, California, USA. Collector(s), Dater(s): Ed Schulman, Tom Harlan.”
#Biondi F, Meko DM, Piovesan G. Maximum tree lifespans derived from public-domain dendrochronological data. iScience. 2023
“Figure 2. Distribution of tree longevity estimates, showing differences between angiosperms (red bars and boxplot) and conifers (green bars and boxplot)”
– Trees are not a real biological category but one of the most successful ideas life has ever had, and many different species developed on their own.
#Kelleher CT. Evolution and Conservation of Trees – A Review of Salient Issues. Annual Plant Reviews Online. 2018
https://onlinelibrary.wiley.com/doi/full/10.1002/9781119312994.apr0621
Quote: “Trees are a charismatic group of plants bound together by life form rather than taxonomy, as the tree form occurs across most phylogenetic lineages of plants. Trees resist a rigorous definition but can broadly be defined as having secondary thickening (wood), large stature, and great longevity.”
– Even today three trillion of them cast their majestic shadows.
#Crowther, T., Glick, H., Covey, K., et al. Mapping tree density at a global scale. Nature. 2015
https://pubmed.ncbi.nlm.nih.gov/26331545/
Quote: “We provide the first spatially continuous map of forest tree density at a global scale. This map reveals that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these trees, approximately 1.39 trillion exist in tropical and subtropical forests, with 0.74 trillion in boreal regions and 0.61 trillion in temperate regions.”
Google Sites
Report abuse