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.