Abstract: Aims Functional trait differences among species are thought to be a prerequisite for niche differentiation. Plant traits are known to vary plastically in response to environmental conditions and different plant neighbours. However, it is not clear to which extent the direction and magnitude of trait variation differ among species representing different growth forms or varying dominance in different environments and how this trait variation affects community-level trait dissimilarity.
Methods In a field experiment, we studied shoot and leaf traits of eight perennial grassland species assigned to two functional groups (grasses vs. forbs) and varying in growth stature (small vs. tall) in monocultures, two- and four-species mixtures that were exposed to two levels of light (shaded vs. full light) and nutrient availability (fertilized vs. non-fertilized) to address this gap of knowledge.
Important findings Fertilization increased leaf greenness and specific leaf area (SLA) as well as tissue nitrogen concentrations. Under shading, changes in morphological and physiological light-acquisition traits as well as increased nitrogen and decreased carbon concentrations in plant tissue indicated larger efforts to acquire light and carbon limitation of plant growth. Similar changes in light-acquisition traits as observed under shading and decreasing shoot biomass also revealed an accelerated carbon limitation at higher species richness (SR) irrespective of the external manipulation of light and nutrient supply. Overall, the direction of trait variation in response to resource availability (shade, fertilization) and SR did not differ between functional groups or growth statures. The magnitude of variation in several traits at different resource availability, however, was larger in grasses than in forbs as well as in small-statured than in tall-statured species. In general, dominant species in terms of aboveground biomass production had taller shoots with higher carbon and lower nitrogen concentrations and allocated less biomass into leaves than subordinates. The expression of leaf traits of dominants (lower SLA and higher leaf greenness than subordinates) indicated their competitive advantage in light acquisition. Shading, as well as fertilization, accentuated trait differences between dominants and subordinates. Fertilization increased community-level dissimilarity in tissue nitrogen concentrations due to differential responses of functional groups and growth statures. However, resource supply did not alter community-level dissimilarity in other traits. Our study shows that a varying extent of trait variation of species belonging to different functional groups or growth statures and varying in their ability to achieve dominance may modulate community-level trait dissimilarity at different resource availability. These results emphasize the importance to consider the context-dependency of trait-based approaches.

Key words: biodiversity, functional groups, functional traits, growth stature, trait dissimilarity