Drought tolerance increases with seed size in a semiarid grassland from southern Mexico

Due to increased reserves available to face adverse conditions, drought tolerance should increase with seed size. This has been confirmed in the laboratory, but under field conditions the pattern has not yet been found. We tested whether drought tolerance increases with seed size under field conditions in a semiarid grassland. We recorded the performance (germination, survival, and growth) of 12 species with different-sized seeds sown under conditions associated with hydric stress: along a natural stress gradient related to soil depth, and under experimentally manipulated irradiance levels that affected evaporation and thus soil humidity. We estimated drought tolerance indices for each performance metric, as well as a multivariate tolerance index that integrated the different components of performance. Each index was regressed on seed size accounting for phylogenetic effects. Seed size had little effect on individual metrics, but the joint evidence from all tests revealed a clear positive correlation with seed size. The multivariate tolerance index also increased with seed size, confirming the expected relationship. This suggests that comprehensive measures encompassing the whole life cycle may be required to characterize drought tolerance properly and may explain why this pattern has remained elusive in most studies. Also, we used a narrow range of seed sizes, so there is the possibility that the relationship between seed size and tolerance holds only for small-seeded species.