Functional trait variation predicts distribution of alien plant species across the light gradient in a temperate rainforest

Several interspecific studies have related functional traits to alien plant invasiveness. Few studies have explicitly addressed how intraspecific variation in functional traits influences alien plant distribution across environments. In an old-growth temperate rainforest, we evaluated alien plant distribution across the light gradient, its relationship with native plant diversity and abundance, and variation in functional traits related to light capture that could explain the ecological breadth of alien plant species across the light gradient. In addition, we estimated possible ecological and phylogenetic constraints to intraspecific variation in these functional traits. In sixty 8-m2 plots we recorded light availability together with richness and abundance of alien and native plant species. We measured in 14 alien plant species three functional traits (specific leaf area, chlorophyll content and root/shoot ratio) and calculated their coefficient of variation. We estimated ecological constraints by measuring correlations among traits that could limit functional responses to light. We assessed phylogenetic constraints by calculating the phylogenetic signal of those functional traits. Richness and abundance of alien plant species were positively associated with light availability, while abundance was negatively related to native plant species abundance. Dominance of alien species increased with light availability, but some of them were dominant even in the shade. Phenotypic variation of functional traits was positively associated with ecological breadth and dominance of alien plant species. We found little evidence of ecological or phylogenetic constraints on functional trait variation. Light availability and functional trait variation correlated with the distribution and dominance of alien plants in this temperate rainforest. Alien plant species showed considerable phenotypic variation in low light, which may facilitate colonization of the forest understory. This process would not be hindered by ecological or phylogenetic constraints. Intraspecific variation in functional traits can render insightful information on current and future alien plant distribution across environmental gradients.