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 .