Ecologists have delineated a variety of mechanisms that can, in principle, favor species coexistence and hence maintain biodiversity. Most mechanisms of coexistence require or imply tradeoffs between different aspects of species performance. However, it remains an open question whether simple functional tradeoffs underlie coexistence mechanisms in diverse natural systems. For a guild of Sonoran Desert annual plants, we combined functional trait measurements, long-term demographic records, and species coexistence theory to determine how functional trait variation relates to population and community dynamics. We conducted sequential harvests across the growing season to estimate relative growth rates and biomass allocation patterns, and we measured leaf-level photosynthetic characteristics related to carbon assimilation under differing temperature and soil moisture conditions. We hypothesized that species would exhibit a tradeoff between growth capacity and low-resource tolerance. We further hypothesized that differences in functional traits underlying the tradeoff would be positively related to differences in long-term population dynamics that are responsible for recovery from low density and hence coexistence. 
Results/Conclusions

Across species, we found a tradeoff between growth capacity and low-resource tolerance. Species with high relative growth rates allocated a large fraction of biomass to photosynthetic surfaces and rapidly deployed large leaf area displays following infrequent, large rainfall events. Conversely, species with low relative growth rates but high integrated water-use efficiency invested a large fraction of leaf nitrogen in the photosynthetic processes that become limiting at low temperatures, which are characteristic of a short time period during and after rainfall events; this optimizes carbon assimilation following small but relatively frequent rain events. The magnitude of difference between species in these key functional traits was related to the magnitude of species difference in demographic response to environmental variation across years.  Variation in growing season precipitation appeared to be the critical environmental variable underlying the demographic decoupling of species.  These results demonstrate how physiological differences in resource uptake and allocation between species lead to the decoupled population dynamics that promote local biodiversity.