Phenological Differences Promote Coexistence in Sonoran Desert Winter Annuals

Identifying trait differences is an important step toward understanding differential demographic responses to the same environmental fluctuations. In the Sonoran Desert, winter annual plants exhibit high demographic variability due to variation in precipitation, and patterns of demographic variability are related to species position along a tradeoff axis between relative growth rate (RGR) and water-use efficiency (WUE). Prior investigation revealed that species with high RGR and low WUE have greater inter-annual variability than species with low RGR and high WUE. In this study, we use long-term census data, climate records, and plot data to investigate timing of germination, reproduction, and senescence of several winter annual species in multiple years to test whether phenology relates to demographic variability and position along the tradeoff axis. We also use climate records to describe germination niches of the species and make predictions regarding future community composition. We hypothesized that seasonal phenology would differ such that demographically 'buffered' species (low RGR-high WUE) would germinate, flower, and senesce earlier in the season due to an ability to utilize small amounts of rain and photosynthesize at low temperatures. In contrast, we hypothesized that the demographically 'variable' species (high RGR-low WUE) would germinate later in the season, only after enough rain had fallen to break seed dormancy. Consistent with our hypothesis, buffered species did germinate and reproduce earlier in the season than variable species. Contrary to our hypothesis, buffered species also survived later into the season. Variable species germinated later, reproduced quickly, and senesced earlier in the season. These results show that phenology promotes coexistence by partitioning resource use. Germination niches and climate data suggest that buffered species may increase in abundance through time.