Henni Ylänne, Johan Olofsson, Lauri Oksanen and Sari Stark
Recently global attention has been directed to high latitudes, where a large share of the worlds’ carbon is stored. The anticipated rise of temperatures could enable higher activity of soil microbes and release some of the carbon to the atmosphere, thus increasing the greenhouse effect. However, several other factors besides temperature control the rates of carbon uptake and release in the tundra. This study tackles the impact of large grazers on the carbon reservoirs of tundra.
Large grazers are found throughout the tundra and they are known to influence vegetation and the functioning of the ecosystem. When occurring at high abundance, they may even induce an alternative ecosystem state, where the plant species composition and ecosystem processes have changed drastically from the non-grazed state. An example of such grazer-induced alternative ecosystem state is the conversion of tundra shrublands or heaths to tundra meadows with higher rates of nutrient turnover. In this study we assessed recent changes in carbon storage along reindeer fences, where a high grazing pressure on one side of the fence had induced the above-mentioned shift in vegetation.
We show that tundra systems have the capacity to adjust to changes in grazing pressure. We report an increase in vegetation dominated by grasses and sedges with increased reindeer numbers and higher trampling intensity. With the changed vegetation, also soil nutrient availability had increased within the past 14 years.
We also show that the “grassification” of tundra shrublands, whether this occurred recently or decades earlier, reduces carbon stored aboveground. However, the impacts of grazing belowground varied between our two study sites. At one site, the grass dominated, grazed area stored equal amounts of carbon compared to the tundra heath. At the other site, there was more carbon in the organic soil under the grass-dominated, grazed area compared to the shrub tundra. We suggest the consequences of grazing depend on the characteristics of the vegetation under light grazing.