Global change predictions indicate future conditions in Mediterranean areas will be drier, with long periods of drought punctuated by extreme rainfall, and with ongoing increases in nitrogen deposition from fossil fuel sources. Carbon sequestration is commonly cited as a way to mitigate the impacts of global change, but there is limited research on how ecosystem level carbon dynamics, including belowground processes, will respond to environmental changes. We used a long-term, altered precipitation and nitrogen input experiment to quantify how Mediterranean coastal sage scrub (CSS) and grassland (GL) carbon stocks respond to global change at the Loma Ridge Global Change Experiment (LRGCE). LRGCE crosses three water levels (ambient, +35%, -48%) with two nitrogen levels (ambient, +6 gN m-2 yr-1) in eight GL replicates and eight CSS replicates, and has been operating since 2006. We measured the following carbon stocks in both CSS and GL, and across the 5 treatments, and 1 ambient control: aboveground phytomass and detritus, belowground phytomass and detritus, and soil carbon stocks to 2m depth. Our research aimed to answer the following questions:
- How do above and belowground C stocks differ between coastal sage scrub and grassland ecosystems?
- How do above and belowground C stocks respond to global change?
- How do global change mediated vegetation type changes (CSS to GL) impact C stocks?
Results/Conclusions
Preliminary results indicate that drought, and combined drought/added N treatments have significant reductions in above and belowground phytomass stocks, as well as reductions in soil C stocks in the CSS treatments. These reductions in C storage were related to decreases in shrub size and cover, and increases in shrub mortality and invasion by annual grasses. The grassland C stocks showed little response to the treatments, but overall stored less carbon in both above and belowground stocks than CSS. These results indicate that global change can significantly reduce CSS carbon storage through both reductions in growth and cover, and through vegetation type changes from CSS to GL, and that CSS carbon storage, including soil C sequestration, is particularly vulnerable to global change.