Data from: The relative contribution of species richness and species composition to ecosystem functioning

The influence of species diversity on ecosystem functioning has been the
subject of many experiments and remains a key question for ecology and
conservation biology. However, the fact that diversity cannot be
manipulated without affecting species composition makes this quest
methodologically challenging. We used partial Mantel tests to evaluate the
relative importance of diversity and of composition on biomass production.
Here, we applied partial Mantel tests (controlling for the other variable)
on two datasets, the Jena (2002–2008) and the Grandcour (2008–2009)
Experiment. In both experiments, plots were sown with different numbers of
species to unravel mechanisms underlying the biodiversity-ecosystem
functioning relationships. Contrary to Jena, plots were not mowed nor
weeded in Grandcour, allowing external species to establish. Based on the
diversity–ecosystem functioning and competition theories, we tested two
predictions: 1) the contribution of composition should increase with time;
2) the contribution of composition should be more important in non-weeded
than in controlled systems. We found support for the second hypothesis,
but not for the first. On the contrary, the contribution of species
richness became markedly more important few years after the start of the
Jena Experiment. This result can be interpreted as species
complementarity, rather than intraspecific competition, being the driving
force in this system. Finally, we explored to what extent the estimated
relative importance of both factors could vary when measured on different
spatial scales of the experiment (in our case, increasing the number of
plots included in the analyses). We found a strong effect of scale,
suggesting that comparisons between studies, and more generally the
extrapolation of results from experiments to natural situations, should be
made with caution.