Influence of two N-fixing legumes on plant community properties and soil nutrient levels in an alpine ecosystem

Abstract

Low nitrogen (N) supply is a limiting factor for plant growth in most terrestrial ecosystems. N-fixing legumes therefore have the potential to facilitate surrounding vegetation by increasing soil N levels. This effect should be especially pronounced in low-productivity habitats where ambient soil N levels are low, such as in alpine areas. We examined whether plant species composition, community diversity measures, and soil N levels differed with and without the presence of two alpine legumes, Oxytropis lapponica (Wahlenb.) Gay and Astragalus alpinus L., in a Dryas octopetala heath at Finse, Norway. Species composition sites, as other plant species were positively associated with the legume. None of the measured community properties differed between plots with and without the presence of Astragalus. The contrasting influence of the two legume species corresponds to our findings of higher soil N levels below Oxytropis, but not Astragalus. Differences in microhabitat N levels may explain why only Oxytropis acts as a facilitator. By affecting local species composition Oxytropis increases spatial heterogeneity, which may increase total species richness of the ecosystem.