The role of intraspecific trait variability and soil properties in community assembly during forest secondary succession

Trait-based approaches applied to community assembly have led to a considerable advance in understanding the drivers that underlie community dynamics. However, species respond to environmental conditions based on traits, and the role of intraspecific trait variability (ITV) in community assembly during succession has not been studied in detail. Here, we explored the effects of environmental conditions on plant trait composition and tested the role of ITV on understanding the assembly processes throughout a longterm forest succession on the Loess Plateau of China. Thirty plots were established across six successional stages, and 15 functional traits of 210 species were measured. Rao's quadratic entropy of each community with and without accounting for ITV was calculated and then compared with random expectation to evaluate the community functional structure. A structural equation model was used to determine the effects of soil abiotic factors on community functional structure. Community functional structure based on multiple traits transitioned from a stochastic pattern in the early community to a significantly overdispersed pattern in the latest climax community, mainly driven by the increasing niche differentiation of plant height and seed mass. In contrast, community functional structure for most chemical and structure traits showed a neutral but increasing clustering pattern throughout succession. We found that consideration of intraspecific variability did not result in the misreading of the assembly process along a successional gradient; consideration of intraspecific variability only increased the divergence of traits for the latest climax community. Soil NH4-N was an especially important factor that influenced community functional structure and ITV during succession. Our results suggest a change in the dominant assembly mechanism from stochastic to deterministic processes and a strong environmental modification that drives community assembly during a long-term forest succession. Ignoring ITV (except for plant height) may be acceptable in understanding community assembly along a local secondary forest succession in a semiarid ecosystem, such as the Loess Plateau.