The effect of biodiversity on productivity changes over time in duckweed communities

Several studies have shown that increased biodiversity can lead to increased productivity in plant communities. This relationship can be explained by a selection effect caused by the dominance of one or a few species that drive productivity, and by a complementarity effect resulting from the use of resources by the different species. However, these effects are likely to change over time, which cannot be observed in short‐term experiments (relative to lifespan) that make up most of the literature on this topic. To study the change over time of the diversity–productivity relationship (DPR) and its underlying mechanisms, we developed an experimental system using small, fast‐generating plants: duckweeds (Lemnaceae). In a growth chamber, we grew four species in monocultures and polycultures with all possible combinations for 60 days (about six generations of growth). At 20‐day intervals, we measured the biomass of each species, as well as their functional traits. Our results show a clear increase in the net effect of biodiversity on productivity over time, mainly caused by an increase in the complementarity effect and the presence of one species in particular. We also show that, in the presence of other species, the plants showed phenotypic plasticity; however, these changes did not contribute to an enhancement of the complementarity effect. Our results reinforce the importance of long‐term DPR studies and underline the effectiveness of duckweeds as an experimental model.