Change of Vegetation Health inside Fire Perimeters in Southern California: An Analysis of Drought and Fire Interactions since 2000

Fires and droughts are two major disturbances altering vegetation conditions in Southern California's chaparral ecosystem. Extended fire seasons, increasing number of mega-fires, and prolonged droughts in California during the last two decades have impacted the growing cycle of plants, especially in the post-fire recovery in areas burned by wildfires. In this study, we investigate the trend of vegetation conditions with and without disturbances from droughts or fires to understand interactions among these factors, which have shaped the chaparral growing cycle. We extracted the preceding and following periods without another fire occurrence inside the burned areas for the fire-active years in Southern California since 2000, using fire history data. Then we compared the Mann-Kendall trend test coefficients of pheno-metrics derived from MODIS reflectance products between the two fire-free periods. Temporal trends (Figure 1) showed that the recent prolonged drought (2012-2015) had a strong impact on the post-fire regenerating process of areas burned in 2006, 2007, and 2009. Although an upward trend was brought by the post-fire regeneration for 5-6 years, the prolonged drought significantly weakened the inter-annually post-fire vegetation growing trend. Compared with the historical fires without a following prolonged drought in the regeneration period, the inter-annual change was also drastic. Spatially, the weakening was more prominent in the outskirts of fire perimeter with a relatively low burn severity, but a great spatial heterogeneity existed across different fires. This study suggests that the overlapping fire disturbances and droughts are major impact factors on the vegetation health, especially during the post-fire regeneration period. However, interactions between droughts and fires led to a complex spatial heterogeneity in the post-fire vegetation growth trend in a more extensive wildland-urban interface due to urbanization.