2015 Conference

Poster Presentation

Southern California is predicted to face decreasing precipitation with increased interannual variability in the coming century. Native shrublands in this area are increasingly invaded by exotic annual grasses, though invasion dynamics can vary by rainfall scenario, with wet years generally associated with high invasion pressure. Interplay between rainfall and invasion scenarios can influence carbon stocks and community composition. Here we asked how invasion alters ecosystem responses under drought versus high rainfall scenarios in either native shrub or exotic grass dominated communities, as quantified by biomass production and decomposition rates. To do this, we performed a rainfall manipulation experiment with paired plots dominated either by native shrubs or exotic herbaceous species, subjected to treatments of 50%, 100%, or 150% of ambient rainfall. The study site was located in a coastal sage scrub ecosystem, with patches dominated by native shrubs and exotic grasses located in San Diego County.

During two growing seasons, we found that native, herbaceous biomass production was significantly affected by rainfall treatment (p<0.05 for both years), though was not affected by dominant community composition. Exotic biomass production showed a significant interaction between dominant community composition and rainfall treatment, and both individual effects (p<0.001 for all). Decomposition rates of the exotic grass were much faster overall than rates observed for native shrub decomposition (p<0.001), though shrub litter decomposition was less affected by rainfall treatment than was exotic grass decomposition. Furthermore, native shrub litter shows significantly faster decomposition with shrub-associated microbial communities, suggesting that recalcitrant litter decomposition is highly affected by microbial communities. In combination, these results show that production and decomposition of exotic species is more sensitive to rainfall than are rates associated with native shrubs. With decreased overall precipitation, greatest carbon storage may occur in shrub-dominated communities, but with increased precipitation, this trend is reversed.