Rare tree species have narrow environmental but not functional niches

Abstract The majority of species in many communities are rare, and it is an enduring challenge to understand the mechanism of species rarity and commonness in local communities. The niche partitioning hypotheses posit that common species occupy wide and core positions, while rare species occupy narrow and outlying niche positions. We test these hypotheses here, simultaneously considering both an environmental and a functional perspective on niches. We examined how niches correlate with species abundance in environmental and functional space by quantifying relationships between species abundance and niche width and niche position. Environmental niches were defined as the section of an environmental gradient where each species occurred, and functional niches using species traits were thought to be related to resource acquisition. Hypotheses were tested using a dataset including functional traits collected from 4,302 individual trees of 423 species and environmental data in one subtropical and one tropical forest in China. Consistent with the niche partitioning hypotheses, rare species in our two study systems tended to occupy the edges of the functional and environmental niche space. This likely allows rare species to avoid competition from dominant species. However, rare species tended to have a similar or larger niche width than common species in functional space, which contrasts with their narrower niche width in environmental space. Synthesis. Our results support the prediction that rare species occupy outlying niches to avoid competition with common species. We found inconsistent evidence, though, about the niche width of rare species. Rare species may be less constrained in functional space, and persist by dint of their functional lability and ability to use a variety of resources. Our work supports previously hypothesized mechanisms underlying local species abundance patterns, but highlights new idiosyncrasies when considering environmental niche or functional space alone. A free Plain Language Summary can be found within the Supporting Information of this article.