Multi-time Scale Features of Fire Weather in Two Major Forests in China during 1961–2020

Globally, there have been frequent occurrences of extreme forest fire incidents in recent years. As one kind of the compound extreme events, the occurrence and spread of forest fires are closely associated with meteorological conditions. Under global warming, investigating the changing characteristics of forest fire risk can provide valuable scientific information for forest fire prevention activities in the context of carbon neutrality. In this study, the daily forest fire danger index (FFDI) was used to measure fire weather, and the applicability and spatial distribution of this index were analyzed. In addition, the linear trend in FFDI and related meteorological factors in two major forest areas during 1961–2020 were analyzed. Finally, the ensemble empirical mode decomposition method was used to reveal the multi-time scale characteristics of FFDI in the two forest areas. The results show that the spatial distribution of FFDI has obvious regional characteristics on seasonal and annual time scales. Northeast China has a high FFDI during spring and autumn, whereas southwest China has a high FFDI during spring and winter. These seasonal variations show a good corresponding relationship with the forest fire prevention period in the two forest regions. The number of stations showing a significant increasing trend in FFDI in each season is around 10%–20%, with the highest in spring (21%). The linear trends in FFDI in the northeastern forest area are not significant in all seasons; however, among relevant meteorological factors, the daily maximum temperature and average wind speed respectively show a significant warming trend and a significant weakening trend in all seasons. The FFDI in all seasons of the southwestern forest area showed a significant increasing trend at a level of 0.1 at least, among which the trends during the spring and winter fire prevention periods were 0.09/10 a (P<0.1) and 0.05/10 a (P<0.1), respectively. The summer, autumn, and winter periods showed a significant warming and drying trend (P<0.05). The interannual variability contributes more than 70% to the evolution of FFDI in the two forest areas. The nonlinear trend of FFDI in the spring and autumn fire prevention periods in northeast China showed a rapid rise at first and then a decline. The nonlinear trend of FFDI in the spring fire season in southwest China changed from a stable stage in the last century to a rapidly increasing trend in the 21st century, whereas the overall trend of FFDI during the winter fire season increased steadily. Therefore, the situation of fire risk in forests of southwest China is increasingly becoming severe.