UC Irvine

The extent to which patterns of germination are influenced by the environment is mediated by germination functional traits related to both temperature and water potential. The co-variance of these traits informs our understanding of how these organisms cope with periods of environmental stress, such as drought. Here we quantify the effect of variation in temperature and water potential on germination for nine coastal sage scrub plant species representing multiple life forms, using a growth-chamber based hydrothermal approach, and compare the results to traits known to relate to growth and water use. We also assess the potential of germination, growth, and water-use traits to inform our understanding of how coastal sage scrub vegetation responds to water limitation in Southern California. We investigated how both regenerative and vegetative traits may influence the evolution of plant life history strategies in this complex and dynamic system. We identified relationships between germination fraction, base water potential required for germination, and water-use efficiency. Specifically, we find that species with low base water potential (ψb(50)) have high water use efficiency (WUE), low thermal time to germination (θT) , and germinate high fractions, while those with high ψb(50) have low WUE, high θT and germination low fractions. Species differences in traits related to water sensing and water-use served as effective predictors of how percent-cover of vegetation changed during a three-year drought period. We found that native plant species which increased cover during drought germinated high seed fractions, had low base water potentials, and were more water-use efficient compared to those that avoided drought through sets of functional traits that related to the life history tactic of escape. Overall, our study helps to further our understanding of plant strategies associated with environmental variation by highlighting key functional traits that may confer tolerance to environmental stress.