Identifying trait differences is an important step toward understanding differential demographic responses to the same environmental fluctuations. In the Sonoran Desert, the 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 ask whether phenological differences may interact with the RGR/WUE tradeoff to allow for differences in resource use within a growing season. We use long-term census data, climate records, and sequential harvests across the growing season 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 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. 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.
Results/Conclusions
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. The variable species germinated later, reproduced more quickly, and senesced earlier in the season. The early germination and reproductive phenology of buffered species enables them to have greater demographic success in dry years, while the delayed germination and reproduction of variable species enables them to have greater success in wetter years. These results show that phenology interacts with functional traits to create different demographic responses to environmental variation, a critical ingredient for species coexistence in variable environments.