Soil nutrient availability determines the facilitative effects of cushion plants on other plant species at high elevations in the south-eastern Himalayas

Background: Cushions are the typical nurse species of high-elevation environments. However, few studies have explored the differences in facilitative power and environmental amelioration between nurse plants of different species that share a common cushion morphology. Aims: To compare the nurse effects of different alpine cushion plants and their mechanism. Methods: We compared the effects of two co-occurring cushion species, Potentilla articulata and Arenaria polytrichoides on species richness, Shannon-Wiener diversity and the evenness of vascular plant assemblages in the Himalayas. In addition, we compared the temperature and the soil nutrients within and outside the two cushion species. Results: The presence of either cushion species significantly increased the species richness and diversity in comparison with the areas outside the cushions; P. articulata cushions were associated with greater species richness than A. polytrichoides. Substrate temperatures were similar under the two species and in open areas, but nitrogen and potassium levels were higher under either of the two cushion species than they were outside. Soil phosphorus and potassium concentrations were significantly higher beneath P. articulata than beneath the A. polytrichoides cushions. Conclusions: Our results indicate that facilitation of local species richness by cushion plants contributes to the structure of the alpine plant community in the eastern Himalayas. The intensity of the nurse effect varies with the soil nutrient level beneath different benefactor species.