Big-sized trees overrule remaining trees' attributes and species richness as determinants of aboveground biomass in tropical forests.

Large-diameter, tall-stature, and big-crown trees are the main stand structures of forests, generally contributing a large fraction of aboveground biomass, and hence play an important role in climate change mitigation strategies. Here, we hypothesized that the effects of large-diameter, tall-stature, and big-crown trees overrule the effects of species richness and remaining trees attributes on aboveground biomass in tropical forests (i.e., we term the "big-sized trees hypothesis"). Specifically, we assessed the importance of: (a) the "top 1% big-sized trees effect" relative to species richness; (b) the "99% remaining trees effect" relative to species richness; and (c) the "top 1% big-sized trees effect" relative to the "99% remaining trees effect" and species richness on aboveground biomass. Using environmental factor and forest inventory datasets from 712 tropical forest plots in Hainan Island of southern China, we tested several structural equation models for disentangling the relative effects of big-sized trees, remaining trees attributes, and species richness on aboveground biomass, while considering for the full (indirect effects only) and partial (direct and indirect effects) mediation effects of climatic and soil conditions, as well as interactions between species richness and trees attributes. We found that top 1% big-sized trees attributes strongly increased aboveground biomass (i.e., explained 55%-70% of the accounted variation) compared to species richness (2%-18%) and 99% remaining trees attributes (6%-10%). In addition, species richness increased aboveground biomass indirectly via increasing big-sized trees but via decreasing remaining trees. Hence, we show that the "big-sized trees effect" overrides the effects of remaining trees attributes and species richness on aboveground biomass in tropical forests. This study also indicates that big-sized trees may be more susceptible to atmospheric drought. We argue that the effects of big-sized trees on species richness and aboveground biomass should be tested for better understanding of the ecological mechanisms underlying forest functioning.

Climatic water availability is the main limiting factor of biotic attributes across large-scale elevational gradients in tropical forests.

Climatic water availability is a key spatial driver of species distribution patterns in natural forests. Yet, we do not fully understand the importance of climatic water availability relative to temperature, and climate relative to edaphic factors for multiple biotic attributes across large-scale elevational gradients in natural forests. Here, we modelled multiple abiotic factors (elevation, climate, and edaphic factors) with each of the taxonomic-related (Shannon's species diversity, species richness, species evenness, and Simpson's dominance) and tree size or biomass-related (individual tree size variation, functional dominance and divergence, and aboveground biomass) biotic attributes through boosted regression trees (BRT) models, using biophysical data from 247,691 trees across 907 plots in tropical forests in Hainan Island of Southern China. The tested multiple abiotic factors explained simultaneously 43, 50, 36, 45, 37, 50, 17 and 46%, respectively, of the variations in Shannon's species diversity, species richness, species evenness, Simpson's dominance, individual tree size variation, functional dominance, functional divergence and aboveground biomass. After the large influences of elevation (i.e. 30.43 to 62.83%), climatic water availability accounted for most (i.e. 15.52 to 25.30%) of the variations in all biotic attributes. Beside the increasing trend with elevational gradients, taxonomic diversity increased strongly with climatic water availability whereas tree size or biomass-related biotic attributes showed strong decreasing and increasing trends. Tree size or biomass-related rather than taxonomic-related biotic attributes also decreased apparently with mean annual temperature. Most of the biotic attributes monotonically increased with soil fertility but decreased with soil pH, whereas soil textural properties had mostly negligible influences. This study strongly reveals that future climate change (i.e. a decrease in climatic water availability with an increase in mean annual temperature) is thus likely to have a substantial influence on the biotic attributes in the studied tropical forests across large-scale elevational gradients.

Tree crown complementarity links positive functional diversity and aboveground biomass along large-scale ecological gradients in tropical forests.

Most of the previous studies have shown that the relationship between functional diversity and aboveground biomass is unpredictable in natural tropical forests, and hence also contrary to the predictions of niche complementarity effect. However, the direct and indirect effects of functional diversity on aboveground biomass via tree crown complementarity in natural forests remain unclear, and this potential ecological mechanism is yet to be understood across large-scale ecological gradients. Here, we hypothesized that tree crown complementarity would link positive functional diversity and aboveground biomass due to increasing species coexistence through efficient capture and use of available resources in natural tropical forests along large-scale ecological gradients. We quantified individual tree crown variation, functional divergence of tree maximum height, and aboveground biomass using data from 187,748 trees, in addition to the quantifications of climatic water availability and soil fertility across 712 tropical forests plots in Hainan Island of Southern China. We used structural equation modeling to test the tree crown complementarity hypothesis. Aboveground biomass increased directly with increasing functional diversity, individual tree crown variation and climatic water availability. As such, functional diversity enhanced individual tree crown variation, thereby increased aboveground biomass indirectly via individual tree crown variation. Additional positive effects of climatic water availability and soil fertility on aboveground biomass were accounted indirectly via increasing individual tree crown variation and/or functional diversity. This study shows that tree crown complementarity mediates the positive effect of functional diversity on aboveground biomass through light capture and use along large-scale ecological gradients in natural forests. This study also mechanistically shows that tree crown complementarity increases species coexistence through maintenance of functional diversity, which in turn enhances aboveground biomass in natural tropical forests. Hence, managing natural forests with the aim of increasing tree crown complementarity holds promise for enhancing carbon storage while conserving biodiversity in functionally-diverse communities.