Interspecific trait differences rather than intraspecific trait variation increase the extent and filling of community trait space with increasing plant diversity in experimental grasslands

Niche differentiation among species is supposed to be reflected in the distribution of trait values in communities. Yet, how species-specific trait expression responds to changing plant diversity and how these changes affect community-level trait distributions is less clear. We used monocultures and mixtures of a large biodiversity experiment (Jena Experiment) to evaluate this question. We applied additive partitioning of net biodiversity effects (NE) into complementarity (CE) and selection effects (SE) to community-weighted trait means (CWM). Between 60-100% of NEs on CWMs (i.e. their deviation from the abundance-weighted means of monoculture trait values) were explained by CEs indicating that trait plasticity of different species contributed to shifts in CWMs. Positive CEs for shoot-architectural traits, tissue elemental concentrations and specific leaf area (SLA) indicated on-average higher values of these traits in the mixtures, while negative SEs in these traits suggested that dominant species showed the opposite response to the mixture-environment. Trait plasticity in response to mixture growth conditions increased species-level trait deviation from CWMs in several traits related to structural stability and SLA, while species-level deviation in shoot length from CWMs decreased. Community-wide occupation of trait space (range and variance of trait values) increased with plant diversity. Trait plasticity in response to the mixture-environment reduced the occupied trait space in several size-related traits, while it increased trait space in SLA. Measures of trait-based niche differentiation did not change (kurtosis, i.e. “peakedness” of trait distribution) or decreased (standard deviation of nearest neighbour distances, SDNN) at increased species richness, indicating that plasticity did not reduce niche overlap. Our results suggest that fixed trait differences among species primarily explain the greater extent and denser filling of community trait space at increased plant diversity. Species-specific trait plasticity in response to increased plant diversity does not increase community-wide trait space or decrease niche overlap between species.