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Ecological effects of drought with a focus on its abiotic drivers: A case study examining Southern California chaparral shrublands
Global-change-type drought has resulted in large numbers of woody plants suffering mortality in California during the most recent droughts from 2012-2018. These droughts are ecological droughts that impact ecosystems and trigger responses. The factors that trigger ecological droughts are not well established. One limitation of studies considering drought is the lack of a clear set of criteria used to define aspects of drought that should be examined when assessing the effect of drought on an ecosystem. The most common way to evaluate drought is to use indices that are calculated from annual metrics such as annual precipitation. Other indices are calculated on an ongoing basis and integrate both temperature and precipitation such as the Palmer Drought Severity Index (PDSI). In this work, I examined other climate extreme indices that can be used to understand woody plant dieoffs. For example, the number of days without precipitation and the intensity of storms, relative to long-term averages, are some factors that may be important in evaluating drought effects on woody plants. I applied these criteria to understand which factors best correspond to recent woody shrub die-offs in southern California chaparral shrublands. I hypothesized that there are aspects of droughts that: 1) drive the response of vegetation and 2) create tipping points that trigger ecological droughts. To test this, I coupled existing remotely-sensed vegetation data from satellites with weather-derived climate indices over 17 rainfall years (2000-2016) from field sites located in the Santa Monica, Santa Rosa, and Tehachapi mountains of southern California. Vegetation indices (NDVI, EVI, LAI, and fPAR) were calculated using Moderate Resolution Imaging Spectroradiometer (MODIS) data, and climate indices, in addition to PDSI, were calculated using daily summaries from PRSIM (Parameter-elevation Relationships on Independent Slopes Model) weather data. I found that the climate in our coastal study region has become more drought prone in the 30-year data set, whereas the desert region has become neither drier nor wetter. PDSI was more predictive of vegetation health than a combined, multiple climate extremes index developed in this study. Vegetation indices were lowest during the most intense drought years, but during longer-term droughts of lower intensity did not show continued decline. Coastal chaparral sites contain high biodiversity and many localized endemic species, which may be threatened by a warming and drying climate. Monitoring these regions using remotely sensed data may help to identify affected areas to inform land management agencies and conservation priorities.
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