Recruitment of the high elevation cushion plant Arenaria polytrichoides is limited by competition, thus threatened by currently established vegetation

Cushion plants have been confirmed to be keystone species of alpine ecosystems. However, the adaptive strategies responsible for their recruitment and persistence remain largely unclear. Also, the effect of competition by species already established in surrounding vegetation is not known. We first assessed population size and the frequency distribution of individuals of different size in communities of the keystone cushion Arenaria polytrichoides Edgew. along an elevational gradient in southwestern China. Furthermore, in controlled experiments the effects of soil quality, light availability, and competition by other species on seedling recruitment and survival was tested. At lower elevation individuals were larger (elder), but population size (density) was lower compared to higher elevation; similarly, adults and old individuals were relatively more frequent at low elevation, whereas juvenile and small individuals were more frequent at higher elevations. Seedling recruitment differed depending on soil origin with seeds sown in soils from low elevation having a higher germination percentage and seedling survival when grown without competition. Competition by other species delayed germination but did not influence the final germination percentage, but it increased seedling mortality and reduced their survival. Full light accelerated the germination process, but did not affect final germination percentage and seedling mortality. Results suggest that the recruitment and persistence of A. polytrichoides is strongly affected by competition, whereas temperature and soil quality are less important. We speculate that when populations of cushion A. polytrichoides are likely to be exposed to higher competition due to environmental changes, this species is in danger of being weeded out.