The effects of density dependence and habitat preference on species coexistence and relative abundance

In plant communities, some mechanisms maintain differences in species' abundances, while other mechanisms promote coexistence. Asymmetry in conspecific negative density dependence (CNDD) and/or habitat preference is hypothesized to shape relative species abundance, whereas community compensatory trends (CCTs) induced by community-level CNDD and heterospecific facilitation are hypothesized to promote coexistence. We use survey data from three 1-ha permanent dynamic plots in a subtropical forest over the course of a decade to find out which of these processes are important and at which life-history stages (the seedling, sapling, and juvenile stages) they exert their effects. CNDD was not related to abundance in any of the life-history stages. Suitable habitats positively influenced plant abundance at all tested life stages, but especially so for juveniles. Community-level CNDD of seedling neighbors was detected at the seedling stage, while heterospecific facilitation was detected across all tested life-history stages. A CCT in seedling survival was detected, but there was no evidence for such trends across the other life-history stages. Altogether, our results suggest that habitat specificity increases the rarity of species, whereas a CCT at the seedling stage, which is likely to be induced by CNDD and heterospecific facilitation, enables such species to maintain their populations.