COS 2-1
Four-species experimental grassland swards improve agronomic yield and resistance to weeds across 31 international sites
Intensively-managed agricultural grasslands are globally prominent ecosystems in which large amounts of energy and nutrients flow, and associated management options have the potential to supply (or degrade) a range of ecosystem services. The ability of more diverse plant communities to acquire and convert available resources to aboveground biomass has considerable relevance to resource use efficiency in agricultural systems, although the results from extensively-managed semi-natural grasslands do not necessarily extrapolate to intensively-managed grasslands.
We used experimental grassland communities to investigate whether multi-species mixtures provide improved productivity and weed suppression over monocultures. We also tested whether these diversity benefits persisted through time and were consistent across a wide geographical scale. A common experiment was established at 31 sites across Europe and Canada. At all sites, mixtures consisted of two grass and two legume species. Fifteen experimental communities were sown with varying levels of evenness (E) at two seed densities. Communities consisted of monocultures (100:0:0:0, E=0), mixtures dominated in turn by each species (70:10:10:10, E=0.64), mixtures dominated in turn by pairs of species (40:40:20:20, E=0.88), and an equi-proportional community (25:25:25:25, E=1). Yield was sampled by mowing, and yield of sown species and weeds measured for up to three years.
Results/Conclusions
Yield of sown species in mixtures exceeded the average yield of monocultures. Transgressive overyielding (where the yield of mixtures exceeded the best-performing monoculture) occurred at 70% of sites with a ratio of mixture to best-performing monoculture = 1.18. Within each year, there was a highly significant relationship (p<0.0001) between sward evenness (quadratic relationship) and the diversity effect (excess of mixture performance over that predicted from the monoculture performances of component species). The diversity effects were positive at all evenness values, although the diversity effect was not significantly different from the maximum diversity effect across a wide range of higher evenness values. In mixtures, median values indicate <4% of weed biomass in total yield (sown species + weeds), whereas the median percentage of weeds in monocultures increased from 15% in year 1 to 32% in year 3. In summary, four-species mixtures generally yielded more than the best-performing monoculture at each site, and mixtures maintained a resistance to weed invasion over at least three years.