(Photo Credit: Layla Dishman)

Fantastic Ferns and Where to Find Them

Ferns are very diverse. - Abby Ireland

Ferns, and Fern Allies, are perhaps the most charismatic plant with their lovely fractal foliage, vibrant verdant hues, and stunning soral patterns. Simultaneously, they are often the most misunderstood plants. Many people, even professional botanists, have stereotyped ferns into being moisture and shade loving plants which can only be found in gardens outside of the forest floor. This sentiment couldn’t be further from the truth; in their 450 million year history, ferns have evolved awe-inspiring adaptations to survive and thrive in a variety of habitats: from high on cliff faces, nestled within the branches of trees, submerged under bodies of fresh and salt water and even in the harshest of deserts.

Epiphytic and Epipetric Ferns

When out botanizing for ferns, one may keep their eyes glued to the forest floor, but by doing so, they risk the chance of not spotting some fantastic ferns hiding above them. Ferns, and other plants, living above the forest floor can be classified into two different categories depending on where they live. If a fern is growing on a tree, then it is called an epiphyte, and if it is growing on a rock outcropping or cliff face it is called epipetric (Zotz, Armenia, and Einzmann, 2023).

These habitats may seem very different, they are quite similar in terms of abiotic factors. For instance, they both receive elevated levels of sunlight, relative to the understory, and provide no soil, which means these plants do not have a reliable source of nutrients or water (Zotz, Armenia, and Einzmann, 2023). In response to these harsh conditions, ferns have evolved various morphological and physiological adaptations.

To cope the increases levels of sunlight and sporadic rainfall, Many epiphytic ferns have evolved leaves with smaller surface areas (Jin et al. 2021). With smaller leaves, they absorb less sunlight which minimize water loss (Jin et al. 2021). Some ferns have evolved even more extreme adaptations to mitigate drought stress, such as the Resurrection fern. If these ferns lose too much water, they will curl up and enter a state of hibernation and awaken once they are rehydrated by a rainstorm, shown in Figure 1.

Figure 1: A specimen of Resurrection Fern (Pleopeltis michauxiana) in both dehydrated and fully hydrated states. (Photo Credit: Dusty Prater)

Figure 2: A staghorn fern, Platycerim andinum, note the clasping leaves that grow upwards (Photo Credit: Carlos Tatsuta).

To deal with limited nutrients, many epiphytic and epipetric ferns have evolved structures to help capture bits of dead leaf material from the host tree, which the fern can then salvage for minerals (Dubuisson, Schneider, and Hennequin, 2009; Hoshizaki, 1972). This can be seen in the charismatic and conspicuous Staghorn fern, as shown in Figure 2. Its circular, clasping mantle leaves wrap around the host plant, creating its own pot to hold soil and water (Hoshizaki, 1972).

An even more extreme form of nutrient acquisition found in the genus Lecanopteris, shown in Figure 3A; these ferns have evolved rhizomes with hollow passages which give ants, Figure 3B, a place to live (Honor, 1993). In exchange for a place to live, these ants pay rent in the form of clumps of soil or decomposing matter, which they bring to the fern (Honor, 1993).

By employing these and many more adaptations, ferns have truly mastered the epiphytic and epipetric niches.

Figure 3A: A specimen of Lecanopteris sinuosa. Note how its rhizome has little brown specs, which is where ants live. (Photo Credit: Carlos Tatsuta).

Figure 3B: An example of one of the genera of ants who colonize these ferns. (Photo Credit: www.AntWeb.org, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=8168886)

Aquatic Ferns

When one thinks of a small creek or lake, “fern habitat” doesn’t immediately spring to mind, but ferns and their close relatives have colonized aquatic habitats. Living in these areas may seem relatively less stressful than living on cliff faces or in tree branches, but this lifestyle has many challenges, such as competition for sunlight and carbon dioxide with algae and fluctuating water levels- floods and droughts (Suissa and Green, 2020). Despite these challenges, ferns have evolved many morphological and physiological adaptations to this ecosystem.

When looking at the members of the aquatic fern family Marsileaceae, Figure 4, one would be shocked at the morphological diversity that can be found here. Ranging from four-leaved clovers to simple blades, even the novice botanist can differentiate between the different genera (Westbrook and McAdam, 2022). These differences did not evolve to make the life of botany students easier, instead they evolved to help these ferns conquer the aquatic habitat.

