Effects of competition on phylogenetic signal and phenotypic plasticity in plant functional traits

Recent studies of communities have examined phylogenetic signal in species' functional traits to infer drivers of community assembly. Phenotypic variation in traits, arising from constitutive genetically based variation and from environmental influences on gene expression, or phenotypic plasticity, could affect inferences about community assembly. We found significant trait plasticity in 12 focal species across four species-interaction treatments grown in four soil environments. Phylogenetic signal in traits was present, but was also dependent on species-interactor treatment, suggesting that phenotypic plasticity and plant neighborhood could affect the ability to detect and interpret community phylogenetic patterns of trait variation. Individuals competing with conspecifics expressed significant divergence in specific leaf area (SLA) relative to when they were grown alone. Combined with the observation that competition is stronger between close relatives than between distant relatives in some soils, these results suggest that trait plasticity may be an adaptive response to competition. To test this hypothesis, we examined total biomass in a pot, relative to the predicted biomass of two individuals grown alone, and related pot biomass to phylogenetic distance of the interactor treatment, as well as to divergence in SLA and root : shoot ratio. Within competition treatments, only plastic divergence in root : shoot ratio in one interactor treatment was correlated with increased productivity, and only in one soil type. We also tested whether, across all treatments, divergence in SLA or root : shoot ratio increased pot productivity. We found that community productivity was positively influenced both by phylogenetic distance to competitor, as well as by divergence in root : shoot ratio due to both plasticity and constitutive differences. Phenotypic plasticity resulting in trait divergence may increase the ability of plants to coexist and may also decrease phylogenetic signal in community assembly at small spatial scales.