# Dual Roles of Water Availability in Forest Vigor:
## 1 Introduction  
Forest ecosystems cover more than 4.1 billion hectares of the Earth's surface [1] and offer products (e.g., food and timber) and services (e.g., sequestering atmospheric carbon dioxide and controlling water erosion) [2, 3], making them crucial to sustainable development. Forest ecosystems are strongly impacted by drought, which is likely to intensify in the future [4, 5]. Despite the importance of water to vegetation, researchers havefound that water supply (e.g., precipitation) exerts two quite different impacts on forest vigor (e.g., Normalized Difference Vegetation Index (_NDVI_) and Gross Primary Production (_GPP_)), i.e., both positive and negative roles (i.e., positive and negative correlations) coexist and their roles depend on the spatial location and timing of precipitation [6; 7]. To date, most studies have concentrated on the positive role of the water supply in vegetation vigor, ignoring the negative role and its underlying mechanism [8; 9]. Revealing the dual roles of water availability in forest vigor is crucial, as it forms the basis for a broad comprehension of the mechanisms involved and will allow us to make better predictions regarding the impacts of climate change. Given that the impacts of water on forest vigor have time-lag [10] and cumulative effects [11; 12], the cumulative effect of water availability should be considered when assessing the forest vigor-water relationship. Previous studies have indicated that forest vigor is affected by water availability not only at the current time (month or season), but also in the preceding time, with the impacts of accumulative water in preceding months probably being greater than those of the current water conditions [13; 14]. The Standardized Precipitation Evapotranspiration Index (_SPEI_) is widely used to reflect the cumulative effect of water availability in terms of the magnitude, accumulative time, and spatial extent [14; 15]. Given that _SPEI_ is a typical meteorological index that contains little information about vegetation, it is commonly combined with a forest vigor index generated from optical remote sensing data, such as the _NDVI_[11; 16]. Green vegetation strongly absorbs radiation in the red wavelength region, but strongly reflects it in the near-infrared region. Additionally, satellite remote sensing data provide information for forest areas that are inaccessible, remote, or difficult to assess for a direct field study. Therefore, remote sensing may be used to monitor vegetation dynamics over large areas at various spatial and temporal scales. Considering this, the satellite-derived _NDVI_ calculated by the reflectivity of red and near-infrared wavelengths is considered a robust indicator for vegetation greenness and vigor at regional and continental scales [17]. The relationship between water availability and forest vigor is usually characterized using [PERSON]'s correlation coefficients [18; 19]. Despite increasing recognition of the negative role (i.e., negative correlations) of water availability in forest vigor [6; 20], the underlying mechanisms are unknown. A negative role means that forest vigor decreases with the increase of water availability, which is inconsistent with the ecology principle that water is a necessary resource for plants. Previous studies indicated that environmental climate, i.e., temperature (\(T\)) and water balance (\(WB\)) (i.e., precipitation minus potential evapotranspiration) and forest type, as well as seasonality, were related to water availability impacts on forest vigor. For example, the sensitivity of forests to water availability decreases with increases in environmental moisture [21], and varies according to the temperature zone and forest type [6; 20]. Some studies have found a negative role of water availability in forest vigor in cold and wet regions [20]. Additionally, the main limiting factors for forest vigor change with the different seasons [8; 9; 19]. Forest vigor during spring and summer is more sensitive to temperature and water conditions [19; 22].