Can complementarity in water use help to explain diversity-productivity relationships in experimental grassland plots?

Positive diversity-productivity relationships have repeatedly been found in experimental grassland plots, but mechanistic explanations are still under debate. We tested whether complementarity for the exploitation of the soil water resource helps to explain these relationships. In the dry summer of 2003, evapotranspiration (ET) was assessed at the Swedish BIODEPTH site using two different approaches: snapshot measurements of canopy surface temperature and simulation of time-accumulated ET by means of a soil water balance model. More diverse plots were characterized by lower surface temperatures and higher accumulated ET. Transgressive overyielding tests revealed that ET in polycultures was higher than in the best-performing monocultures, but this pattern was reversed at high degrees of water stress. Our results indicate that a more complete exploitation of soil water by more diverse grassland systems is on the one hand likely to be a driver for their increased biomass production, but on the other hand causes the more diverse communities to be affected earlier by drought. Nevertheless, the results also suggest that productivity may (at least partially) be maintained under dry conditions due to the higher likelihood of including drought-tolerant species in the more diverse communities.