## 3 Results
### Boreal Vegetation Phenology Changes
#### 3.1.2 Comparison of Phenology Long-Term Trends During the Two Periods  
To compare the spatiotemporal changes in boreal vegetation phenology during the two periods, we examined the linear trends in regional mean and pixel-level phenology metrics of SOS and EOS from 1982 to 2000 and 2001 to 2018. Our findings reveal significant shifts in phenology trends across various vegetation communities during these two periods (Figure 5). While the SOS consistently advanced in both periods, the trend was more pronounced in P2. Shrublands, particularly composite with Arctic tundra in high latitudes, exhibited the most substantial increase in SOS advancement rates (\(-\)1.77 \(\pm\) 0.80 days dec\({}^{-1}\)) in P2, surpassing those in P1 (\(-\)0.5 \(\pm\) 0.1 days dec\({}^{-1}\)) by 2.5 times. Spatially, SOS advancement trends intensified significantly in Western Canada, Alaska, and Northern Siberia during P2. In contrast to SOS, EOS shows divergent shifts of linear trends for different vegetation types during the two periods. For forestlands and grasslands, primarily located at lower latitudes, EOS delaying rates slowed in P2. However, shrulbands in the high northern region experienced an enhanced EOS delay of 1.75 \(\pm\) 1.01 days dec\({}^{-1}\) in the recent period. Spatial patterns indicate a deceleration of EOS delaying rates in Northern America and Europe, but a substantial increase in Northern Siberia during P2. Consequently, the area of forestlands with extended EOS decreased by 17% in P2, while the area of shrulbands with extended EOS increased by 40%.  
Figure 5: Long-term trends in pixel-level and regional mean SOS and EOS of boreal vegetation during the two study periods. Panels **(a,b)** present the time series and linear trends of regional mean SOS and EOS, respectively. Panels **(c)** presents three landcover types in our study. Panels **(d–g)** present the spatial distributions of linear trends in phenology metrics (SOS, EOS) for the two distinct periods, respectively. The accompanying bar chart (**d–g**) depicts the frequency of pixels with positive and negative trends for three vegetation types. The right panel displays the linear trend values for each study period, with R\({}^{2}\) values in brackets.  
The length of the LOS, determined by the combined effects of the SOS and the EOS, plays a critical role in regulating vegetation canopy photosynthesis duration and overall annual productivity. Over the entire study period (1982-2018), LOS exhibited substantial extension for all three vegetation types: forests (2.90 \(\pm\) 0.48 days dec\({}^{-1}\)), shrublands (2.95 \(\pm\) 0.61 days dec\({}^{-1}\)), and grasslands (2.31 \(\pm\) 0.42 days dec\({}^{-1}\)). When comparing the two distinct periods, divergent trends emerged for different vegetation types and ecological zones. While the LOS extension rate for forests decreased by 44% from P1 (2.75 \(\pm\) 1.46 days dec\({}^{-1}\)) to P2 (1.54 \(\pm\) 1.25 days dec\({}^{-1}\)), shrublands and grasslands demonstrated significantly accelerated rates in P2. Shrublands, predominantly found in high latitudes, experienced a notable reversal from a declining trend (\(-\)0.38 \(\pm\) 1.83 days dec\({}^{-1}\) in P1) to a pronounced extending trend (3.52 \(\pm\) 1.46 days dec\({}^{-1}\) in P2). Statistical analyses revealed a 10% decrease in the area of forestlands with positive LOS trends in P2, whereas shrubland areas with such trends increased by 32%. Spatial patterns, as illustrated in Figure 6, show a marked shift in positive LOS trends, which were primarily concentrated in Eurasia. Notably, European forestlands saw a decrease in positive LOS trends, while Siberian shrublands experienced an increase during the recent period. Histograms depicting spatial trends for forestlands, shrublands, and grasslands provide further clarity on these phenomena.