## 3 Results
### Spatiotemporal Variation Characteristics of Vegetation in Xinjiang  
#### 3.2.1 Spatiotemporal Variation Analysis of Vegetation NDVI and SIF  
Using NDVI and SIF data from 2000 to 2020, the mean values of NDVI and SIF over the past 20 years were calculated (with all negative values set to zero during processing). This analysis produced interannual variation (Figure 5) and spatial distribution (Figure 6) maps for the multiyear average NDVI and SIF, where NDVI represented vegetation structural conditions, and SIF reflected vegetation physiological conditions. \begin{table}
\begin{tabular}{c c c c c c} \hline
**SPEI-n** & **Ecological Zone** & **Z** & **\(p\)** & **SLOPE** & **Variation Trends** \\ \hline \multirow{8}{*}{SPEI-03} & R1 & \(-1.55\) & \(>\)0.05 & \(\leq\)\(-0.0005\) & Slight deterioration \\  & R2 & \(-1.56\) & \(>\)0.05 & \(\leq\)\(-0.0005\) & Slight deterioration \\  & R3 & \(-0.48\  
The interannual time series of the NDVI in Xinjiang revealed an upward trend in the annual mean NDVI, which was largely consistent with the trend observed in the SIF time series. During the study period, the highest and lowest annual mean NDVI values were recorded in 2017 (0.1534) and 2001 (0.1272), respectively. Meanwhile, the mean SIF value increased from 0.0770 in 2000 to 0.0827 W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\cdot\)sr\({}^{-1}\) in 2020. It reached a peak of 0.0859 in 2016 and a minimum of 0.0734 in 2010. The average NDVI was 0.140, while the average SIF was 0.0792 W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\cdot\)sr\({}^{-1}\) over the study period. Drought events were identified for NDVI during 2007-2011 and 2012-2016. In contrast, SIF experienced three periods of decline: 2002-2007, 2009-2013, and 2013-2016. These periods aligned closely with the timing of the drought events identified by SPEI-12 and SPEI-24, suggesting that vegetation growth was affected by drought conditions. Furthermore, the intervals of the minimum values in NDVI and SIF were nearly identical, with SIF showing more  
Figure 5: Inter-annual variation analysis of vegetation NDVI and SIF. **(a)** Annual mean NDVI with trend line and confidence interval. The trend line (red) represents the linear trend, with the equation y = 0.0012x + 0.1276y = 0.0012x + 0.1276y = 0.0012x + 0.1276 and a correlation coefficient of 0.8929. **(b)** Annual mean SIF with trend line and confidence interval. The trend line (red) shows the linear trend, with the equation y = 0.0005x + 0.0741y = 0.0005x + 0.0741y = 0.0005x + 0.0741 and a correlation coefficient of 0.7521. Blue triangles represent observed data, and the shaded area indicates the confidence interval around the trend line. Figure 6: Spatial distribution of mean NDVI and SIF in Xinjiang from 2000 to 2020. **(a)** Spatial distribution of mean NDVI, representing vegetation structural conditions across Xinjiang. The color bar indicates NDVI values, with yellow to dark green representing increasing vegetation coverage from 0.0 to 1.0. **(b)** Spatial distribution of mean SIF, indicating vegetation physiological activity levels. The color bar reflects SIF values, ranging from 0.00 to 0.20 W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\cdot\)sr\({}^{-1}\), with dark green areas showing higher fluorescence. -frequent extremes than NDVI. The years in which SIF displayed minimum values coincided more closely with the drought events identified by the SPEI, indicating that SIF was more sensitive to drought than NDVI. From the spatial distribution across Xinjiang (Figure 6), areas with high vegetation coverage (NDVI > 0.6) occupied the smallest proportion, mainly in ecological regions R1 and R3, accounting for only 1.21% of the total area of Xinjiang. Moderate coverage zones (NDVI 0.3-0.6) are primarily concentrated in R1 and R2 and extend into R2 and R5, with a few on the edges of the R4 desert region, comprising 10.96% of the area of Xinjiang. Low coverage areas (NDVI 0.1-0.3) represent 23.46% of the total area and are the most common vegetation type.