Abstract
Forest productivity (increment of above-ground biomass) is determined by biodiversity but also by stand structure attributes. However, the relative strengths of these drivers in determining productivity remain controversial in subtropical forests. In this study, we analyzed a tree growth data from 500 plots with in a 20 ha mature subtropical forest in eastern China. We used spatial simultaneous autoregressive error models to examine the effects of diversity variables (species richness, evenness, and composition), stand structural attributes (stand density, tree size range and diversity), environmental factors (topography and soil), and initial above-ground biomass (AGB) on productivity. We also applied structural equation models to quantify the relative importance of diversity, stand structure, environmental factors, and initial AGB in determining forest productivity. Our results showed that stand structure together with diversity and initial AGB governed forest productivity. Tree size diversity (DBH Shannon’s diversity index) had the largest positive effect on forest productivity. These results provide new evidence that structural explanatory variables have greater contributions to productivity for mature subtropical forests, strongly supporting the niche complementarity hypothesis. Our work highlights the importance of tree size diversity in promoting high forest productivity, and suggests that regulating and conserving complexity of forest stand structure should be among the most important goals in subtropical forest management.
Highlights
The relationship between biodiversity and productivity is a central topic in both theoretical and applied ecology [1]
Two fundamental mechanisms wildly used to explain the positive effects of species diversity on forest productivity are the niche complementarity and the selection effect hypothesis [3,7,8,9]
Based on linear regression tests, bivariate relationships indicated that forest productivity was positively associated with 7 of 11 predictor variables and negatively associated with 1 of 11 predictor variables (Figure 2)
Summary
The relationship between biodiversity and productivity is a central topic in both theoretical and applied ecology [1]. Increasing evidence suggests that the positive effect of species diversity can be influenced or neutralized by structural diversity, in forest ecosystem which has complex vertical and horizontal structures [4,5,6] It remains far from clear what the major drivers of forest ecosystem productivity are. On one hand, increasing diversity may lead to niche overlap (species that make use of the same resources), intensifying inter-species competition [13]; on the other hand, ontogenetic niche differences (resource requirements vary with individual size) of intraspecies are ignored [6] This may explain why the relationship between species diversity and productivity/above-ground biomass sometimes does not support the niche complementarity hypothesis in high diversity natural forest ecosystems, where negative [4,14], or non-significant relationships are found [15,16]
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