Multi-trait phenotypes drove compositional change
The weak influence of any single trait on abundances indicates that assembly occurred along multiple environmental axes, sometimes in opposing directions. Corroborating this, no combination of a single trait with a single environmental gradient explained net compositional change in either fragments or contiguous forest. However, in contiguous forest, trait syndromes shaped compositional variation along a composite environmental gradient defined by elevation, soil fertility and climatic water deficit (RLQ axis 1). Sites with higher elevation, soil fertility and lower water deficit had species with lower SLA and smaller seeds, the latter being consistent with trait-abundance associations (see above). These results suggest the need to examine the role of trait coordination in determining species’ responses to multiple stressors (Díaz et al., 2015; Shen et al., 2019).
Elevation was the only standalone gradient that significantly explained trait-mediated compositional change, but only in contiguous forest. Read together with trait-abundance associations, compositional change across the elevation gradient was driven by multiple environmental factors acting in concert (Figure S2). In contrast with contiguous forest, trait-environment linkages did not mediate compositional change across fragments. Thus, changes in the abundances of species with different traits along climate gradients did not translate into changes in community-level trait composition in fragments. In particular, fragments being located at higher elevations on average than contiguous forest may have dampened the effects of elevation on trait-mediated changes in composition. Trait-environment linkages may also be disrupted if fragments experience more flux than contiguous forests in species’ recruitment and survival (Collins et al., 2017; Laurance, 2002). Stochastic population dynamics or impact of local microclimate on species performance could obscure or override the role of larger-scale trait-environment linkages that otherwise mediate community assembly.