Trait assembly of woody plants in communities across sub-alpine gradients: Identifying the role of limiting similarity

Questions Plant species can be assembled into communities through habitat filtering or species competition, but their relative roles are still debated. We do not know whether there is limited similarity between co-existing species when accounting for the parallel effect of abiotic habitat filtering and biotic competition. By accounting for such effects, we test the predictions of three theories (classic niche theory, the Hubbell neutrality theory and the Scheffer and van Nes theory) of community assembly. Location Two vegetation transitions (a grazing gradient and a timber line ecotone) in a sub-alpine area of western Sichuan Province, China (31 degrees 51'N, 102 degrees 41'E). Methods We used a null model to investigate the above plant community assembly theories on two sub-alpine gradients of woody vegetation. In the null model, species traits were constrained between the maximum and minimum trait values of observed communities to test the principle of limiting similarity between co-existing species by testing for even spacing of traits. We analysed traits characterizing growth strategies of stems, leaves and twigs, measured at the level of individuals in situ. Results After accounting for variations in trait range, it became evident that six out of eight traits showed significantly uneven spacing within some plots, notably towards the forest end of the gradient, i.e. under increased competition pressure among woody plants. The Wilcoxon rank test showed that seven out of eight traits were significantly unevenly spaced within plots. The two transitions studied showed surprisingly similar patterns, despite their dissimilar precise drivers. Conclusion Our results support the recent theory of Scheffer & van Nes, which suggests that competition could result in a clumped pattern of species along a niche axis.