Hélène Barthelemy, Sari Stark, Minna-Maarit Kytoviita and Johan Olofsson
The Arctic tundra is a vast and pristine landscape under harsh climate. Due to low temperature and short summers, decomposition and mineralisation of organic matter in the soil is very slow. Plant growth is thus strongly limited by the availability of nutrients, especially soil nitrogen. Plant competition for this single resource should consequently be high. Still, the arctic vegetation is very diverse and grasses, herbs, shrubs, mosses and lichens typically co-occur in close proximity.
Resource partitioning between plants in the same community, for example the uptake of different forms of nitrogen, is commonly seen as one of the main mechanisms for plant coexistence in nutrient-poor environments. Indeed, most arctic plants differ greatly in their uptake affinity for the different forms of soil nitrogen.
Herbivores, by consuming plants, trampling the ground vegetation and by depositing nutrient-rich dung and urine, have strong effects on arctic ecosystem functioning and often increase the abundance of mineral nutrients in the soil. They have great potential to affect the proportions and forms of nitrogen in the soil.
We studied the long-term effect of reindeer grazing on plant resource acquisition in two tundra sites with low and high grazing intensity. Under low reindeer grazing, the nutrient-poor tundra vegetation is dominated by shrubs and mosses, and under high reindeer grazing the tundra vegetation has been transformed into a nutrient-rich tundra where grasses and herbs dominate. We examined the isotopic signatures of nitrogen (δ15N) of five common plant species in the two habitats; differences in δ15N signatures indicating the uptake of different source of nitrogen in the soil.
We found large differences in plant δ15N signatures under low reindeer grazing suggesting that plants, in the nutrient-poor tundra, are strongly partitioning the available forms of nitrogen in the soil. By contrast, under high reindeer grazing, the differences in plant δ15N signatures were strongly reduced indicating that coexisting plants are using more similar forms of nitrogen. Our work thus brings strong evidence that herbivores are not only changing plant community composition and increasing nutrient availability, but also affecting plant nutrient uptake.
Image provided by authors.