Plants

66-0 mya Cenozoic Era

Soil Survival Grass-Herbivore Symbiosis Soil animals Ground-dwelling beetles

Make up your own mind about the impact of the K-Pg extinction on plant life...

Survival

"By analysing two angiosperm mega-phylogenies containing approximately 32 000–73 000 extant species, here we show relatively constant extinction rates throughout geological time and no evidence for a mass extinction at the K-Pg boundary. Despite high species-level extinction observed in the fossil record, our results support the macroevolutionary resilience of angiosperms to the K-Pg mass extinction event via survival of higher lineages."
(Thompson & Barahona 2023)

Extinction

"We review emerging paleobotanical data from the Americas and argue that the evidence strongly favors profound (generally >50%), geographically heterogeneous species losses and recovery consistent with mass extinction. The heterogeneity appears to reflect several factors, including distance from the impact site and marine and latitudinal buffering of the impact winter. The ensuing transformations have affected all land life"
(Wilf et al 2023)

Without the dinosaurs, plant life had an opportunity to flourish during the Cenozoic era, with nearly every plant living today having its roots then.

Long phylogenetic fuse

Angiosperms (flowering plants) were both diverse and widespread (at the family level) by the Middle Cretaceous, but they didn’t diversify into the species richness of today until the early Cenozoic Era - the Palaeocene Period. This called the “long phylogenetic fuse”, between the first divergence of angiosperms from their sister group (the stem node), and then when the group began to diversify in earnest (the crown node). The time-lag differed, with the shortest lags in temperate and arid biomes compared with tropical biomes. (Raminez-Barahona et al 2020).

I think some of this can be explained as the continents remained apart and drifted away. All the Angiosperm 'families' would have been on most of the continents but as they drifted the new environments would have encouraged 'speciation'. The adaptation to changing soil conditions could also explain both the timing (delay) and the geographical heterogeneity observed in the diversification of flowering plants.

Flowering plants were part of the underbrush in dense forests, along with ferns and horsetails and included Lacinipetalum spectabilis, a fossil flower that grew in Patagonia, Argentina, 64 mya and colonised coastal southern South America.

The evolution and diversification of flowering plants were also influenced by their interactions with other organisms, such as the newly arrived herbivores, pollinators, and pathogens, which themselves were influenced by soil conditions. Some plants developed specific chemical defences against herbivores or specific traits to attract pollinators that thrived in particular soil environments.

Soil & Flowering plants

The spread of flowering plants contributed to the development of new soil types.

Diversify

The composition and diversity of soil microbial communities (such as mycorrhizal fungi and bacteria) also played a role in the diversification of flowering plants. Angiosperms evolved diverse relationships with these microbes, which could help them acquire nutrients more efficiently, protect against pathogens, or even promote growth. These relationships were often highly specific to particular soil types or conditions. Therefore, the presence of certain microbes in specific soils could influence the success and spread of particular flowering plant families.

Different soils

Soils in tropical regions were often more weathered and nutrient-poor than those in temperate zones, which could have driven different patterns of diversification among plant families. Certain angiosperm lineages that could tolerate or adapt to these varied soil conditions were able to diversify rapidly and occupy new ecological niches. Conversely, others that were less adaptable or had more specialized soil requirements would have experienced delayed diversification.

New soil types

The rise of flowering plants itself contributed to the development of new soil types. The evolution of deeper-rooted plants (like many angiosperms) altered soil structure and nutrient cycling. This deeper rooting allowed angiosperms to access nutrients previously unavailable to shallower-rooted plants, further driving the diversification of species.

Some angiosperms altered the acidity or organic matter content of soils, making it more suitable for themselves or excluding competitors. This "plant-soil feedback," could lead to localized diversification as plants adapted to particular soil conditions expanded into new ecological niches. As these plant species spread, they modified the soil further, influencing the types of plants that could colonize those areas next.

