Aims: Fine roots play an important role in the biogeochemical cycles of terrestrial ecosystems and are vital for understanding forest ecosystem functioning and services. Higher plant species diversity has been largely reported to increase aboveground community biomass, but how biodiversity affects fine-root production and the related mechanisms in forests remain unclear. In this study, we aim to answer two questions: (i) does fine-root production increase with tree species richness? (ii) Can this effect be explained by niche complementarity among species?
Methods: We analyzed data from a large forest biodiversity experiment (BEF-China) with 5-year-old trees. Fine-root growth was measured as standing biomass and annual fine-root regrowth was estimated using ingrowth cores. Moreover, relative yield was calculated to test whether over- or under-yielding occurred when mixtures were compared with the average monoculture of the species included in the mixtures. We calculated functional diversity for fine-root (≤2mm in diameter) traits by Rao’s quadratic entropy index for each species mixture. The effects of manipulated tree species richness and identity on fine-root traits were analyzed with linear mixed-effects models. Mixed models were also used to test the relationships between tree species richness and fine-root standing biomass, annual regrowth and vertical heterogeneity.
Important Findings: Fine roots of more than one species were found in half of the soil cores in mixtures indicating that belowground interactions in these young forest stands occurred much earlier than canopy closure. We found significant differences among species in fine-root traits such as diameter and specific root length (SRL), which suggested different resource-use strategies and niche partitioning among species. Mean fine-root diameter of species ranged from 0.31 to 0.74mm, mean SRL ranged from 12.43 m·g−1 to 70.22 m·g−1 and mean vertical distribution index β ranged from 0.68 to 0.93. There was a significant positive relationship between species richness and the evenness of the vertical distribution of fine-root standing biomass. Moreover, marginally significant positive relationships existed between species richness and standing biomass as well as annual regrowth of fine roots. Relative yields and Rao’s quadratic entropy index were both not significantly affected by species richness. However, the relative yield of fine-root standing biomass was marginally correlated with Rao’s quadratic entropy index, implying that belowground niche complementarity between species does contribute to diversity effects. In conclusion, our study showed positive effects of species richness on the filling of soil volume by fine roots in the studied experimental forest communities. This has positive effects on fine-root standing biomass and may also lead to increased aboveground biomass.