Species Trait Dissimilarity and Hierarchy
We focused on six key functional traits here: leaf area (LA; cm2), specific leaf area (SLA; cm2 g-1, calculated as leaf area/dry mass), leaf dry matter content (LDMC; g g-1, calculated as leaf dry mass/fresh mass), wood density (WD; g cm-3, calculated as trunk wood dry mass/fresh volume), wood dry matter content (WDMC; g g-1, calculated as dry wood mass/fresh wood mass) and tree maximum height (Hmax; m) for each of the 80 species in this study. These traits represent leading axes of ecological variation among tree species that have been previously implicated in interspecific variation in resource use efficiency, species interactions, and life history strategies and are frequently used in analyses of the functional structure of forest communities (Kraft & Ackerly, 2010; Kunstler et al., 2016; Li et al., 2018). Specifically, LA is important for energy balance and hydraulic architecture (Ackerly & Cornwell, 2007). SLA and WDMC are key elements of the leaf and wood economic spectrum and correlate with procurement of resources (Chave et al., 2009; Wright et al., 2004). LDMC is indicative of a plant species’ resource use strategy that links to the trade-off between a rapid assimilation and growth (Díaz et al., 2004). WD is significant in relation to growth, stress-tolerance and survival rates (Chave et al., 2006). Hmax is a key determinant of light competition (Westoby, Falster, Moles, Vesk, & Wright, 2002). Data on these traits on species-level were collected and measured from the HSD plot (He, Chen, Zhao, Cornelissen, & Chu, 2018), in which Hmaxwas estimated by averaging the top 1% tallest trees for each species in the plot.
We calculated two kinds of species differences based on each single trait: absolute trait distance and hierarchical trait distance, to evaluate the effects of trait dissimilarity and trait hierarchy on interspecific spatial associations, respectively (Kunstler et al., 2012). Absolute trait distance between species i and j was calculated as |ti -tj | where ti and tj are the functional trait values of the respective species, whilst hierarchical trait distance was calculated asti -tj . In both trait distance measures, species i is the focal species in correspondence to that in the spatial point pattern analysis. As species’ niche dissimilarity might be better measured by a multi-trait than by a single trait approach (Kraft et al., 2015), we also computed species absolute distance in a multidimensional space defined by the 6 (centered and standardized) functional traits .