Newly evolving flowering plant species often had specific nutrient requirements or tolerances, leading to the rise of various plant communities in different soil environments. The availability of essential nutrients could determine which species could establish and diversify in a particular area.

The biologically significant event during this Cenozoic Era was the transition of a widespread and equable ‘hothouse’ climate to a largely seasonal ‘icehouse’ climate’. During the early part of the era, forests overran most of North America. However as the climate cooled, forests died off. This opened up land. Among the common plant life were pines, mosses, oaks and … for the first time - grasses

Grassland

Grasses only arrived between 55-70mya ((Kellogg 2001) although some say around 55-34 million years ago during the Paleogene period. They are very successful all over the world, accounting for 40% land area. especially nowadays where humans have taken control. The grass family includes all the major cereals, such as wheat, maize, rice, barley, and oats, and most of the minor grains as well, such as rye, common millet, finger millet, and many others that are less familiar. It also includes such economically important species as sugar cane and sorghum.

Grass soils enable about half the amount of soil life compared with forest, but twice that of arable-ploughed landCountryside Survey 2007 Chapter 8

Grasses increased dramatically after the meteors, although few have made a direction connection What impact does the meteor impact have on soil rhizospheres?.

Grasses are quite different from trees, and it is grass which now becomes the dominant plant form on the planet.
Do soil organisms and creatures play a part too?

The world's first wheat, peas, cherries, olives, rye, chickpeas and rye were cultivated from wild plants found in Turkey and the Middle East, which we'll talk about later. For now we’ll stick to green green grass. It seems the success of the grasses at that time is related with embryo development, structure of the fruit and silica accumulation. (Kellogg 2001 Evolutionary history of the grasses).

Roots

Like all monocots, grasses have fibrous roots, and their's are more fibrous.

There is much evidence about the benefits of grass roots in stopping erosion, and on the resistance of topsoils. (Baets et al 2005)

There is even an "algorithm, inspired from the reproduction and root system of the general grass plants. " (Akkar & Mahdi 2017.

"Research advances and challenges in understanding soil reinforcement by fibrous roots bridge concepts from soil science, plant biology, geography and geotechnical engineering. Most work on soil reinforcement by roots focuses on woody species, with much less research on herbaceous species such as grasses and major crops." (Loades et al 2013)

Grass Mycorrhiza

Grass roots are all completely fibrous. This will make a difference in water runoff. Water goes down tree roots quickly, so hold the water well. Pasture roots are OK, a bit like a fibrous ball, but have not got the depth to hold water like trees do. But many form a beneficial association with mycorrhizal fungi, similar to the ones we saw first about 400mya, with the AM fungi. These we saw this relationaship was actually a tri-symbiosis, with the springtails and was quite ‘generalist’. The key charcters were not confined to specific roots, fungi or springtails. This makes them more adaptable to changing conditions, in this case, grass. The resulting structure with AM fungi would be good for soil formation. There is a medium permissiveness for mycorrhiza in Festuca grass (Corcoz et al 2022)

Clay

Grasses would have brought about several key changes in soils and clay composition due to enhanced silica inputs from 'phytoliths' (see right), increased chemical weathering from organic acids and root exudates, physical disruption of minerals via dense root systems and alteration in the soil's cation balance through nutrient cycling. These processes would likely have led to faster mineral weathering, more diverse clay mineral assemblages, and the promotion of certain types of clays (e.g., smectite, kaolinite) depending on the local environment and climate.

The advent of grasslands represents an important shift in the interaction between vegetation and the Earth's soils, with far-reaching impacts on the geochemistry of clays. For instance...

1. Silica Input:

Phytoliths: One of the most distinctive new features that grasses introduced to the environment is the production of silica bodies called phytoliths (above). Phytoliths are microscopic particles made of silica (SiO₂) that form within grass cells. As grass decays, these phytoliths accumulate in the soil, increasing the silica content. This would have significantly affected the mineralogical composition of clays, with potential transformations in clay structures or the creation of new types of silicate minerals.

