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Abstract

Niche differentiation, assumed to be a key mechanism of species coexistence, requires that
species differ in their functional traits. So far it remains unclear to which extent trait plasticity
leads to niche shifts of species at higher plant diversity, thereby increasing or decreasing
niche overlap between species. To analyse this question it is convenient to measure niches
indirectly via the variation in resource-uptake traits rather than directly via the resources
used. We provisionally call these indirectly measured niches trait-based niches. We studied
shoot- and leaf-morphological characteristics in seven legume species in monoculture and
multi-species mixture in experimental grassland. Legume species varied in the extent of
trait variation in response to plant diversity. Trait plasticity led to significant shifts in species
niches in multiple dimensions. Single-species niches in several traits associated with height
growth and filling of canopy space were expanded, while other niche dimensions were compressed
or did not change with plant diversity. Niche separation among legumes decreased
in dimensions related to height growth and space filling, but increased in dimensions related
to leaf size and morphology. The total extent of occupied niche space was larger in mixture
than in the combined monocultures for dimensions related to leaf morphology and smaller
for dimensions related to whole-plant architecture. Taller growth, greater space filling and
greater plasticity in shoot height were positively, while larger values and greater plasticity in
specific leaf area were negatively related with increased performance of species in mixture.
Our study shows that trait variation in response to plant diversity shifts species niches along
trait axes. Plastically increased niche differentiation is restricted to niche dimensions that
are apparently not related to size-dependent differences between species, but functional
equivalence (convergence in height growth) rather than complementarity (divergence in
traits associated with light acquisition) explains increased performance of legumes
in mixture.