Eight years of selection in a biodiversity experiment leads to differentiation into monoculture and mixture types in 12 grassland species

Background/Question/Methods

Biodiversity–ecosystem functioning experiments have shown increasing species or functional diversity has positive effects on productivity and that the magnitude of these effects increases over time. This increase in productivity has been interpreted as a potential consequence of increasingly complementary resource use, or pathogen regulation of plant species or functional groups. However, such increases could also be due to selection for increased "combining ability" or for reduced defence in high-diversity plant communities. We tested this novel hypothesis by comparing the performance in monocultures vs. 2-species mixtures of plants collected after 8 years in plots of a grassland biodiversity experiment containing monocultures or polycultures of species of one functional group or of four functional groups (grasses, legumes, tall herbs, small herbs). We used community aboveground biomass as performance measure for monocultures and 2-species mixtures and calculated complementarity effects for the latter as measure of combining ability. We also tested if plants derived from polycultures or grown in 2-species mixtures showed greater "character displacement" than plants derived from monocultures or grown in monocultures, respectively. Finally, we tested if plants derived from polycultures showed different metabolomic signals than plants derived from monocultures.

Results/Conclusions

We found monocultures produced the highest yield if they contained plants collected from monocultures and 2-species mixtures produced the highest yield if they contained plants collected from polycultures, indicating that selection for monoculture or mixture types had occurred in the biodiversity experiment. However, there was no indication of increased complementarity effects in mixtures of plants derived from polycultures, suggesting that not evolution for increased combining ability but possibly evolution for reduced defence (and thus increased growth) explained the observed effects on community performance. Consistent with these suggestions, we did not observe any character displacement between species in mixtures yet found clear signals of different metabolic composition of plants derived from polycultures vs. monocultures.

We conclude that evolution in biodiversity experiments may favor well-defended genotypes in monocultures, which are often more affected by pathogens than polycultures. In polycultures, genotypes with lower defence but stronger growth may be favored. Together, these effects provide an additional potential explanation for the increase in biodiversity effects over time in grassland experiments.