DIFFERENCES IN THE TIMING OF GERMINATION AND REPRODUCTION RELATE TO GROWTH PHYSIOLOGY AND POPULATION DYNAMICS OF SONORAN DESERT WINTER ANNUALS

Premise of the Study: Trait differences can promote distinct survival and fecundity responses to environmental fluctuations. In a Sonoran Desert winter annual plant community, we have identified a tradeoff between relative growth rate (RGR) and water-use efficiency (WUE) that predicts interannual variation in reproductive success. Here we test the hypothesis that traits underlying RGR and WUE differences are linked to seasonal phenology. Methods: We use long-term demographic data and finer-scale, short-term data to investigate timing of germination, reproduction, and death of several winter annual species in multiple years in open and under-shrub habitats. We hypothesized that species with high WUE and less interannual demographic variability would have life cycle transitions early in the winter to spring growing season. This would be due to an ability to use small amounts of rain and photosynthesize at low temperatures. By contrast, we hypothesized that species with low WUE whose survival and reproductive rates vary greatly from year to year would have life cycle transitions later in the season. Key Results: In any given year, species with high WUE germinated and reproduced earlier in the season than species with low WUE, whereas low-WUE species germinated later and had shorter reproductive phases. Conclusions: Our results demonstrate a direct relationship between phenology and physiological trait differences. This link between phenology and physiology is of interest because it clarifies the mechanism by which trait differences determine species' relative abundances.