Abstract
The classical environmental control model assumes that species diversity is primarily determined by environmental conditions (e.g., microclimate and soil) on the local scale. This assumption has been challenged by the neutral theory that assumes that the maintenance of biodiversity mainly depends on the ecological drift and dispersal limitation. Understanding the mechanisms that maintain biodiversity depends on decomposing the variation of species diversity into the contributions from the various components that affect it. We investigated and partitioned the effects of the biotic component (productivity, forest spatial structure) and the environmental component (topography and soil fertility) on the distribution of tree species richness jointly (the combined effect of environment and biotic process) and separately (the effect of environment or biotic process alone) in 25 permanent plots of 600 m2 in a subtropical evergreen broadleaf secondary forest in southern China. The analysis was also completed for trees at different growth stages based on diameter breast height (young trees: 5 cm ≤ DBH < 10 cm, mature trees: 10 cm < DBH ≤ 20 cm, old trees: DBH > 20 cm) within each plot. Our results indicated that (1) tree species richness had significant negative relationship with productivity and a unimodal relationship with its spatially structured distribution; (2) biotic and environmental factors both have significant influence on species richness and jointly explain ~60% of the variation for the overall tree assemblage, and the variation explained by the two components jointly increased across growth stages (34%, 44%, and 75%, respectively); (3) additive variation partitioning revealed that the tree species richness was dominantly controlled by environmental factors (32%), while the biotic component also independently contributed a non-negligible effect (16%); and (4) the dominant fraction changed from the biotic component to the environmental component across growth stages. Results suggest that the tree species richness may be governed from neutral process to environmental control during tree life span in subtropical evergreen broadleaf secondary forests.
Highlights
Understanding the connection between maintaining biodiversity and ecosystem processes has long been a major focus of research in ecology [1,2]
We examined four inter-related questions as follows: (1) How does productivity influence tree species diversity in subtropical secondary evergreen broadleaf forests? (2) Does tree spatial aggregation have a significant role in explaining tree species diversity? (3) How do biotic and environmental factors jointly affect the tree species diversity and what is the relative influence of either factor? (4) How do the above relationships change across growth stages within a forest?
Relationships between the above broadleaf ground biomass (AGB) and richness were shown for different sizes of trees old trees separately
Summary
Understanding the connection between maintaining biodiversity and ecosystem processes has long been a major focus of research in ecology [1,2]. Existing studies in forests have obtained equivocal results of the factors affecting community species richness, and most are attributed to four major processes: selection, drift, speciation, and dispersal [3]. Niche differentiation has traditionally been used as an explanation for the maintenance of local diversity in multispecies communities [4]. Focusing on the selection process, the variation in environmental conditions may be responsible for the species diversity through species–habitat associations. This is the classical environmental control model [5]. Studies of niche differentiation in tree species communities focus on species’ partitioning along gradients of environment heterogeneity, especially on topography and soil nutrient on local scales in forests [6]
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