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Abstract

Several multi-year biodiversity experiments have shown positive species richness–productivity relationships
which strengthen over time, but the mechanisms which control productivity are not well
understood. We used experimental grasslands (Jena Experiment) with mixtures containing different numbers
of species (4, 8, 16 and 60) and plant functional groups (1–4; grasses, legumes, small herbs, tall herbs)
to explore patterns of variation in functional trait composition as well as climatic variables as predictors
for community biomass production across several years (from 2003 to 2009). Over this time span, high
community mean trait values shifted from the dominance of trait values associated with fast growth to
trait values suggesting a conservation of growth-related resources and successful reproduction. Increasing
between-community convergence in means of several productivity-related traits indicated that environmental
filtering and exclusion of competitively weaker species played a role during community assembly.
A general trend for increasing functional trait diversity within and convergence among communities suggested
niche differentiation through limiting similarity in the longer term and that similar mechanisms
operated in communities sown with different diversity. Community biomass production was primarily
explained by a few key mean traits (tall growth, large seed mass and leaf nitrogen concentration) and
to a smaller extent by functional diversity in nitrogen acquisition strategies, functional richness in multiple
traits and functional evenness in light-acquisition traits. Increasing species richness, presence of
an exceptionally productive legume species (Onobrychis viciifolia) and climatic variables explained an
additional proportion of variation in community biomass. In general, community biomass production
decreased through time, but communities with higher functional richness in multiple traits had high productivities
over several years. Our results suggest that assembly processes within communities with an
artificially maintained species composition maximize functional diversity through niche differentiation
and exclusion of weaker competitors, thereby maintaining their potential for high productivity.