Cultivar mixtures increase stability and productivity over time through asynchrony and complementarity

Crop cultivar mixtures commonly increase productivity in the short term and stabilize and increase productivity in the long term, but these effects can be highly variable, and this may be related to a poor understanding of the underlying mechanisms for such diversity effects. We explored mechanisms for the effects of cultivar mixtures on the temporal yield stability and crop productivity trends in a seven-year (2016-2022) field experiment with maize in Northwest China. Across all cultivar mixtures over the seven years, grain yield and aboveground biomass increased by 5.6% and 3.6%, respectively, compared to the monocultures. To investigate changes in temporal yield stability over the seven years, we calculated temporal yield stability by using three-year rolling windows, resulting in five consecutive three-year rolling windows. Our results showed that temporal yield stability in cultivar mixtures increased during the later years (2019-2022), compared to the monocultures. Over the seven years, grain yield and aboveground biomass outperformed monocultures by 35% and 38%, respectively, compared to the first year. Complementarity effects were strong and increased over time. The mean values of functional traits changed in response to mixtures, leading to plant height and ear height traits correlating positively with complementarity effects, which were correlated with temporal yield stability. Asynchrony, or variation in the responses of cultivars to environmental fluctuations, was higher in four- and eight-cultivar mixtures than in two-cultivar mixtures, and negatively correlated with the temporal deviation in yield. These results indicated that large differences in mean trait values among cultivars, or those that express dynamic trait responses to diversity, can increase complementarity effects and asynchrony, producing more productive and stable crops. This increases our understanding of how intraspecific diversity might contribute to sustainable agroecosystems.