## 4 Discussion
### Impact of Flash Drought on NDVI  
In order to manage the effects of flash droughs, it is critical to identify plant responses to these events since this can serve as the scientific foundation for the development of an early warning system [24]. In this study, we used the NDVI to detect if there is any influence of flash drought on the forest of northeast China. We observed the negative response of the forest NDVI toward flash drought through the overall increasing trend in the NDVI during the decreasing trends in the proportion of flash drought from 2000 to 2020, but there was no exact variation in the response of the NDVI to flash drought in individual years. Likewise, through a comparison of years with more and a smaller proportion of flash drought events, we found differences in the NDVI values. The NDVI, which is sensitive to vegetation water stress, can decline rapidly at the time of sudden onset of flash droughs when plant suffers soil moisture deficits. However, the magnitude of this decline depends on the intensity and duration of the flash drought; longer and more severe events may result in larger negative NDVI anomalies [60; 61]. Even though flash droughs only last a short while, they have a significant influence on vegetation, according to previous research and practical cases [62]. The main cause of plant stress during flash droughs is the rapid depletion of root-zone soil moisture, which causes the plants to close their stomata to conserve water, reducing photosynthesis and productivity [57; 63]. Observations of vegetation during the early stages of flash droughs, when there is temporarily enough soil water available, have shown positive responses (such as enhanced evapotranspiration), which most likely lead to the subsequent loss of soil moisture. However, as the soil moisture levels continue to drop, flash droughs have detrimental impacts on plants, including reductions in photosynthetic activity and canopy dieback [22]. Through an analysis of the FLUXNET observations, the evaluation of vegetation's response to flash drought revealed that changes in the stomatal conductance and water usage efficiency lead to decreased gross primary production (GPP) during the flash drought period [26]. In addition, another study found a significant decline in the normalized difference vegetation index (NDVI), gross primary productivity (GPP), and sun-induced fluorescence throughout the dry period while investigating a typical flash drought in the United States [64]. Likewise, while studying the effects of flash drought on vegetation in the Loess Plateau, China, they observed the negative correlation of flash drought and the NDVI in the northwest part of Loess Plateau. Similarly, research during the flash drought period also showed a continuous decline in the NDVI anomaly and plant canopy surface water, indicating a significant effect on vegetation [65]. Researchers used a variety of observational data, such as the gross primary production (GPP) and leaf area index (LAI), to examine how ecosystems responded to flash droughs in Europe between 2001 and 2019. They discovered that, in arid regions, a sudden shift to water-stressed conditions occurred within a few weeks, which had a negative impact on the vegetation status and decreased carbon uptake, particularly in the early stages of drought [22]. Using solar-induced chlorophyll fluorescence to monitor how plant ecosystems responded to the flash droughs that occurred in the Hai River Basin, China, between 2001 and 2019, it was shown that the flash drought occurrence had a major impact on vegetation production, as evidenced by 40% of the basin experiencing losses in gross primary productivity (GPP) [23]. Due to the complexity of the interactions, vegetation ecosystems might respond differently to flash droughs depending on the particular characteristics of the place and the type of vegetation there [66].