Both diversity and functional composition affect productivity and water use efficiency in experimental temperate grasslands

Many experiments have shown that biodiversity promotes ecosystem
functioning and stability and that this relationship varies with resource
availability. However, we still have a poor understanding of the
underlying physiological and ecological mechanisms driving diversity
effects and how they may interact with soil nutrient availability. We
collected data in a grassland experiment factorially manipulating
fertilization, species richness, functional composition
(slow-growing vs. fast-growing species), and functional diversity
in resource economic traits. We measured aboveground productivity,
nitrogen (N) uptake, photosynthesis, and water use efficiency by combining
a 15N labelling approach with productivity, gas exchange, and
stable isotope measurements in three years differing in rainfall. We found
that sown species richness increased aboveground productivity, N uptake
and photosynthesis, suggesting that species richness is the most important
driver of ecosystem productivity and nutrient cycling. Similarly,
photosynthesis was affected by functional composition but not by
functional diversity. Water use efficiency was reduced by sown species
richness for communities dominated by slow growing species but not for
communities dominated by fast growing species. Fertilization increased
productivity, N uptake and water use efficiency. The positive effects of
high species richness on ecosystem functions were independent of fertility
levels. Synthesis. Our results provide evidence that high species richness
in temperate grasslands could enhance productivity and reduce the negative
impacts of drought events. Multiple factors and community characteristics
are important in driving enhanced ecosystem functioning in biodiverse
grasslands and seem to affect functioning and stability through different
mechanisms.