Variation in resource availability is known to affect plant communities and climate change is predicted to affect the frequency and intensity of precipitation events, affecting the availability of water to plants. Annual plants may be especially sensitive to intra-annual variation in precipitation as they germinate and reproduce in a given year. Altered precipitation regimes may affect annual communities by influencing germination and growth. I conducted a series of experiments using annual agricultural weed species to determine whether species differed in their response to precipitation variability in terms of their germination, growth and abundance. I also used a long-term data set of species abundances in an annually-tilled community from the Long Term Ecological Research project at the Kellogg Biological Station to determine how the biomass, richness, and abundance of the most common species in this community varied with variation in initial precipitation and temperature conditions. I also experimentally manipulated variability in the field throughout the growing season for two common species and developed a simple mathematical model to explain differences in their responses. Results from the two greenhouse studies revealed that the germination and initial growth of annual weed species is sensitive to variation in the distribution of precipitation. Species responses were not identical suggesting that variability in precipitation could affect the relative abundance of species. Results from the long-term data analysis revealed that community productivity, species richness, and species abundances varied in response to precipitation (and to a lesser extent) temperature conditions in the week following tillage. Interestingly, community biomass decreased but species richness increased in response to increased early season precipitation. These results were confirmed in a field experiment where precipitation and temperature conditions were varied for just the week following tillage. The field experiment also showed that species responses to precipitation varied and competitive interactions early in the year had persistent effects on the community, including the abundance of the dominant species. In a second field experiment I found that responses of two dominant species in this community to precipitation variability throughout the growing season depended on the community context (monoculture vs. mixtures). My results indicate that variation in precipitation, especially early in the growing season can have significant effects on community production and species composition in annual weed communities. In addition, the responses of species to this environmental variability differed and appeared to be mediated by the precipitation driven changes in the composition of the community.. These results highlight the importance of understanding how current and predicted variation in precipitation affects individual species both directly and indirectly to predict how communities will respond to climate change.
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