Abstract
A combination of drought and high temperatures (“global-change-type drought”) is projected to become increasingly common in Mediterranean climate regions. Recently, Southern California has experienced record-breaking high temperatures coupled with significant precipitation deficits, which provides opportunities to investigate the impacts of high temperatures on the drought sensitivity of Mediterranean climate vegetation. Responses of different vegetation types to drought are quantified using the Moderate Resolution Imaging Spectroradiometer (MODIS) data for the period 2000–2017. The contrasting responses of the vegetation types to drought are captured by the correlation and regression coefficients between Normalized Difference Vegetation Index (NDVI) anomalies and the Palmer Drought Severity Index (PDSI). A novel bootstrapping regression approach is used to decompose the relationships between the vegetation sensitivity (NDVI–PDSI regression slopes) and the principle climate factors (temperature and precipitation) associated with the drought. Significantly increased sensitivity to drought in warmer locations indicates the important role of temperature in exacerbating vulnerability; however, spatial precipitation variations do not demonstrate significant effects in modulating drought sensitivity. Based on annual NDVI response, chaparral is the most vulnerable community to warming, which will probably be severely affected by hotter droughts in the future. Drought sensitivity of coastal sage scrub (CSS) is also shown to be very responsive to warming in fall and winter. Grassland and developed land will likely be less affected by this warming. The sensitivity of the overall vegetation to temperature increases is particularly concerning, as it is the variable that has had the strongest secular trend in recent decades, which is expected to continue or strengthen in the future. Increased temperatures will probably alter vegetation distribution, as well as possibly increase annual grassland cover, and decrease the extent and ecological services provided by perennial woody Mediterranean climate ecosystems as well.
Highlights
CMIP5 climate models consistently project future warming and drying in most Mediterranean climate regions through the poleward-expanding subtropical subsidence [1,2,3]
Climatic water deficit (CWD), which combines the influences of temperature and precipitation, has been shown to be an important driver of woody plant decline and mortality in drought [94], which is consistent with the results presented here
We utilized a bootstrapping regression to investigate the effects of changing temperature and precipitation in modulating the drought sensitivity of Mediterranean ecosystems in Southern California
Summary
CMIP5 climate models consistently project future warming and drying in most Mediterranean climate regions through the poleward-expanding subtropical subsidence [1,2,3]. Previous studies have suggested that the impacts of climate fluctuations on ecosystem processes become less predictable as climate extremes increase in frequency [4,5]. Will habitat characteristics, potentially to the detriment of some species. Mediterranean climate regions are diversity hotspots, with large numbers of endemic and threatened species dependent upon specific habitat types [6]. There has been increasing concern about a shift in Mediterranean climate ecosystems from “near-neutral” to a long-term carbon source, due to climate extremes or other disturbances in the near future [9,10,11]
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