At the end of the last Ice Age about 12,500 years ago, PEI would have looked more lunar than Earthly: no vegetation, just clay, silt, sand, gravel, boulders, and bedrock. Over thousands of years, this sterile landscape developed into rich soil and temperate forests, with lichens playing an important role in kicking things off.
You’ve likely seen lichen-encrusted rocks like this one, but have you ever thought about what’s going on here? This is an early stage in a long, slow process that turns inorganic rock into living, organic soil. It may not look like much is happening, but those lichens are busy!
Imagine our barren post-glacial landscape. Further south, fungi and algae are releasing spores, some of which get carried north by wind to land on our newly-exposed rock and glacial till. When the right spores meet up (often with the help of rain or meltwater), they form lichens.
Lichens get all their nutrition from air, water, and sunlight rather than from the substrate, which means they can grow directly on rock. As they grow, they produce oxalic, carbonic, and other weak acids that make the rock more soluble and start to break it down in a process called chemical weathering.
This chemical weathering starts to create cracks and holes in the rock. Lichens take advantage of those, sending in root-like structures called ‘rhizines’ or ‘hyphae’ (depending on the type of lichen). Lichens and their rhizines & hyphae expand when wet; repeated cycles of expansion and contraction inside those cracks and holes weaken and break the rock. This is helped along by the freeze-thaw cycles of rain and meltwater, as anyone familiar with potholes knows all too well. Taken together, the expansion and contraction of lichens, rhizines & hyphae, and water help break down rock via physical weathering.
As the lichens are slowly breaking rocks into smaller pieces, they are also adding organic matter. When they die, lichens decompose and begin to create a thin layer of soil on which mosses, algae, fungi, and small plants can start to grow. Additionally, organic matter decomposes more slowly in the presence of lichens. This means when those mosses, algae, fungi, and small plants start to decompose, their organic matter is more available to future generations than it would have been without the lichens.
Finally, lichens trap particles from both air and the weathering rock. In this way, elements such as calcium, iron, magnesium, phosphorus, potassium, and sulfur are added to the newly-forming soil. In some cases, lichens are able to make these elements more available to plants, as well as create clay minerals that contribute to healthy soil.
So, there’s a lot going on in this seemingly serene scene, albeit very slowly. Depending on the type of rock, it may take centuries or millennia for a diversity of higher plants to move in. This particular rock was exposed when European settlers began to farm the land around it nearly 200 years ago; while lichens have established a good foothold, it will be several more centuries before it becomes soil.
Keep your eyes out for lichens around you and be sure to thank them for their roles in creating PEI Untamed!
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