Communities are almost always found to have many rare species and a few common ones. The question of why few species are common and others are rare is still a subject of ongoing debate. Negative density dependence (NDD) has been recognized as a major process for maintaining species diversity by limiting survival rates of focal species through negative interactions with neighboring plants, especially with conspecifics. Recent studies suggest that different strength of NDD among species influence the relative abundance of species and shape the species abundance distribution in forests. Also the process that affect abundance and rarity could be trait-based. But the relationship between functional traits and negative density-dependence is lacking. Different species may develop different traits to against natural enemies as well as competition from neighboring plants. Understanding the links between NDD and species functional traits will help us know more about the rules of species abundance distribution and has the potential to reveal the mechanisms that structure local communities. To figure out how plant functional traits affect species abundance via NDD, we conducted our study in the 50-ha HSD forest dynamics plot in a subtropical forest at Guangdong China. We collect abundance and functional traits data of 234 species. To evaluate the strength of NDD, we first calculate each species’ sapling death rates during two censuses and then use a generalized linear mixed-effect model to access the relationship between sapling death rates and number of adults around saplings within 20 meters. The random coefficients we extracted from the model represent the strength of NDD. Finally, we use structural equation models (SEMs) to explore the causal relationships among species functional traits, NDD and species abundance.
Results/Conclusions
From SEMs we find that, 1) Species abundance is positively correlated with strength of NDD, rare species suffer stronger NDD (cor = 0.545, p < 0.001). 2) Species functional traits can influence species abundance directly and indirectly through the strength of conspecific negative density dependence (CNDD). 3) 58.2% variation of species abundance can be explained by our SEM model (RMSEA = 0.071). Our results demonstrate that species abundance is determined by species functional traits via the strength of NDD, especially CNDD. One photosynthesis trait (maximum photosynthesis rate) and five leaf traits (leaf C concentration, leaf N concentration, leaf P concentration, specific leaf area (SLA), leaf dry matter content (LDMC)) are most important functional traits we find in this analysis.