Phenotypic plasticity may mediate habitat filtering in a forest edge community

A key process determining community assembly is habitat filtering, which reduces the range of trait values and thus leads to trait convergence. Habitat filtering is often assumed to involve a reduction in species richness, but such trait convergence could also result from phenotypic plasticity. By allowing more species in the community to show the "right" phenotype and pass the filter, phenotypic plasticity could maintain species richness despite trait convergence. If phenotypic plasticity plays this role, environmental changes should modify trait distribution but not species composition or richness. We tested this hypothesis in a forest edge community recently invaded by the alien shrub <i>Rubus praecox</i>. We measured functional traits beneath <i>Rubus </i>canopy (<i>Rubus</i>-invaded community) and in adjacent microsites naturally free from <i>Rubus </i>(original community), comparing trait dispersion and community structure parameters. We also characterized the microenvironments to seek functional explanations for the observed phenotypic changes. The functional dispersion parameter FDis showed habitat filtering in the <i>Rubus</i>-invaded community (trait convergence), while in the original community it did not differ from a null model. Species richness and composition did not differ between the two communities. Soil characteristics were similar in both communities but light availability was greatly reduced in the <i>Rubus</i>-invaded community. A Principal Component Analysis of community-weighted means of functional traits showed that phenotypic changes between communities were mostly consistent with functional responses to shading, including an increase in specific leaf area and a reduction in flavonoid concentration in the <i>Rubus</i>-invaded community. We verified the conditions necessary to infer that phenotypic plasticity mediates the outcome of habitat filtering driven by <i>Rubus </i>in the study system, i.e., plasticity would explain species persistence despite trait convergence. Phenotypic plasticity, often subsumed within intraspecific variation in community assembly studies, should be considered individually to improve our understanding of the causes and consequences of habitat filtering.