71 Biotic interactions limit species richness in an alpine plant community, especially under experimental warming Siri L. Olsen and Kari Klanderud S. L. Olsen (siri.lie.olsen@umb.no) and K. Klanderud, Dept of Ecology and Natural Resource Management, Norwegian Univ. of Life Sciences, PO Box 5003, NO-1432 Ås, Norway. he determinants of local species richness in plant communities have been the subject of much debate. Is species richness the result of stochastic events such as dispersal processes, or do local environmental filters sort species into communities according to their ecological niches? Recent studies suggest that these two processes simultaneously limit species richness, although their relative importance may vary in space and time. Understanding the limiting factors for species richness is especially important in light of the ongoing global warming, as new species establish in resident plant communities as a result of climate-driven migration. We examined the relative importance of dispersal and environmental filtering during seedling recruitment and plant establishment in an alpine plant community subjected to seed addition and long-term experimental warming. Seed addition increased species richness during the seedling recruitment stage, but this initial increase was cancelled out by a corresponding decrease in species richness during plant establishment, suggesting that environmental filters limit local species richness in the long term. While initial recruitment success of the sown species was related to both abiotic and biotic factors, long-term establishment was controlled mainly by biotic factors, indicating an increase in the relative importance of biotic interactions once plants have germinated in a microhabitat with favourable abiotic conditions. he relative importance of biotic interactions also seemed to increase with experimental warming, suggesting that increased competition within the resident vegetation may decrease community invasibility as the climate warms. What determines local species richness in plant communi- ties is a central and much debated question in ecology. On one hand, neutral theory states that species richness in a given plant community is mainly governed by stochastic events, such as dispersal from the regional species pool (Caswell 1976, Bell 2001, Hubbell 2001). On the other hand, niche theory claims that diversity is mainly deter- mined by local environmental filters, such as micro-climate and biotic interactions, which sort species into communi- ties according to their ecological niches (Hutchinson 1959, MacArthur and Levins 1967, Chase and Leibold 2003). hese theories are not necessarily mutually exclusive, and according to Lortie et al. (2004) local species richness may be simultaneously governed by stochastic processes, abiotic environmental filters such as temperature and soil characteristics, and biotic interactions, which includes both competition and facilitation among plants and inter- actions with other organisms (see also Belyea and Lancaster 1999). However, the relative importance of the different factors determining species richness may vary in space and time (Lortie et al. 2004). he most common way to determine whether seed dis- persal or local environmental filters limit local species rich- ness, is to conduct seed-addition experiments. Meta-analyses of such seed-addition experiments suggest that seed limita- tion is widespread both in populations of individual species (Turnbull et al. 2000, Clark et al. 2007) and plant commu- nities as a whole (Myers and Harms 2009), indicating that dispersal from the local and regional species pool is an important limiting factor for species richness. Although the importance of environmental filtering has been hypoth- esized to increase compared to seed limitation over the lifespan of a species (Clark et al. 2007), the duration of most seed-addition experiments is too short to determine whether the relative importance of seed limitation and environmental filtering change over time (Turnbull et al. 2000, Clark et al. 2007, Myers and Harms 2009). Hence, long-term studies are needed to examine the relative impor- tance of dispersal and environmental filtering for species richness at different stages of the plant life cycle. he relative importance of the different limiting factors for species richness may also change with climate warming, both as a direct response to increased temperatures and indirectly through altering of biotic interactions (Lortie et al. 2004). Such changes may in turn allow new species to establish in resident plant communities, possibly leading to major changes in community species richness and com- position. his is already happening in alpine habitats, where Oikos 123: 71–78, 2014 doi: 10.1111/j.1600-0706.2013.00336.x © 2013 he Authors. Oikos © 2013 Nordic Society Oikos Subject Editor: Bente Graae. Accepted 8 May 2013