# Stronger Cumulative than Lagged Effects of Drought on Vegetation in Central Asia
## 1 Introduction  
Vegetation is an integral element of land ecosystems and has a major role in global carbon cycling, energy exchange, geochemistry, and the restoration of surface ecosystems [1]. It can also provide a record of hydrological climate change [2]. Drought is one of the major limiting factors for terrestrial ecosystems, with significant ecological and human amenity impacts [3]. An important feature of drought is low soil water use efficiency, which is usually caused by reduced rainfall and increased evapotranspiration [4]. The future strength and frequency of global drought and heat events are expected to rise [5, 6, 7],and within the context of heating, a plant's survival depends on its ability to tolerate drought [8]. Drought intensification can pose a serious threat to vegetation productivity [9], causing systematic and abrupt changes in ecosystem structure and function [10], especially in moisture-scarce arid regions [11; 12]. Therefore, understanding the process of vegetation responses to drought events can help illuminate the mechanisms by which terrestrial ecosystems respond to future climate variability and suggest effective measures for ecosystem conservation [13; 14; 15]. Many studies have analyzed the mechanism by which drought impacts vegetation, but the majority of these have concentrated on the current month to determine the time at which drought events occurred, neglecting the fact that prior drought also has an impact on current vegetation development [16; 17; 18]. Previous research has found that drought has a lagged effect on vegetation development [19; 20; 21; 22], which refers to early drought events having a certain impact on the present vegetation growth; for example, vegetation growth is more sensitive to drought at specific earlier time points than at current time points [23; 24; 25]. However, the drought effect on vegetation lag has reached inconsistent conclusions in different regional studies, and relatively few research in arid areas where drought events are frequent have been conducted. Thus, studying the lagged effect of drought can offer a scientific foundation for local eco-environmental protection in arid areas. In assessing the responses of vegetation to drought, drought duration is an essential aspect of the response of vegetated ecosystems; therefore, the cumulative drought effect cannot be ignored [26; 27; 28]. Cumulative drought effects consider drought conditions in the previous consecutive time period based on a lagged effect, which means that the currently developing vegetation has been affected by the cumulative effect of drought in the past few months [29; 30; 31]; for example, the accumulation of drought can have a doubling effect on vegetation density and leaf abundance [32]. The cumulative effect of drought can be used to more fully comprehend the responses of vegetation to drought [33; 34]. The majority of previous studies only considered a single cumulative drought effect and drought-lagged effect, and they have rarely synthesized lagged effects in comparison to cumulative effects [35; 36; 32]. Therefore, this study compared lagged and cumulative effects of drought on vegetation and determined which drought effect had a greater impact on vegetation cover. Gross primary productivity (GPP) of terrestrial ecosystems is the total amount of carbon fixed by terrestrial vegetation ecosystems through vegetation photosynthesis and is a fundamental component of the global carbon cycle [37; 38]. Recently, an increasing number of studies have used GPP to assess vegetation productivity [39; 40; 41]. The results of these evaluations vary for each product. For example, the MODIS GPP shows a high correlation with flux tower GPP in terms of vegetation types, but it underestimates GPP for eight vegetation types, whereas the solar-induced chlorophyll fluorescence (SIF) dataset's GOSIF data show higher accuracy in estimating single-tower GPP and the seasonal cycle of GOSIF is consistent with flux tower GPP.