# Climate Warming Dominates Vegetation Productivity in the Hanjiang River Basin, China
## 4 Discussion
### Response Mechanisms of Vegetation Productivity to Climate Change in the HJRB  
In the context of global warming, the HJRB also experienced a warming trend, particularly in the central part of the basin (Figure 11). Despite this warming trend, there was no significant increase in Pre in the HJRB. In fact, the northeastern part of the downstream basin, specifically the Nanyang Basin, experienced a significant decrease in Pre, exacerbating the region's aridity. As a result, the increased Tem and SSD led to a significant increase in PET near the Nanyang Basin. However, despite the gradual growth of GPP in the downstream Nanyang Basin, it still exhibited a significant upward trend. We hypothesize that this is mainly attributable to the continuous enhancement in agricultural productivity in the downstream over the past centuries, along with extensive irrigation in the region, which compensated for the regional aridity. Consequently, a significant negative correlation was observed between Pre and GPP in the Nanyang Basin (Figure 8). Regarding the impact of drought on GPP in the HJRB, a previous study indicated that the basin experienced relatively severe droughts in 2001-2002, 2006-2008, and 2012-2014. During our study period (2000-2020), reduced GPP was observed in 2001, 2011, 2014, 2017, and 2019 (Figure 5), suggesting that drought primarily influenced short to medium-term GPP variations in the HJRB. In the long term, the water conservation functions of the upstream forests and the Danjiangkou Reservoir in the middle reaches ensured a stable water supply for the HJRB. Furthermore, the study observed that despite significant differences in seasonal precipitation within the HJRB, the seasonal drought conditions remained relatively stable. As the temporal scale increased, the seasonal variation of the SPEI in the HJRB became more stable, with SPEI12 remaining around 0 throughout the year (Figure 6). This indicates that despite the evident temperature rise in the study period, the regional long-term hydrometeorological conditions in the HJRB have remained relatively stable. SPEI1 and SPEI3 were lower from February to May compared to other months, suggesting a lack of spring precipitation affecting vegetation growth. Additionally, the study indicated that the mISDI in the downstream and upstream valleys of the HJRB exhibited a significant downward trend [59]. This aligns with our trend analysis of SPEIs, illustrating that, in the context of continuous warming temperatures, there is a significant drought risk in the downstream HJRB, particularly during the relatively dry spring season. This also explains why croplands in the upstream HJRB have a significant correlation with precipitation, while in the downstream, the correlation is not significant, indicating that downstream croplands rely more on irrigation than precipitation. Among the various climate factors, WS and SSD significantly influence variations in GPP in the HJRB. A study focusing on the global vegetation response to WS variations suggests that decreased WS in water-limited areas can reduce vegetation transpiration rates, thus limiting vegetation growth [60]. Conversely, in wind-resistant forest areas, increased WS can enhance material exchange and promote vegetation growth. Similar patterns were observed in the HJRB. In the mid-high altitude forest areas of the basin, the decline in WS reduced regional vegetation transpiration rates, impacting material exchange in these areas. Furthermore, a trend of increasing WS from October to the following March was observed in the HJRB, while WS continued to decrease during the summer when vegetation activity is at its peak. This undoubtedly affected regional vegetation material exchange, resulting in a significant overall negative correlation primarily observed in forested areas. The region's SSD experienced a considerable decrease, which would impact solar radiation and consequently affect vegetation photosynthesis.