Climate change is projected to increase the frequency of both extreme wet and extreme dry events, especially in parts of the Northern Hemisphere. This is especially important in California annual grasslands where water is the primary limiting resource, with a significant amount of work showing strong correlations between grassland composition and productivity with variability in precipitation. Whether responses to precipitation are mediated by interactions with other species however remains to be understood. Some evidence supports a strong role for indirect effects of climate mediated by grass competition, but other evidence supports a greater role for the direct effects of climate. Importantly, annual forbs in this community exhibit a range of strategies to cope with precipitation variability and competition. Annuals with more stress tolerant traits have lower relative growth rates to deal with stressors like drought (“drought tolerators”), while species with higher relative growth rates rely heavily on persistent seed banks to buffer their populations from declining during these stressful years (“drought avoiders”). This study uses a manipulative field experiment that alters rainfall and exotic grass presence to understand how demographic rates of native annual forbs with contrasting drought strategies respond to changes in precipitation both directly and indirectly via competition.
Results/Conclusions
The first year of this experiment revealed that drought avoiding forbs varied more by watering treatment and were more negatively affected by grass competition across treatments than drought tolerant forbs. Grass competition both increased mortality and lowered seed set in these forbs. Additionally, the indirect effects of grass competition were more important for these forbs than for drought tolerant forbs. Drought avoiding forbs displayed higher mortality in the drought treatment when grass was present, but this negative interaction between watering treatment and grass presence disappeared in the watered plots. Conversely, demographic rates of drought tolerators did not vary much by watering treatment or grass treatment, nor did their response to grass competition vary by watering treatment. Grass competition is thus a more important factor for drought avoiding forbs than drought tolerating forbs both directly and indirectly. While watering does seem to alleviate the negative effect of grasses for these forbs, drought tolerators will likely be more resilient to future rainfall variability than drought avoiders given their ability to withstand both drought and grass competition.