Figure 4: A carosel of the three genera of the fern family Marsileaceae. First, the Regnellidium, followed by Marsilea then Pilularia. (Photo Credit: Dusty Prater)

For the ferns, Marsilea and Regnellidium, researchers found that these ferns have a high stomatal density and very dynamic stomata– in terms of being open or closed when different environmental triggers occur. This allows these ferns to maximize their photosynthesis without worrying about water loss because of their moist environment. On the other hand, Pilularia, with their simple stipes, had lower densities of stomata and less responsive stomata to their ephemeral environment. Despite having less reactive stomata, Pilularia gets by fine, because it keeps its stomata open even when fully submerged, always letting carbon dioxide diffuse into its tissues (Westbrook and McAdam, 2022). Together these two groups of ferns show two very different lifestyles but are both suited to take advantage of their aquatic abode.

Figure 5: Isoetes engelmannii in a small creek; note it's grass-like appearance (Photo Credit: Alan Cressler).

Another great adaptation to the aquatic world can be found in the Isoëtes- a fern ally. These small plants that look like grass are found worldwide in freshwater ecosystems, as shown in Figure 5. In an area where Isoëtes coexist with algae, the algae will outcompete the Isoëtes for carbon dioxide, so members of this genus have evolved a CAM-like photosynthetic pathway (Suissa and Green, 2020).

In this pathway, Isoëtes will close their stomata during the day, as the algae are gobbling up all available carbon dioxide, but open them at night, when all the algae don’t have a competitive edge, and absorb as much carbon dioxide as they can. When the sun rises the next morning, they will close their stomata and do photosynthesis on their stored carbon dioxide, outsmarting the algae (Suissa and Green, 2020).

From unique morphological structures to innovative physiological pathways, ferns have evolved ways to master the aquatic domain.

Mangrove Ferns

When paddling through a swamping mangrove, keep an eye on the brackish flats as you may encounter some of the few fern species that have colonized this marine habitat. In addition to the stresses in normal aquatic environments, ferns growing in mangroves have an additional factor in the form of extreme salinity. Fortunately, a particular genus of ferns has evolved to cope with this salty habitat.

Figure 6: A specimen of Acrostichum aureum growing in a mangrove swamp off the coast of Florida (Photo Credit: Alan Cressler).

The genus Acrostichium, shown in Figure 6, is a cosmopolitan genus with members found in mangrove and swampy, coastal areas in the tropics around the world. Interestingly, this genus of ferns has similar traits to the aquatic ferns but has evolved many traits associated with salt storage that we see in other mangrove angiosperms (Akinwumi et al. 2022). For instance, to help cope with increased levels of sunlight and the hyper salinity of their environment, Acrostichium has evolved denser hypodermis tissue than other ferns (de Arruda, da Costa Lima, de Paiva Farias, 2021) . This tissue layer helps insulate the mesophyll tissue layer, the layer housing most of the photosynthetic machinery, from intense solar radiation; it also doubles as a place where the plant can store excess salt that its roots absorbed. Similarly, these mangrove ferns have evolved deposits of calcium oxalate crystals- which few ferns have. These deposits help to trap other minerals, such as salt and heavy metals, which allow these ferns to thrive in these brackish areas (de Arruda, da Costa Lima, de Paiva Farias, 2021). Together, these handful of traits have helped this genus colonize and thrive in these brackish areas.

Desert Ferns

The last fantastic fern is quite an extradentary one, even amongst the ones described, as it lives in the most extreme habitat- the desert. The deserts of North America seem like the least habitable area for ferns with intense solar radiation and near constant dry conditions, but many species of ferns have found a way to survive this harsh environment.

At first thought, there is an obvious Achilles’ Heel for ferns in terms of fully colonizing this harsh landscape, their lifecycle is dependent upon water. As any botany student, worth their salt recalls, ferns require a thin film of water to be present for the sperm to swim to the egg in order to complete their lifecycle. This may seem like a dead end for ferns, but many ferns have evolved ways around this by going through apomixis.

Figure 7: A specimen of Notholaena standleyi growing in the Arizona desert (Photo Credit: Alan Cressler). Note the fuzzy nature of the white trichomes on the fronds.

Figure 8: A diagram of fern apomixis (Adapted from Amanda Grusz). The yellow arrow represents diplospory and the pink arrow represents apogamy. Note the conservation of the ploidy number throughout the process.

Apomixis literally means without mixing, specifically no mixing of genetic information of sperm and egg. For this system to work, two separate biological processes must align; this is process is outlined in Figure 8.

First, the fern must produced unreduced spores; in other words, the spores must be the same ploidy level as the sporophyte— typically spores are half the ploidy level of the sporophyte. This process is called diplospory and shown as a highlighted yellow arrow in Figure 8. Now, that spore must survive and germinate into a gametophyte. Once at this stage the second process must align, generating a sporophyte from the gametophyte tissue without the mixing of gametes. This process is called apogamy and shown as a pink arrow in Figure 8 (Grusz, 2016).

