In California annual grasslands, water is a key limiting resource, with a significant amount of work showing strong correlations between grassland composition and productivity with variability in precipitation. How plant community responses to variation in water availability are mediated by interactions among species, however, remains to be understood. In particular, what is the relative importance of direct climate effects such as drought versus indirect effects of climate through plant competition? Importantly, the annual forbs that make up most of this community exhibit a range of strategies to cope with precipitation variability and competition with high-density exotic annual grasses. Annual forbs with more stress tolerant traits have lower relative growth rates to deal with stressors like drought, while less drought-tolerant species with higher relative growth rates generally rely on persistent seed banks to buffer their populations from declining during these stressful years. In this field experiment, we manipulated a factorial combination of rainfall (augmented, control, reduced) and exotic grass presence (weeded and unweeded) to measure how demographic rates of native annual forbs with contrasting life-history strategies respond to precipitation both directly and indirectly via competition.
After the first year of data collection, we found that the effects of water availability and competition on annual forbs depended strongly on the species’ functional traits. Stress tolerant species exhibited higher mortality and lower seed set in grass treatments, but were not significantly affected by drought treatments. Stress avoiders, in contrast, had highest mortality and lowest seed set in drought treatments. Indirect effects of treatments on mortality were not significant but grass competition had an indirect negative effect on seed set only in stress tolerant species. This work suggests that a forb population’s response to climate and competition will vary by life history strategy with the direct effects of a drier climate being more important for stress avoiding forbs and the indirect effects through competition with grasses playing a larger role in stress tolerant forbs. We expect therefore that increasing climatic variability will shift both taxonomic and functional composition of these grassland plant communities.