California recently experienced one of the most severe droughts in its history, resulting in extensive dieback of chaparral vegetation in the State’s Mediterranean regions. Though chaparral species are adapted to annual summer drought, the duration or intensity of a drought may exceed the adaptive capabilities of even these plants. The impacts of the recent severe drought were further exacerbated in many chaparral ecosystems by the occurrence of wildfires. The most notable of these fires in southern California were the 2017 Thomas Fire (281,893 acres) and the 2018 Woolsey Fire (96,949 acres).
U.S. Geological Survey (USGS) researchers used remote sensing data to map vegetation dieback in southern California shrubland landscapes and determine the relationship between dieback and patterns of fire severity in the Woolsey and Thomas fires. Specific research goals of the project included 1) mapping the spatial extent of chaparral dieback within the perimeters of the fires prior to their respective burns by calculating the difference in annual NDVI (Normalized Difference Vegetation Index) between the year before the drought and 6 years after the drought, 2) assessing the effects of potential drivers of chaparral dieback, and 3) evaluating the effect of vegetation dieback on burn severity using differenced Normalized Burn Ratio (dNBR) data from the Monitoring Trends in Burn Severity project.
The study found that drought-induced dieback was extensive in chaparral areas that later burned in the Thomas and Woolsey fires and was likely a major contributor to the size of these fires. The highest levels of dieback were found at lower elevations, which is a particular concern because low-elevation chaparral is often adjacent to urban environments where ignition sources are common and infrastructure is most at risk. These fires serve as a test of the usefulness of this approach for evaluating relationships between severe drought, vegetation dieback, and subsequent fire severity, and for determining to what extent that information could be used to inform land and fire management activities in the region.
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Chaparral dieback within the 2017 Thomas Fire and 2018 Woolsey Fire perimeters, calculated as the difference in mean annual normalized difference vegetation index (NDVI) between 2010 and 2016 (difference in NDVI [dNDVI]) in California. Positive values indicate a decrease in live vegetation plus other human disturbances that reduced the amount of live biomass. The associated manuscript is available here.
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