## 4 Discussion
### Changes in Growing Season GPP Trends and Reasons for Drought Responses  
GPP exhibited a unimodal pattern of variation during the 2009 growing season in Xinjiang, peaking at 82.189 gC m\({}^{-2}\)a\({}^{-1}\) in July (Figure 6); this was primarily attributed to the high summer temperatures, abundant precipitation, and robust photosynthesis of Xinjiang vegetation. However, as drought intensity increased, the GPP exhibited a declining trend. First, persistently high temperatures reduce stomatal conductance in vegetated leaves, contributing to a decline [54]. Second, in arid areas experiencing water scarcity, plants regulate their stomata to minimise water loss, decreasing vegetation GPP [55].  
Regarding spatial distribution, the vegetation GPP pattern in Xinjiang exhibited notable heterogeneity, with higher values in the north and lower values in the south. This spatial heterogeneity was influenced by regional hydrothermal conditions and the zonal differentiation of vegetation types, which is consistent with previous research findings [56]. The peak GPP values estimated in 2009 were predominantly distributed in the Iii Valley, northern Tianshan Mountains, and oases around the Tarim Basin. This distribution pattern was partially attributed to the effects of global climate trends towards \"warming and humidification\", coupled with the enhanced surface runoff resulting from melting snow and ice in the Tianshan Mountains [57], notably enhancing the vegetation cover in these regions. In addition, ecological protection measures have supported increased vegetation cover in Xinjiang [58]. Although the overall GPP value was relatively low in southern Xinjiang, vegetation growth in specific areas, such as the oasis region of the Tarim Basin, was also relatively good because of the abundance of water resources and the effective implementation of ecological protection measures. Notably, in recent years, Xinjiang has converted agricultural land back to forests and initiated comprehensive sand prevention and control measures, resulting in a substantial increase in forest coverage from 1.03 to 5.06% (available at [[https://t.m.china.com.cn/convert/c_v1](https://t.m.china.com.cn/convert/c_v1) Ekj317.html]([https://t.m.china.com.cn/convert/c_v1](https://t.m.china.com.cn/convert/c_v1) Ekj317.html), accessed on 20 July 2024). Climate change, human activity, and their synergistic effects are the primary drivers of large-scale vegetation recovery and improvement in Xinjiang.  
This study revealed a lagged response pattern of vegetation GPP to the SPEI in Xinjiang. Specifically, GPP responded most significantly to SPEI09 (Figure 7a-e). In addition, the response of GPP to changes in the SPEI differed considerably across vegetation types, with grass land exhibiting the highest sensitivity (Figure 7f). [PERSON] et al. [59] demonstrated that water stress negatively affects vegetation growth. This viewpoint is supported by other studies, particularly those focusing on GPP in most grass land areas in Central Asia, which showed that grass land has a cumulative response to drought conditions, with the primary concentration of this cumulative effect occurring over extended periods [60]. The lagging effect of vegetation on drought is predominantly attributed to PRE scarcity in arid zones, which makes it difficult for surface vegetation to obtain sufficient water support during the growing season. Consequently, vegetation tolerates prolonged and intense drought through long-term adaptive mechanisms.