Writer: Evra Haspolat
Soil is formed when the main material, rocks, goes through a crumbling process caused by weather conditions and natural erosion. Many of us are familiar with the concept of soil conservation, but what often goes unnoticed is the vital role played by a specific organism in safeguarding the stability and well-being of soil. Soil lichens are organisms that grow on the soil's surface, playing a vital role in stabilizing the ground and preventing erosion. They also contribute to many more things. So in this article, we will talk about the quiet engineers of nature, the soil lichens.
What is a Soil Lichen?
A lichen ("λειχήν" in Greek) is a compound organism made up of two species which are fungus and cyanobacteria or green algae that live in a mutualistic relationship. The fungus provides a structure for the algae to live in, while the algae provide food for the fungus ("Lichen - The little things that matter (U.S. National Park Service)," 2018). They dominate 8% of the world's land surface, mainly in Arctic and Antarctic regions, tundra, high mountain elevations, and as components of dryland crusts (P, 2013).
Soil Lichen Types
There is more than one type of lichen. The species that live in the soil are separated from each other by their different features and there are primarily 3 types of soil lichens.
1- Foliose Lichens
Foliose lichens are large and leafy like lettuce, reaching diameters of several feet in some species, and are usually loosely attached to the surface on which they grow. These lichens have a distinct top and bottom side and can be leafy and flat, or full of ridges and bumps ("Lichen biology," 2024),(Milow et al., 2022),("Foliose thallus | biology," 2024).
Figure 1: An example of what a foliose lichen looks like (Dillman, n.d.).
2- Fruticose Lichens
Fruticose lichens can be pendant and hair-like. Moreover, they might be bushy, spindly, stringy, branching, swag-like, or cup-shaped. Many fruticose lichens have round branches that have a central core and others are hollow in the middle. Other fruticose lichens such as Cladonia, Ramalina, and Usnea have flat branches that tangle up with each other. ("Lichen biology," 2024),("Fruticose lichens - Mount Rainier National Park (U.S. National Park Service)," 2020).
Figure 2: Example of fruticose lichens, left to right: toy soldiers, pixie cups, and reindeer lichen (Vecchio Photos, n.d.).
3- Crustose Lichens
Crustose Lichens are attached to their surface that the contact is so intimate that they are practically inseparable from the substrate. They are crusts as the name refers. They form a crust over a surface, like a boulder, the soil, a car, or roof shingles. They can come in many bright, vibrant colors like sunny yellow, orange, and red, as well as grays and greens (Milow et al., 2022), ("Lichen biology," n.d.).
Figure 3: Many colorful crusts on a rock (St. Claire, n.d.).
Figure 4: Lecanora garovaglii, gray crust, with other crusts, on a rock (Wagner, n.d.).
Figure 5: Pleopsidium chlorophanum, yellow crust, with other crusts, on a rock (Wagner, n.d.).
Ecological Roles of Lichens
Even though these three types of liches vary in physical characteristics, their role in ecosystems is similar. Lichens are significant components of BSCs (Biological Soil Crusts), which form a protective layer over the soil. This layer binds soil particles together, thus preventing erosion caused by wind and water, which is especially important in arid and semi-arid environments where soil is more vulnerable (Büdel et al., 2009). A study conducted in 2021 by Wenwen Xu, highlights how areas with well-developed BSCs, including lichens, show significantly reduced rates of soil erosion compared to areas without these crusts.
Furthermore, lichens contribute to the soil's ability to retain moisture by creating a microenvironment that reduces evaporation. The physical structure of BSCs helps in trapping water, which can be gradually released into the soil. The study (by Wenwen Xu) discusses how lichens improve soil water infiltration, which is necessary to replenish groundwater and support plant growth, especially in ecosystems with less rainfall.
Soil lichens also provide “nitrogen fixation” to the environment. Nitrogen fixation is an important process in the nitrogen cycle. In this process, the N2 from the air is reduced to NH3 (or NH4+), which can be further converted to nitrogen-containing organic compounds, such as proteins and nucleic acids. Lichens play a crucial role in nutrient cycling, especially in nitrogen fixation. The decomposition of lichen material provides soil fertility by releasing essential nutrients into the soil (Wang & Yu, 2023).
The most significant property of soil lichens is to be a part of the soil formation process. They are vital in the colonization of bare rock. By contributing to soil formation by releasing its chemical compounds to break down rocks into smaller bits that lead to the formation of fine earth, soil liches are vital for the ecosystem. They increase the fertility of the soil by trapping water, dust, and silt, creating a microhabitat for other microorganisms and invertebrates (Beckett et al., 2013), (Hnatuik, n.d.).
Figure 6: Steps in the development of soil (Thompson Rivers University, n.d.).
Threats to Soil Lichens
Similar to other living creatures, soil lichens are threatened by climate change. Climate strongly influences the lichen community. Increasing temperatures cause physiological stress and slow growth in lichens, while the decrease in cold and moist habitats makes survival even more difficult. Decreased humidity leads to water stress, affecting metabolic activities during dry conditions. Air pollution, especially from gases such as sulfur dioxide, negatively affects lichen growth, and increased nitrogen deposition can reduce biodiversity.
A few lichen species can tolerate large fluctuations in climate, but most require more specific regimes. Even a 1 °C shift in mean annual temperature can drastically increase or decrease the probability of finding certain lichens (U.S. Forest Service, n.d.).
Lichens are facing multiple threats, not just from climate change but also from air pollution. They play a crucial role in "fixing" atmospheric nitrogen into forms that can be used by lichens themselves, as well as other plants and animals. As increasing nitrogen- and sulfur-containing air pollutants, the ecological impacts rise. Sensitive lichens such as diatoms, bryophytes, ectomycorrhizal fungi, and alpine plants are among the first affected. The organisms sensitive to air pollution play a crucial role in the ecosystem. Any harm to these organisms can have a ripple effect, negatively impacting other species that rely on them for food, habitat, or protection. (U.S. Forest Service, n.d.).
Lichens serve as reliable indicators of air pollution and play a significant role in tracking climate change and biodiversity. By monitoring the composition of lichen communities, valuable insights into the impact of these environmental challenges can be gained. Despite being threatened by these issues, lichens aid us in monitoring them through the density of their presence. (U.S. Forest Service, n.d.).
Conclusion
Lichens, with their delicate and intricate forms, stand as a stunning testament to the intricate balance of the natural world. These organisms play a crucial role in our environment, as they safeguard the soil, maintain its moisture, foster the creation of new soil, and create habitats for a myriad of creatures. Additionally, they contribute to providing clean air for humans and other organisms and serve as vital indicators of environmental changes such as climate variations and air pollution.
References
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Büdel, B., Darienko, T., Deutschewitz, K., Dojani, S., Friedl, T., Mohr, K. I., Salisch, M., Reisser, W., & Weber, B. (2009, February). Southern African biological soil crusts are ubiquitous and highly diverse in drylands, being restricted by rainfall frequency. PubMed.
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