The crust that forms on the top layer of the soil that is exposed after a glacier retreats is a rich, important place and can support new plant growth in a tough alpine environment.
A study published in the Canadian Journal of Biology suggests that biological soil crusts can help larger plants grow and colonize the area, a process called succession. The authors, Katie Breen and Esther Lévesque of the University of Québec (Trois-Rivières), found that the land covered by biological crusts after a glacier retreats usually supports more plants than places that aren’t covered by soil crusts. The most dominant and thriving plant species can usually be found there, like Salix arctica, a tiny low shrub that grows in Arctic regions.
In the middle of the 19th century, after the end of the Little Ice Age, temperatures increased, which led to a decrease in the mass of glaciers in the Canadian High Arctic. As glaciers retreated, microorganisms and plants had new opportunities to colonize the surface that appeared. The primary colonization of the barren terrestrial environment usually starts on the microbial scale, which is an often-overlooked fact in vegetation studies. The first to move in are the pioneering organisms, such as green algae, lichens, mosses, fungi and heterotrophic bacteria.
As time goes by, the pioneering organisms in the soil can form a solid yet flexible layer no more than 1 cm deep close to the upper layer surface, called the biological or microbiotic soil crust. The microbiota nurtured in the biological soil are very resilient and can survive the most extreme living conditions on earth, such as glacial ice. However, it’s harder for larger plants to grow in the High Arctic; they favor habitats with higher soil temperature, lower wind speed, higher soil moisture content, and increased soil nitrate level.
Luckily, biological soil crusts can provide higher plants with all the necessary growing conditions. Cyanobacteria, a type of bacteria, are able to fix nitrogen in soil crusts and improve nutrients levels in soil; some crusts have a gluey composition, which helps the soil retain moisture and protect it from erosion by wind and water. The rougher surface created by soil crusts is able to absorb more sunlight and thus increase temperature. The process of plants helping each other grow is called facilitation.
In the early stage of succession, soil crusts are comparatively thin. Within four years of glacier retreat, the plant densities above the crusts are low. Nevertheless, as time goes by, the crusts help the plants grow and the variety of plants increases. Surprisingly, the researchers discovered that a few specific species benefit the most from soil crusts than other species. Those species are represented in much higher densities than the others and account for more of the land cover, such as Dryas integrifolia, a tiny shrub in the rose family. Dominant and long-lived species also seem to do especially well in the crust environment.
According to the authors, as global temperature continues to grow, more glaciers are going to melt in the future and continue to make impacts on the development of communities left in the wake of glaciers. This trend may potentially influence the direction of succession. The study refers to this process as the “greening of the north.”