We sought to assess effect of plant environmental adaptation strategies and evolutionary history and quantify the contribution of ecological processes to community assembly by measuring functional traits and phylogenetic composition in local forest community. We selected 18 dominant tree species in a Lithocarpus glaber–Cyclobalanopsis glauca evergreen broad-leaved forest and measured nine leaf functional traits and phylogenetic data of each species. We analyzed the variation in traits and trade-off relationships, tested phylogenetic effects on leaf functional traits, explored the influence of phylogeny and environment on leaf functional traits, and distinguished the relative effects of spatial and environmental variables on functional traits and phylogenetic compositions. The results showed the following: (i) Leaf traits had moderate intraspecific variation, and significant interspecific variation existed especially among life forms. (ii) Significant phylogenetic signals were detected only in leaf thickness and leaf area. The correlations among traits both supported “the leaf economics spectrum” at the species and community levels, and the relationships significantly increased or only a little change after removing the influence of phylogeny, which showed a lack of consistency between the leaf functional trait patterns and phylogenetic patterns. We infer the coexistent species tended to adopt “realism” to adapt to their habitats. (iii) Soil total potassium and phosphorus content, altitude, aspect, and convexity were the most critical environmental factors affecting functional traits and phylogenetic composition. Total environmental and spatial variables explained 63.38% of the variation in functional trait composition and 47.96% of the variation in phylogenetic structures. Meanwhile, the contribution of pure spatial factors was significantly higher than that of the pure environment. Neutral- theory-based stochastic processes played dominant roles in driving community functional trait assembly, but niche-theory-based determinative processes such as environmental filtering had a stronger effect on shaping community phylogenetic structure at a fine scale.

How soil quality and microbial communities change in conjunction with stand age in plantations is poorly understood. Here, we evaluated soil quality by using an integrated soil quality index (SQI) and traced the paralleled shifts in fungal community composition by high-throughput sequencing in a chronosequence of Chinese fir (Cunninghamia lanceolata) plantations (stand age of 3, 16, 25, 32, >80 years). Soil properties showed pronounced changes with stand age in the top 0-5 cm. The most prominent increase from 3 to >80-year-old stand was for soil organic carbon (SOC, by 2.1-times), total nitrogen (TN, 1.9-times) and available phosphorus (AP, 2.2-times). SQI increased logarithmically with stand age, with sharper change seen in the 0-5 cm layer than in the 5-15 cm layer. Mycorrhizal fungi increased in abundance initially in younger stands, but then they were gradually replaced by saprotrophic fungi in older stands due to the increase in litter input, which sustains saprotrophs. The positive correlation between saprotrophic fungi and the key soil quality indicators, such as TN, AP and NH4+, showed that higher soil quality was tightly linked with the enrichment of decomposers. Mycorrhizal taxa, such as orders Sebacinales, Thelephorales and Russulales, were positively correlated with acid phosphatase mobilizing P from organic matter. This suggests that the establishment of mycorrhizal fungi sustains tree productivity in younger stands under low soil quality. We conclude that the increase in soil quality throughout the development of Chinese fir plantations is closely linked with the observed transition of fungal communities from mycorrhizae to saprotrophs.

Complementarity in resource use leading to increased resource partitioning is the most commonly proposed mechanism for explaining the positive relationship between plant diversity and productivity. However, we still have a poor understanding of the relationship between plant diversity and root biomass. We used molecular method to identify tree species and to estimate the biomass of fine root (≤ 2 mm in diameter) for each tree species in soil cores sampled from the plots along a tree species gradient elaborated in subtropical forests. Our objectives were to examine whether spatial resource partitioning and symmetric proliferation are responsible for the relationship between aboveground tree species richness (SRA) and fine root biomass. We found that increasing SRA led to higher fine root biomass and a support for symmetric proliferation strategies, but this pattern only appeared in nutrient-rich upper soil layer. Structural equation modelling (SEM) indicated that stand density was the dominant factor to mediate SRA effects on fine root biomass. Specifically, fine root biomass depended on the SRA × stand density interaction, with lower biomass at lower density and low richness, and this effect disappeared in higher density forests. Overall, we found inconsistent support for the vertical niche partitioning, indicating that greater soil volume filling is not the reason for belowground overyielding pattern. Alternatively, density-dependent biotic interactions affecting tree recruitment are an important driver affecting productivity in diverse subtropical forests but the usual root distribution patterns in line with the resource partitioning hypothesis are unrealistic in contexts where soil nutrients are heterogeneously distributed.

Complementarity in resource use leading to increased resource partitioning is the most commonly proposed mechanism for explaining the positive relationship between plant diversity and productivity. However, we still have a poor understanding of the relationship between plant diversity and root biomass. We test whether the hypotheses of spatial resource partitioning and symmetric proliferation are responsible for the phenomena that aboveground tree species richness (SRA) increases fine root (≤ 2 mm in diameter) biomass. We found that increasing SRA led to higher belowground biomass and a support for symmetric root proliferation strategies, but this pattern only appeared in the more nutrient-rich upper soil layer. Fine root biomass depended on the SRA × tree density interaction, with lower biomass at lower density and low richness, and this effect disappeared in mixtures with high density. The results indicate that density-dependent biotic interactions affecting tree recruitment are an important driver to influence productivity.