# Vegetation Growth Response and Trends after Water Deficit Exposure in the Loess Plateau, China
## 1 Introduction  
As global warming intensifies, extreme weather and climate events are becoming more frequent [1]. These events include cumulative water deficit events, which seriously threaten terrestrial ecosystems and human societies, making water deficit an increasingly prominent global issue [2]. Extreme droughts, in particular, have the potential to alter the carbon balance of terrestrial ecosystems and may cause them to transition from carbon sinks to carbon sources [3]. This carbon-climate feedback effect can lead to increased droughts and the irreversible degradation of ecosystems [4; 5; 6]. However, different terrestrial ecosystems respond to climate change in different ways [7], and the response of vegetation growth to drought exhibits significant spatiotemporal heterogeneity [8]. Under drought conditions, vegetation experiences water stress, which causes stomata to close and reduces the rates of photosynthesis, leading to a decline in primary productivity [9]. Consequently, drought can result in a growth slowdown, degradation, or even death of the vegetation [10; 11]. Therefore, studying the vegetation response to drought is crucial to understanding theunderlying mechanisms and characteristics of vegetation response to climate variability in the context of global climate change [3]. The relationship between drought and vegetation has been extensively explored [8; 12; 13]. Drought is essentially characterized by a water deficit event, and the vegetation's response to such events is not always straightforward, as plant species exhibit different levels of resistance and resilience to water deficit due to their unique physiological responses [14; 15]. The physiological characteristics of vegetation are mainly influenced by the climatic conditions of the different vegetation types in various climatic zones [16]. Moreover, the response of vegetation to drought is affected by drought characteristics and climatic environments [17], making it a complex process influenced by many eco-physiological and climatic factors [18; 19]. To precisely monitor the overall climate condition and depict the temporal and spatial distribution of drought, a drought index that synthesizes the effects of temperature, precipitation, and radiation on vegetation is imperative [20]. These indices, such as the Moisture Index (MI) [21], the Palmer Drought Severity Index (PDSI) [22], the Standardized Precipitation Index (SPI) [23], and the Standardized Precipitation Evapotranspiration Index (SPEI) [19], are widely used and have been found to be well-correlated with vegetation indicators such as the NDVI, EVI, the Leaf Area Index (LAI), and Gross Primary Productivity (GPP) [24]. Notably, the SPEI holds an advantage over other drought indicators as it combines potential evapotranspiration and precipitation data to gauge the severity of drought [19]. Furthermore, it can indicate drought conditions at different timescales [25]. The timescale of drought, which refers to the duration from the onset of vegetation water deficit (VWD) to the determination of vegetation change, has been widely used in aridification studies [5; 8; 26]. Vegetation growth responses to climate change have been widely studied at global and regional levels. However, the effects of water limitation on vegetation growth in arid and semi-arid areas and how they will respond to climate change remain to be thoroughly explored. The Loess Plateau is particularly susceptible to frequent droughts caused by climate warming, which has had an increasing impact on vegetation [27]. The moisture required for vegetation growth in this region mainly comes from soil moisture, which is unevenly distributed due to the deep soil layer and buried groundwater [28].