Recent droughts have precipitated large-scale woody plant die-offs. These woody plants dominate the systems in which they inhabit. Many studies have sought to distinguish the traits of species that resist drought while fewer studies have focused on aspects of drought that could affect drought resistance of species.
Drought is commonly defined by annual metrics such as precipitation totals or Palmer Drought Severity Index (PDSI). While these are useful metrics, they may have limitations because aspects of droughts that occur on shorter-than-annual time scales may affect how species respond to drought and because droughts can interact with several other variables.
In the context of climate change, we examined if the climate in Southern California’s chaparral region has become extreme. We used a multiple climate extremes index (MCEI) and PDSI to predict the response of chaparral vegetation to drought to assess if the MCEI was predictive of vegetation health.
Climate extreme indices were calculated from PRISM climate data. Using principal components analysis, a subset of climate extreme indices for each site were used to develop site-specific MCEIs. Yearly MCEIs and PDSI were correlated to yearly-averaged Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices to understand the relationship between climate and vegetation.
Results/Conclusions
Over the past 30 years, wet and dry rainfall years have become more extreme for the coast of Southern California while they have not become more extreme for the desert region of Southern California. For most sites, the relationship between vegetation indices and the PDSI was significantly positive, which means that when drought severity is high, vegetation is less green, has lower absorbance of photosynthetically active radiation, and less leaf area. For most sites, MCEI was not significantly associated with rainfall-year, yearly-averaged vegetation indices. These results suggest that intra-annual extreme weather events may not have as large of an impact on vegetation as do periods of either intense drought or intense wet.
Recent studies of multi-year droughts have identified drought duration as being a key factor triggering widespread woody plant die-offs. While this may be true in some cases, for chaparral, extreme low values for traditional drought metrics explain most of the observed mortality responses.
In the context of climate change, as droughts become more common and precipitation more erratic, an integrated understanding of how extreme climate events and traditional drought characteristics influence ecosystems will increase the predictive power of vegetation models and aid natural resource management.