Predawn And Mid-Day Water Potentials To Measure Response To Altered Precipitation Regimes In South Coastal Chaparral Species

The chaparral of Southern California is predicted to encounter a climate shift toward increased drought, in addition to higher interannual variation in rainfall. We sought to determine how native chaparral species will respond to these climate shifts by evaluating their ecophysiological responses to experimentally altered precipitation regimes. To do this we applied 0, 50, 100, 150, or 200% of ambient precipitation in a randomized block design over 30 experimental plots (3.4m x 3.7m). We hypothesized that we would find the lowest soil water availability in the drought plots, but that adult shrubs with deeper roots would be less influenced the experimental treatments. To test these hypotheses we measured pre-dawn and mid-day water potentials for each species. Using a pressure chamber, we evaluated the water status of plants when at equilibrium with the soil (pre-dawn) as well as their tolerance to water stress while photosynthetically active (mid-day). Surprisingly we found that both adult and juvenile shrubs encountered lower soil moisture availability under lower precipitation treatments, evidenced by more negative predawn water potentials (p=0.006)_. We found that juvenile shrubs exhibited higher mid-day water-stress than adults, but while mid-day water stress was greatest for adults of all species under the lowest precipitation treatment (most negative mid-day water potentials) adult mid-day water stress was not significantly different under the other conditions. These results confirm that chaparral species are extremely drought tolerant, but that juvenile stages are more sensitive, such that multiple low rainfall years could prevent establishment and initiate long-term changes in species composition.