## 1 Introduction
Currently, there also remains an argument regarding the precise definition of a flash drought. Although flash droughts were divided into two types, heat wave flash drought and precipitation deficit flash drought [29], the composite drought occurrences outlined in this definition do not capture the swift onset of flash droughts, and certain brief periods of intense heat and drought are not ruled out. According to [30], flash drought was defined as an event where the percentile of the soil moisture averaged over 5 days drops from 40% to 20% in less than 20 days. Likewise, another study employed the Standardized Evaporative Stress Ratio (SERS) derived from the pentad averages [17], and they categorized events with change in mean SERS resulting in a reduction in the drought index of less than 25% of the climate state as flash droughts. While both concepts capture the swift onset of flash droughts, neither of them distinctly delineates the decline and end of these events, thus hindering further research on analyzing their duration and impacts. Drawing from prior studies, considering both the onset and recovery phases of flash droughts, a flash drought event was characterized as a rapid drop in the pentad average percentile of the soil moisture from above 40% to below 20%, with an average decline rate exceeding 5%, and the drought should last for a minimum of 15 days. The end of the flash drought is marked when the percentile of soil moisture returns to above 20% [30]. China is very vulnerable to the impacts of climate change, seeing considerable increases in temperature and decreased precipitation, so flash drought has been occurring more frequently in China. In China, the average flash drought's frequency increased by 109% between 1979 and 2010 [31]. From 2000 to 2016, the frequency of China's flash droughts varied from 6% to 20%, with the northern and southern regions of the country being more susceptible to these occurrences [32]. Warming and shifting wetting patterns have been shown to intensify flash drought in regions like the Loess Plateau and the central Qinghai-Tibet Plateau [33]. Flash droughts are primarily concentrated in the northwest, northeast, southwest, and middle-lower Yangtze River regions [34]. During the summer of 2013, a sudden and severe drought in southern China resulted in the destruction of about two million hectares of crops in the provinces of Hunan and Guizhou, affecting 13 provinces, and during mid-to-late 21 st century, the effects of flash droughts have become increasingly significant [30; 35]. There is a pressing need to deepen our understanding of flash drought. The scientific community needs to make advances in this field in order to develop a better understanding of flash drought events, which encompass a variety of impacts and pose challenges for sub-seasonal prediction [19]. Furthermore, most of the papers related to flash drought have focused on only plant-growing seasons. This paper explored flash droughts that occur throughout the year. In general, flash drought is more common in plant-growing seasons, but the study of year-round flash droughts aids in understanding the effects they have on plants and the natural environment [36]. Similarly, broader research on China's flash drought has clarified the reasons behind flash drought events as well as their trends across different regions of the country, but we did not find many papers that focused solely on flash drought in northeastern China, which not only faces a high risk of drought but also covers a vast forest area of the entire nation. After over a century of climate study, northeast China is one of the world's most significant warming zones and is quite vulnerable to global warming. In the context of forests in this region, where climate change is already altering the composition and health of the trees, it is imperative to look into whether flash droughts happen in this region and their associated impacts [37].