More silica in the soil can lead to the formation of certain smectite or kaolinite clay minerals, which have high silica content.

2. Root Exudates and Organic Acids:

Root Exudation: Grass roots, like many plants, release organic compounds (e.g., organic acids, sugars, amino acids) into the soil. These exudates can chelate metals and alter soil pH, leading to enhanced weathering of primary minerals and affecting the formation and transformation of clays.

Organic acids such as oxalic or citric acid can dissolve minerals like feldspar or mica, releasing elements like aluminum and silicon, which can then contribute to clay formation.

3. Physical Root Activity:

Bioturbation by Roots: Grass roots grow densely in the soil, penetrating deeper and breaking up rock and minerals through physical processes. This physical root-induced bioturbation enhances the breakdown of primary minerals (like feldspar, quartz, or volcanic ash) into secondary clay minerals. Such physical disruption may have increased the weathering rates, accelerating the formation of clay minerals like illite or montmorillonite.

4. Increased Organic Matter:

Organic Matter Accumulation: Grasslands contribute to the build-up of organic matter through dead plant material and root turnover. We have seen how this increased organic content can affect the structure and composition of clays in several ways:

- Organic matter forms complexes with clay minerals, stabilizing them and affecting their chemical reactivity.
- The decomposition of organic matter releases nutrients and ions into the soil that interact with clay minerals, potentially leading to the formation of different types of clay, such as humic clays.

5. Soil pH and Chemical Weathering:

Grass-dominated ecosystems tend to produce slightly more acidic soils due to the decomposition of organic matter and root exudates, which can influence the chemical weathering processes that generate clay minerals. Acidic soils promote the leaching of cations like calcium, magnesium, sodium, and potassium, which can lead to the formation of kaolinite or other weathering-resistant clays.

6. Calcium Cycling and Calcite Formation:

In some environments, grasses can promote the formation of calcium carbonate (calcite) layers in soils, particularly in arid or semi-arid regions (which would evolve more prominently after grasses spread from 23 - 5mya (Miocene). This process can influence the balance of cations in the soil, particularly affecting the ion-exchange properties of clays.

7. Aluminum and Iron Mobility:

The organic acids exuded by grass roots can also mobilize aluminum and iron in the soil, which are essential components of many clay minerals. This mobility could have contributed to the development of certain aluminum- or iron-rich clays like gibbsite or chlorite.

Pollen

The earliest firm records of grass pollen are from South America and Africa, between 60 and 55 million years ago (Jacobs et al 1999) This fits with The genetic evidence of the world's four major grains — wheat, rice, corn and sorghum — is that they evolved from a common ancestor weed that grew 65 million years ago. Does it tell us anything about why where the grass started to take over. Was grass better adapted in to the area around that decimated by the asteroid?

What pollen says

Pollen is very distinctive between plant families and species - grass pollen can be easily recognised in mixed pollen samples becuase it is roughly spherical in form and has a single pore on its surface Using the pollen grains as an indicator "pollen grains give upper and lower bounds for the date of the ancestor of the grasses. Based on the fossil record, this ancestor lived before 55 million years ago but probably after 70 million years ago". (Kellogg 2001 Evolutionary histroy of the grasses).

Pollination is by wind. It seems to make sense that they took over as they do not need insects for pollination, like the flowering plants, as there would not be as many insects humming about. Instead they are pollinated by wind. After the big extinction 250mya the trees took 6-7 million years to recolonise, whereas these grasses would be able to take over that spare soil much quicker.

Open environment

“Grasses are predominately wind pollinated and able to survive seasonal climates, leading to reasonable speculation that grasses first evolved in an open environment, near a forest margin or in warm temperate, subtropical, tropical dry, or seasonally dry, environment" (Jacobs et al 1999)

We take grassland as being here forever, as they now cover around 1/3 world’s surface as Savannas, grasslands and other grass dominated ecosystems. They support vast numbers of herbivorous mammals, so this period is often known as the Age of the Mammals. Think of all the cows, sheep, deer wandering the world. We also know them as beef, mutton and venison, giving you an idea of their role. They produce a lot more poo evenly distributed. See how soil animals get on in grass.

