## 4 Discussion
### Interannual Variation and Trend of WUE  
Interannual variation (IAV) serves as an indicator of the sensitivity of terrestrial ecosystems to climate change [74]. Nevertheless, the patterns and underlying drivers of IAV in WUE on the QTP remain elusive. A higher IAV indicates greater temporal instability, while a lower IAV implies relatively stable conditions over time. An analysis of IAV for GPP, ET, and WUE across the entire QTP reveals that IAV\({}_{\text{GPP}}\) = 0.047, IAV\({}_{\text{ET}}\) = 0.032, and IAV\({}_{\text{WUE}}\) = 0.027. The spatial distribution of IAV for GPP, ET, and WUE is presented in Figure 7. The spatial patterns of GPP and WUE exhibit similarities, with their IAV gradually diminishing from southeast to northwest. This suggests a higher sensitivity to climate change in the southeastern region of the QTP, as compared to the northwest. In contrast, ET exhibits an opposing spatial trend, with IAV increasing from southeast to northwest. This indicates that ET is more stable in the eastern part of the QTP, whereas it is more profoundly influenced by climate change in the western region. In addition to that, studies have also shown that large-scale climate oscillations, particularly the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO), can alter local climatic factors over the QTP, resulting in significant spatial heterogeneity in temperature and precipitation changes across the region. These changes indirectly affect vegetation growth on the plateau [75].  
Figure 6: Correlation of GPP and ET with MAT and MAP. (**(a)** shows the spatial distribution of GPP and MAT correlation over multiple years; **(b)** presents the spatial distribution of GPP and MAP correlation; **(c)** illustrates the spatial distribution of ET and MAT correlation; **(d)** depicts the spatial distribution of ET and MAP correlation. In each figure, the upper left corner indicates the significance of the growth trend at a \(p\)-value of 0.1, with green representing areas of significant change and red indicating areas of non-significant change).  
The stability of climate change was assessed by analyzing the IAV of temperature and precipitation using a 10-year moving windows approach. Previous studies have identified 2000 as a pivotal year for abrupt climate change in the QTP [76]. The IAV of temperature and precipitation before 2000 is referred to as IAV_MAT\({}_{\text{before}}\) and IAV_MAP\({}_{\text{before}}\), and after 2000 as IAV_MAT\({}_{\text{after}}\) and IAV_MAP\({}_{\text{after}}\). Before 2000, the IAV of temperature and precipitation in the 10-year moving window shows a significant increasing trend (IAV_MAT\({}_{\text{before}}\) slope = 0.008, \(p\) < 0.01; IAV_MAP\({}_{\text{before}}\) slope = 0.004, \(p\) < 0.01), indicating a steady increase in temperature and precipitation, accompanied by growing interannual variation. However, after 2000, the IAV in the 10-year moving windows of both temperature and precipitation exhibits a significant decreasing trend (IAV_MAT\({}_{\text{after}}\) slope = \(-\)0.004, \(p\) < 0.01; IAV_MAP\({}_{\text{after}}\) slope = \(-\)0.002, \(p\) < 0.01), suggesting that although the continued increase in temperature and precipitation, the interannual differences are gradually diminishing, leading to a more stable climate, as shown in Figure 8a. A similar trend is observed for GPP, ET, and WUE, with the IAV of these variables within the moving windows showing a significant upward trend before 2000, followed by a significant downward trend after 2000. These findings highlight the substantial impact of the interannual variability (IAV) of climate factors on GPP and ET, which further influences the changes in WUE [77], as shown in Figure 8b.