Effects of altered precipitation on decomposition rates in coastal sage scrub

Background/Question/Methods

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. Altered precipitation can directly impact rates of microbial decomposition through changes in soil moisture, and indirectly though shifting litter quality and quantity. Here we asked how invasion alters decomposition rates under drought versus high rainfall scenarios in either native shrub or exotic grass dominated communities. 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 and decomposed litter from both a native shrub and exotic grass for 1 year. 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. 

Results/Conclusions

We found that decomposition was significantly affected by rainfall treatment, though the magnitude depended on the litter substrate type and dominant community composition. Litter from an exotic annual grass decomposed faster under treatments receiving greater rainfall (p=0.0286), but decomposition rates were not affected by dominant community composition. Decomposition rates of litter from the dominant native shrub showed a significant interaction between dominant community composition and rainfall treatment (p=0.003). Shrub litter decomposed much faster at “home” under a shrub community rather than in an exotic grass dominated community, though this home field advantage was stronger in drought rather than high rainfall scenarios. In the “home” scenario, rainfall treatment did not affect decomposition rates. This suggests that shrub-dominated communities may have associated microbial communities with greater fungal:bacterial ratios, and therefore be less sensitive to soil moisture and have greater capacity to digest recalcitrant litter fractions. In combination, these results show that decomposition of exotic species litter and microbial communities associated with above-ground exotic dominated communities are more sensitive to rainfall than are rates associated with native shrub communities and litter, likely a result of shifts in microbial community composition.