Together these processes of diplospory and apogamy can work together to make a fern life cycle without the need of a thin film of water, allowing ferns to successfully reproduce in the harsh desert.

Such a combination of events may seem unlikely, but extensive research has shown that both diplospory and apogamy are common and can be induced under certain environmental conditions (Grusz et al. 2021). Also, phylogenetic studies have shown that this pattern of apomixis has evolved several times independently in major fern linages especially those in desert regions (Grusz, 2016; Albertini et al. 2019).

Now that ferns have developed a foothold in the desert, other adaptations to this habitat can take hold. First, one will find these desert ferns have a greatly reduced leaf area compared to similar species in other areas, this helps to limit water loss and to decrease the amount of tissue exposed to solar radiation. Another fascinating feature is that these ferns produce tons of hairs, called trichomes, as shown in Figure 7. These are interesting structures as they house tiny packets of chemicals which act as a chemical sunscreen to help protect what leaf tissue from solar radiation.

Together, through the evolution of apomixis and other adaptations to beat the desert heat, these ferns have truly conquered this habitat.

Conclusion

In conclusion, ferns exhibit a wide breath of ecological niches, from the highest branches and cliffs to the aquatic and marine worlds and even the driest deserts. They have achieved this ecological success through countless adaptations, by modifying the classic fern shape to help them combat these extreme environments, evolving novel physiological processes to counter both biotic and abiotic stressors, and even undergoing fundamental changes to fern biology to suit their environmental needs. Truly, ferns are very diverse.

References

Zotz, Gerhard, Lisa Armenia, and Helena JR Einzmann. "A new approach to an old problem: how to categorize the habit of ferns and lycophytes." Annals of Botany 132.3 (2023): 513-522.
1. (Zotz, Armenia, and Einzmann, 2023)
. Jin, Dong‐Mei, et al. "Functional traits: Adaption of ferns in forest." Journal of Systematics and Evolution 59.5 (2021): 1040-1050.
1. (Jin et al. 2021)
Dubuisson, Jean-Yves, Harald Schneider, and Sabine Hennequin. "Epiphytism in ferns: diversity and history." Comptes rendus. Biologies 332.2-3 (2009): 120-128
1. (Dubuisson, Schneider, and Hennequin, 2009)
Hoshizaki, Barbara Joe. “Morphology and Phylogeny of Platcerium Species.” Biotropica, vol. 4, no. 2, 1972, pp. 93–117. JSTOR, https://doi.org/10.2307/2989731
1. (Hoshizaki, 1972)
Honor Gay, Rhizome structure and evolution in the ant-associated epiphytic fern Lecanopteris Reinw. (Polypodiaceae), Botanical Journal of the Linnean Society, Volume 113, Issue 2,October 1993, Pages 135–160, https://doi.org/10.1111/j.1095-8339.1993.tb00335.x
1. (Honor, 1993)
Suissa, J.S., and Green, W.A. CO2 starvation experiments provide support for the carbon-limited hypothesis on the evolution of CAM-like photosynthesis in Isoëtes. Annals of Botany. 127(1), 2020, 135-141
1. (Suissa and Green, 2020)
Westbrook, Anna S., and Scott AM McAdam. "The poisoned chalice of evolution in water: physiological novelty versus morphological simplification in Marsileaceae." American Fern Journal 112.4 (2022): 320-335.
1. (Westbrook and McAdam, 2022)
Akinwumi, Kazeem A., et al. "Acrostichium aureum Linn: traditional use, phytochemistry and biological activity." Clinical Phytoscience 8.1 (2022): 18.
1. (Akinwumi et al. 2022)
de Arruda, Emília Cristina Pereira, Gustavo da Costa Lima, and Rafael de Paiva Farias. "Leaf anatomical traits and their ecological significance for Acrostichum aureum (Pteridaceae), a remarkable fern species growing in mangroves." Aquatic Botany 171 (2021): 103379.
1. (de Arruda, da Costa Lima, de Paiva Farias, 2021)
Grusz, Amanda L. "A current perspective on apomixis in ferns." Journal of Systematics and Evolution 54.6 (2016): 656-665.
1. (Grusz, 2016)
Grusz, Amanda L., et al. "A drought‐driven model for the evolution of obligate apomixis in ferns: evidence from pellaeids (Pteridaceae)." American Journal of Botany 108.2 (2021): 263-283.
1. (Grusz et al. 2021)
Albertini, Emidio, et al. "Did apomixis evolve from sex or was it the other way around?." Journal of experimental botany 70.11 (2019): 2951-2964.
1. (Albertini et al. 2019)

Page updated

Google Sites

Report abuse