Plasticity of functional traits of forb species in response to biodiversity

In spite of increasing recognition that intraspecific variation may play an important role for niche differentiation, which is regarded as a promoter of species coexistence, the extent and structure of functional trait variation in response to plant neighbor diversity is poorly understood. We studied the plasticity of functional traits in vegetative and reproductive shoots of 27 non-legume forb species with different growth forms (reptant, rosulate, semirosulate) in experimental grasslands (Jena Experiment) of varying species richness and functional group composition (with and without legumes). Traits related to whole-shoot structure differed strongly among forb species with different growth forms, while leaf traits associated with light acquisition (specific leaf area, foliar δ13C values) and traits associated with nitrogen nutrition (shoot biomass:N ratios, leaf nitrogen concentrations, foliar δ15N values) were highly plastic within forb species. Plant height generally increased with increasing species richness. Plasticities to increased species richness in leaf traits (leaf length, SLA, foliar δ13C) varied among growth forms and depended on developmental stage. The presence of legumes generally increased plastic responses in light-acquisition traits in the same direction as increasing species richness. Greater tissue nitrogen concentrations and unchanged foliar δ15N values of forb species in the presence of legumes suggested that the fertilizing effect of nitrogen-fixing legumes was due to the supply of unconsumed mineral nitrogen. Stronger correlations between trait means and trait plasticities in size-related traits suggested a functional convergence in response to light competition. A more variable spectrum in the plasticities of traits not associated with plant size indicated a greater functional separation among species. Our results suggest that both interspecific differences and intraspecific trait plasticity affect niche partitioning among forb species and are important for their coexistence in multi-species assemblages.