There is wide consensus that increasing species diversity often promotes ecosystem functions in grasslands, such as biomass yield. However, over time, sward diversity may be reduced through competitive interactions among species. The dynamics of multi-species grassland systems over time and their role in sustaining higher yields through increased diversity are still poorly understood. We investigated the development of species’ relative abundances in intensively managed agricultural grassland mixtures over three years to identify drivers of diversity change and their links to yield diversity effects. A continental-scale field experiment was conducted at 31 sites using 11 different four-species mixtures each sown at two seed abundances. The four species consisted of two grasses and two legumes, of which one was fast establishing and the other temporally persistent. We modelled the dynamics of the four-species mixtures, and tested associations with diversity effects on yield.
Results/Conclusions
We found that species’ dynamics were primarily driven by differences in the relative growth rates of competing species, and secondarily by density dependence and climate. The temporally persistent grass species typically had the highest relative growth rates and hence became dominant over time. Density dependence sometimes induced stabilising processes on the dominant species and inhibited shifts to monoculture. Legumes persisted at most sites at low or medium abundances and persistence improved with higher site annual minimum temperature. Significant diversity effects were present at the majority of sites in all years and the strength of diversity effects was improved with higher legume abundance. Observed diversity effects, when legumes had declined, may be due to (i) important effects of legumes even at low abundance, (ii) interaction between the two grass species or (iii) a store of N because of previous presence of legumes. Our work strongly supports the sowing of diverse mixture swards that include legumes over the long-standing practice of sowing agronomic grass monocultures. Careful and strategic selection of the identity of the species used in mixtures can help maintain species diversity and preserve the strength of yield increases associated with diversity.