Local canopy diversity does not influence phenotypic expression and plasticity of tree seedlings exposed to different resource availabilities

Species diversity seems to favour the environmental heterogeneity and habitat complexity, which in turn favour the selection of greater levels of phenotypic plasticity of plant species. While recent evidence suggested that local species diversity and environmental heterogeneity act as selective pressures over mean traits and plasticity for herbs, which adjust their traits to the identity and species richness of their neighbors, little is known for tree species. Here, we investigated shifts in the phenotypic expression and plasticity of seedlings of two functional groups (broadleaved and conifers) coming from communities with different canopy species richness in two contrasting forest types (a hemiboreal and a Mediterranean forest). We carried out a greenhouse experiment to test the response of seedlings to different light and nutrient availability, measuring different functional traits related to resource acquisition. Our results indicated that seedlings coming from monospecific and more diverse communities had similar phenotypic expression and response capacity to the study treatments. This finding suggests that canopy species richness did not exert a detectable selective pressure on tree phenotypes and plasticity at this early stage. Additionally, we found great differences in the phenotypic expression and plasticity between the two major functional groups (conifers and broadleaved species). Specifically, we found that broadleaves reached higher mean values in key functional traits by a greater plasticity to both light and nutrient treatment than coniferous species, mainly in the Mediterranean forest. This finding suggests that broadleaved species potentially have a higher capacity to respond to future changing environmental conditions than conifers at early stages, conferring an advantage that can be crucial for the species competitive ability for resources and, therefore, for survival.