# Diverse Roles of Previous Years' Water Conditions in Gross Primary Productivity in China
## 1 Introduction  
The frequency of extreme weather events such as drought, heatwaves and heavy precipitation gradually increases under global climate change [1, 2, 3, 4], which seriously affects the composition, structure, function and biodiversity of terrestrial ecosystems [5, 6, 7, 8]. Gross primary productivity (GPP), which is the total amount of organic matter produced per unit time and unit area by green plants through photosynthesis using water and carbon dioxide as raw materials [9], plays an important role in the carbon cycle of terrestrialecosystems [10]. Water is indispensable in this process and is also the main component of plant cells. Once water levels fall below a certain threshold, the resulting drought often causes food shortages in agricultural systems [11; 12] and affects the stock volume [13], leaf area index [14] and biomass of forest ecosystems. It is also possible that water stress leads to hydraulic failure and carbon hunger that reduces vegetative productivity and increases the risk of forest fires [15], thus changing the carbon budget of terrestrial ecosystems [4; 16]. Therefore, studying the response of GPP to water deficit is of great significance for understanding the carbon absorption process of terrestrial ecosystems and estimating ecosystem productivity with a land surface carbon cycle model. Previous studies have shown that abiotic factors (temperature, precipitation, etc.) and biological factors (forest age, tree height, stock volume, etc.) have significant effects on the ecosystem. Among these, abiotic factors are the determinants of ecosystem productivity on a regional scale [17; 18; 19; 20]. Vegetative growth (such as gross primary productivity) is mainly restricted by abiotic factors such as temperature, water and radiation [21; 22]. The restrictions of abiotic factors on vegetative productivity vary from region to region under the influence of climate and the geographical environment [22; 23; 24]. The influence of water on vegetative growth in different regions is controversial. In the Mediterranean region [21; 22], the Alps [25], the Southwestern United States [26; 27] and other regions, water is the most important climate factor limiting vegetative growth. The sensitivity of tree growth to water is greater than to temperature and other climate factors. Water deficits caused by drought can have serious negative impacts on crop yield, vegetative productivity and the formation of tree volume [23]. However, vegetative growth is mainly restricted by solar radiation in Western Europe, tropical rain forests and parts of Southern China [19; 27; 28; 29]. For example, [PERSON] et al. (2017) [19] found that vegetative productivity in Southern China was more sensitive to solar radiation than precipitation, and the decrease in forest primary productivity was mainly due to the reduction in solar radiation (82%) rather than the influence of drought (18%). There is little significance to the study of drought in regions where water is not limited, and it has been ignored in many studies. Therefore, it is necessary to consider the spatial scale of differences in vegetation sensitivity to water when studying the effect of drought. The influence of water deficit on vegetation growth has a time-lag effect [7; 13; 30] with lags of one to four years on a global scale [31; 32]. For instance, some research that used satellite-derived normalized difference vegetation index (NDVI) data found the response of trees to water is related not only to the current water conditions but also to past precipitation and temperature, which shows a clear time lag [33] and legacy effects [34; 35]. The frequency, severity, duration and timing of water deficits have a profound influence on vegetative growth [30; 36]. [PERSON] et al. (1997) [37] investigated the interannual lag of vegetation's response to temperature and found differences in the time-lag effects among different ecosystems.