# Diverse Roles of Previous Years' Water Conditions in Gross Primary Productivity in China
## 4 Discussion
### The Time-Lag Effect of Water Conditions on GPP  
Revealing the impact of water stress on GPP is quite important under the background of global climate change. However, due to the time-lag effect [7; 13; 30; 38], the cumulative effect [36] and the legacy effect [42; 43], the influence of water stress on vegetation growth is very complicated as the frequency, severity, duration and timing of water deficits have a profound influence on vegetative growth [30]. In this study, we applied a random forest method to quantify the relative importance of current and previous years' water stress. Furthermore, we revealed the role of the previous year's water stress on GPP, especially the importance of the combination of previous and current years' water conditions. In this study, the relationship between GPP and water deficit intensity in previous years and the current year was established by setting suitable scenarios. Through the relationships identified in the different scenarios, we revealed some rules governing the influence of water conditions in the previous years and the current year on GPP. The results showed that the water conditions of the current year had the greatest impact on GPP. [PERSON] et al. (2018) [30] also found that the water deficit conditions of the current hydrological year had a greater impact on trees than the conditions of the previous hydrological year. The deficit or surplus of water supply in the current year will directly affect the physiological processes of the plant. Plants typically adjust the physiological function of their water conduction system to adapt to water stress [7; 59; 60]. Water stress limits the photosynthetic rate of plants, thereby reducing vegetative productivity. The aggravation of water stress may also damage the biochemical processes related to respiration and photosynthetic carbon storage, directly forcing plants to engage in adaptive strategies such as defoliation and strengthening of the root distribution [61]. Sufficient water allows an adequate level of photosynthesis for plants and promotes the enhancement of vegetative productivity when water conditions are wet. Our study also showed that the water conditions in previous years, particularly in the first previous year, are also important for GPP, which shows a clear time-lag effect of water conditions on vegetation and is similar to other studies [13; 38; 62; 63]. The reason may be that water in the previous year tended to permeate through the soil and became soil water which would definitely influence the ecosystem [55]. [PERSON] et al. (2018) [13] found that forests of different stock volume levels were affected differently by the water in the previous years, presumably because of their varying ability to absorb underground water. Therefore, the water conditions in previous years could be better for predicting vegetation GPP and its interannual variation. Although plants can adapt to water stress through self-regulation, growth is still affected to some extent when plants are subjected to severe water stress. In other words, water stress will have a time-lag effect on vegetative growth after severe drought events [7; 41]. We found that the water conditions in the first previous year had a significant impact on GPP when the water conditions in the current year were normal or wetter (i.e., one-year time lag). [PERSON] et al. (2017) [64] found a one-year time lag in the response of tree growth to the climate in the circumboreal region, which probably reflected an adaptation of trees to environments with more severe water stress by carrying over fixed carbon from one year to following years [65]. Wet conditions in two consecutive years can often accelerate the growth of vegetation [66]. However, once a strong drought occurs, there is a strong legacy effect from the first previous year due to the sensitivity of the vegetation to drought [31; 36; 67]. This shows the resilience of vegetation.