Subarctic Ground Cover Potentially Contributes to Biotic Resistance Against Invasions by Non-Native Plants
This study demonstrates that native cryptogam ground cover in the Canadian subarctic inhibits the germination and survival of non-native plants by altering moisture and soil access, suggesting that disturbing this cover increases ecosystem vulnerability to biological invasions.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Imagine the Earth's far northern regions as a quiet, frozen fortress. For centuries, this place has been surprisingly safe from the "invaders" that plague warmer parts of the world—those pesky non-native plants that take over gardens and forests. Why? Well, the weather is just too harsh, and not many people live there to accidentally bring new seeds along. But as the planet warms and more humans travel north, this fortress is getting a little leaky. Scientists are worried that these hardy, non-native weeds might finally find a way to move in and change the landscape forever.
To understand how this might happen, we need to look at the ground itself. In the Arctic and subarctic, the soil isn't usually bare dirt. Instead, it's covered by a thick, living carpet made of mosses and lichens (tiny, ancient plants that look like fuzzy green blankets or crusty gray rocks). This "ground cover" is the neighborhood's main security system. It can act like a cozy blanket that keeps things warm and wet, but it can also act like a heavy, impenetrable wall that stops seeds from ever touching the soil they need to grow. The big question scientists are asking is: Is this mossy carpet strong enough to keep the new, unwanted plants out, or is it just a soft landing spot for them?
This is exactly what researchers Noam Michael Harris and Peter M. Kotanen set out to investigate. They wanted to see if this native ground cover acts as a shield against invasive plants in the subarctic. To find out, they didn't wait for nature to take its slow course; they built a mini-universe in a greenhouse at the University of Toronto. They gathered two types of native ground cover from the wild near Churchill, Manitoba: a fluffy moss called Hylocomium splendens and a lichen called Cladonia stellaris. Then, they played a game of "seed vs. carpet" using three common, weedy non-native plants: common plantain (Plantago major), dandelion (Taraxacum officinale), and field pennycress (Thlaspi arvense).
The scientists ran three different experiments to see how the ground cover affected these invaders. First, they tested the "physical barrier" idea. They planted seeds in pots with bare soil, thin layers of moss/lichen, and thick layers. The results were clear: the invaders loved the bare soil. When the seeds had to push through a thick 6-centimeter layer of moss or lichen, their chances of sprouting dropped significantly. It's as if the ground cover was a heavy blanket that the tiny seeds couldn't wiggle under to reach the dirt.
Next, they wondered if the ground cover was just making the soil too dry or too wet. They set up pots with different amounts of water—some dry, some average, and some very wet—and covered them with the moss and lichen. Again, the invaders struggled. Even when the soil was moist, the seeds fared much better on bare dirt than on top of the mossy or licheny mats. Interestingly, the ground cover didn't seem to change the moisture levels in a way that helped or hurt the seeds specifically; rather, the physical presence of the carpet itself seemed to be the main problem.
Finally, they checked if the plants could survive after they had already sprouted. They took tiny seedlings and transplanted them directly onto the moss and lichen mats, giving them no direct contact with the soil. The results were dramatic. Most of the seedlings died on the moss and lichen. For example, only 7% of the dandelion seedlings survived on the lichen, compared to 62% on bare soil. The moss and lichen mats seemed to be a hostile environment for these young plants, perhaps because they couldn't get the nutrients or stability they needed.
So, what's the takeaway? The paper suggests that this native ground cover is indeed a powerful defender. It acts like a biological wall, making it very hard for non-native plants to get their roots into the soil or survive once they do. The authors found that disturbing this ground cover—perhaps by driving vehicles over it or building roads—could accidentally tear down the fortress walls, making the northern ecosystems much more vulnerable to invasion. While the study was done in a controlled greenhouse and not in the wild, the evidence points to a clear conclusion: keeping that mossy, licheny carpet intact is likely one of the best ways to keep the northern wilderness safe from unwanted guests.
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