Plasticity in plant functional traits is shaped by variability in neighbourhood species composition

Plant functional traits can vary widely as a result of phenotypic plasticity to abiotic conditions. Trait variation may also reflect responses to the identity of neighbours, although not all species are equally responsive to their biotic surroundings. We hypothesized that responses to neighbours are shaped by spatial community patterns and resulting variability in neighbour composition. More precisely, we tested the theoretical prediction that plasticity is most likely to evolve if alternative environments (in this case, different neighbour species) are common and encountered at similar frequencies. We estimated the frequencies of encountering different neighbour species in the field for 27 grassland species and measured the aboveground morphological responses of each species to conspecific vs heterospecific neighbours in a common garden. Responses to neighbour identity were dependent on how frequently the experimental neighbours were encountered by the focal species in their home community, with the greatest plasticity observed in species that encountered both neighbours (conspecific and heterospecific) with high and even frequency. Biotic interactions with neighbouring species can impose selection on plasticity in functional traits, which may feed back through trait divergence and niche differentiation to influence species coexistence and community structure.