Seedling drought response to temperature highlights the role of germination niche in community dynamics of winter annual plants

Background/Question/Methods

Germination timing is a crucial life-history trait that determines an individual plant’s environment, especially in variable environments with unpredictable rainfall amount and timing, as is the case for winter annual plants in the Sonoran Desert.  It has been shown that species differences in germination niche are a primary driver of community dynamics in this system.  Additionally, there is a known link between germination niche and a physiological tradeoff between relative growth rate (RGR) and water use efficiency (WUE), which promotes coexistence by partitioning limited resources across time.  In short, WUE species are cold-adapted and generally more resistant to drought, while RGR species do best when conditions are relatively warm and wet.  In this experiment, we sought to elucidate the link between germination niche and seedling survival in relation to functional strategy.  We performed a growth chamber experiment in which seedlings of five species of winter annuals were subjected to drought in three temperature treatments, which represent the spectrum of average temperatures that seedlings may experience throughout a winter growing season.  Growth and mortality rates and functional traits of individuals were measured.  

Results/Conclusions

Results show that drought resistance was greatest in the cold treatment and least in the hot treatment.  This was primarily due to a reduced soil moisture evaporation rate in colder temperatures.  Mortality patterns within treatments are predicted by each species’ germination niche, with the overall pattern being that species which are known to germinate more readily in cold, medium, or hot temperatures survived longest in that treatment.  Also, growth and mortality patterns among species within a treatment are reflective of their position along the WUE-RGR tradeoff.  Overall, high WUE species survived relatively longer than the high RGR species in the cold treatment and the opposite result was found in the hot treatment.  These findings suggest a close adaptive correspondence of germination niche with seedling and adult performance adaptations that are responsible for community dynamics in this system.