Both competition for water and phenological variation are important determinants of plant community structure, but ecologists lack a synthetic theory for how they affect coexistence outcomes. We developed an analytically tractable model of water competition for Mediterranean annual communities to answer the following questions: 1) Can tradeoffs associated with water competition explain the high diversity observed in phenologically structured plant communities in nature? 2) Under what conditions do these mechanisms foster coexistence vs. competitive exclusion? We modeled a system where all water arrives early in the season and species vary in their ability to grow under drying conditions. As a consequence, species differ in growing season length, and compete by shortening the growing season of their competitors. This phenological variation and its association with competitive dynamics have been investigated in several past empirical studies, providing key real world tests for the models we develop here.
We found that our model replicates and offers mechanistic explanations for patterns observed in empirical studies of how phenology influences coexistence among Mediterranean annuals. In the model, phenological differentiation arises naturally through interspecific variation in the critical soil water threshold and a transpiration-driven, monotonic decrease in water content through the growing season. When accompanied by a decreasing, concave-up tradeoff between growth rate and access to water, the resulting phenological nesting of competitors can maintain unlimited species diversity under simple but realistic assumptions. High diversity is possible because: 1) later plants escape competition after their earlier-season competitors have gone to seed and 2) early-season species are more than compensated for their shortened growing season by a growth-rate advantage. Together, these mechanisms provide an explanation for how phenologically variable annual plant species might coexist when competing only for water.