As new areas become exposed by glacier retreat, plants begin to colonize them. Do the different species support or compete with one another? A recent study in the Journal of Vegetation Science follows the interactions of the circumpolar moss Silene acaulis, a type of cushion plant, with other secondary species like the buckwheat Bistorta vivipara in a southern Norway glacier, which has been retreating.
To answer this question, Kari Klanderud and her colleagues from the Norwegian University of Life Sciences demarcated five transact areas that were of increasing distance from the Midtdalsbreen glacier. Areas further away from the glacier represent better environmental conditions for growth as abiotic stress decreases, showing an environment gradient. The further from the glacier, the longer the area has been exposed, resulting in more advanced colonization. The abundance of buckwheat and the number of species of secondary plants within and outside of cushion plants were analyzed by conscientiously photographing the plant species in those areas. Soil temperature, moisture, organic content and pH measurements were also taken to examine if cushion plants modify the abiotic environment.
“We chose to work on S. Acaulis because it is common in alpine and arctic areas worldwide,” lead author Klanderud told Glacierhub. The cushion plant, a fascinating species that can survive in harsh climates, is commonly found in exposed habitats such as alpine tundra and places of cold-air drainage such as glacial moraines. It resembles a large green mat that can grow up to three meters in diameter. As a pioneer species in alpine habitats, the cushion plant nobly optimizes environmental conditions to facilitate growth of secondary plants like the buckwheat. This kickstarts the process of primary succession. These ungrateful secondary plants will continue to shamelessly grow, dominate and ultimately replace their pioneers so that eventually a community with larger species variety is achieved.
Why is the cushion plant able to survive the harsh conditions in the first place? Lawrence Walker, a professor at the University of Nevada who specializes in plant ecology, told Glacierhub, “Their compact growth form preserves heat, which leads to a longer growing season and minimal frost damage during summer months. The cushion growth form also avoids breakage of stems from strong winds.” The benefits of being compact is not only self-serving, its structure also facilitates secondary plants’ growth and survival by buffering extreme soil temperatures. “Even pollinating insects may find refuge in the cushion,” says Walker.
In an environment with limited space and resources for growth, it is every plant for itself. The cushion plant allows other plants to grow within them and in turn compete for nutrients. There must be a threshold for its altruism if the cushion plant wants to survive in the face of the buckwheat and other secondary plants. As Walker explains, “It is common that nurse plants (ones providing protection for small individuals of other plants) can later be outcompeted by the plant that they nursed.”
Must the cushion plant really engage in negative interactions to impede the growth of its ‘child’, the buckwheat to survive? Biologists have discovered that relations of plants vary depending on the level of stress. Coined the stress-gradient hypothesis, this means that competition between plant species is strongest during favorable environmental conditions, but these species will support one another when the going gets tough. With decreasing biotic stress, plant interactions tend to shift from facilitative to competitive.
Indeed, for sites close to the glacier that represent the harshest abiotic conditions, the buckwheat performed better, as shown by bigger leaves when it is grown between the stems of the cushion plant. These sites are characterized by fewer organisms which accentuates the harsh conditions. Soils closer to the glacier contain less organic matter due to a shorter lifespan of the ecosystem present. The plants also lack support and shelter from one another to moderate the environmental conditions. “The very dense and dome-shaped cushion modifies the microclimate and thus the growing condition for other plants,” Klanderud explained.
There was limited difference in buckwheat growth performance in more favourable environments further from the glacier. In this case, the cushions support the theory above. However, during conditions of low abiotic stress, it still has a “conscience,” shifting only from facilitative to neutral interactions instead of negatively affecting the performance of the buckwheat. In terms of secondary plant species diversity, a trend of higher species richness was observed within the cushions across all the sites, with cushions buffering extreme soil temperatures as the main abiotic reason.
While glacier retreat often has a negative connotation, it can also represent new opportunities for plants and other species to build communities in newfound lands. Nonetheless, one thing is for sure – survival is key, prompting cooperation when needed but also knowing when to draw the line, just like humans.