Metabolism

With the arrival of grassland, it is time to recap on the metabolism involved in soil processes.

Macdfadyen constructed a balance sheet for the total annual flow of energy through a meadow exploited by grazing cattle. Of the total energy captured by photosynthesis, less than one seventh is respired by plants about 2/7 consumed by herbivores and about four sevenths are exploited by organisms engaged in the decomposition of plant material after it is dead.

He analysed the proportion of the total energy handled by soil animals engaged in what he calls, ‘the decomposer industry’, into four components - herbivores, predators large decomposers (earth movers) and small decomposers. The material exploited by the decomposers goes back to the soil and on to plants making the organic cycle, and energy along with it.

He went even further and compared the flow for a grasshopper (herbivore) compared with an oribatid mite (detritus feeder). The mites seem able to process proportionately more material and energy with much less biomass than the herbivore.

Macfadyen A 1964 Energy Flow in ecosystems and its exploitation by grazing P 3-20 in D J Crisp (ed) Grazing in Terrestrial and Marine Environments Blackwell

Grass seems to have the ability to recover from being trampled in. Grasses and herbivores developed a very successful relationship. The herbivores ate the grass, digested it and pooed it back to the soil for the breakdown process to transfer into nutrients. Since then grasses have come to take over vast tracts of the world. The vast tracts of grass, called prairies, have a deeper organic layer than trees.

Fungi-Bacteria ratio

The ratio of fungi to bacteria is characteristic to the type of system. Grasslands have bacterial-dominated food webs - that is, most biomass is in the form of bacteria. Forests tend to have fungal-dominated food webs. The ratio of fungal to bacterial biomass may be 5:1 to 10:1 in a deciduous forest and 100:1 to 1000:1 in a coniferous forest.

Again this makes sense. Fungi would have been hit badly by the asteroid and the ensuing fires, but the bacteria less, so that the newly emerging grasslands would tend to be dominated by bacteria carrying out similar decomposing functions.

Detritus feeders

“On the whole detritus feeders abstract rather little energy in proportion to the material they handle but the effects of their activities are important because the material is usually made more accessible to attack by bacteria and fungi especially when, as is the case of soil, it is both moistened and aerated”

Macfadyen A 1964 Energy Flow in ecosystems and its exploitation by grazing P 3-20 in D J Crisp (ed) Grazing in Terrestrial and Marine Environments Blackwell

Legumes

We saw that legumes emerged around 100mya, but the legume family massively expanded soon after the 66 mya extinction and nowadays has six subfamilies and over 19 500 species. What was it about this period that encouraged legumes? It could help solve the classic conundrum that "despite a massive body of knowledge, the long-standing objective to engineer the nitrogen-fixing nodulation trait on non-leguminous crop plants has not been achieved yet". (Huisman & Geurts 2020). That is agricultural scientist's Holy Grail

Increased weathering

“The increasing abundance of N2-fixing legumes in tropical forests amplified silicate weathering rates by increased input of fixed nitrogen (N) to terrestrial ecosystems via interrelated mechanisms including increasing microbial respiration and soil acidification, and stimulating forest net primary productivity. We suggest the high CO2 early Cenozoic atmosphere further amplified legume weathering. Evolution of legumes with high weathering rates was probably driven by their high demand for phosphorus and micronutrients required for N2-fixation and nodule formation" (Epihov et al 2017) ‘Their high demand for phosphorus’ has important implications to this day.

Fossil and phylogenetic evidence indicates legume-rich modern tropical forests replaced Late Cretaceous (100-66mya) palm-dominated tropical forests across four continents between 58-42 Mya. They may have helped weather underlying rocks making for the creation of more soil.

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