Drought-related wildfire accounts for one-third of the forest wildfires in subtropical China

The threat of wildfires to human safety and ecosystem services is widely known to increase due to climate change characterized by drought. In subtropical China, seasonal drought during the non-monsoon period contributes to increase of fire risk. However, both the relationship between drought and wildfire and the extent of drought effect are poorly understood. Based on satellite active fire detections and monthly drought metrics, we quantified the impact of drought on forest wildfire frequency in subtropical China during 2001-2020, using a Bayesian hierarchical modelling framework. We observed excess wildfire, defined here as above normal wildfire activity level, is significantly correlated to drought across subtropical China, and drought-wildfire relationship showed threshold behavior that the probability of excess wildfire increased significantly when the vapor pressure deficit (VPD) crossed 0.24 standard deviations of its long-term average. From 2001 to 2020, excess wildfire caused by drought accounted for approximately 31 % of the total number of forest fire points during the fire season (November to May). The drought-related excess wildfire was concentrated in 2007-2010 and 2014, and their spatial distribution showed a dipole between the western and eastern subtropical China in individual years. Moreover, we evaluated the spatiotemporal sensitivity of wildfire to drought by Bayesian model coefficients. There was higher sensitivity than average level of the entire region in 54 % of subtropical China, especially in northwestern Yunnan. Our results indicate that drought plays a dominant role on wildfire regime at the regional scale in subtropical China. As the drying trend intensifies in the future, wildfires will show different patterns due to the large differences in the sensitivity of wildfire to drought in subtropical China.