## 4 Discussion  
### Phenology Changes  
In this study, we observed divergent changes in boreal vegetation phenology from 1982 to 2018. The extension rate of the length of the LOS for forestlands has slowed in the past two decades. [PERSON] et al. [18] suggested that, since the late 1990s, this tendency of SOS advancement has been weakened in over 60% of boreal forests, particularly in Siberia (\(-0.58\) day yr\({}^{-1}\) for 1982-1997 vs. \(-0.17\) day yr\({}^{-1}\) for 1982-2013). Our results also indicated that a stagnation in the rate of EOS delay was another key factor contributing to this phenomenon. Unlike the slowing trend in LOS for forestlands, in our study, the extension rates for grasslands and shrublands increased markedly. These changes were primarily driven by earlier SOS and delayed EOS. However, other results based on the satellite record and ground-based observation [18; 27; 49; 50] showed that the phenology (SOS) change rate slowed down in recent years. [PERSON] et al. [51] reported that SOS advanced significantly before 2000 (3.1 days per decade) but exhibited no significant trend after 2000 (0.18 days per decade) in the boreal region, and our findings contradict these results. We suggested that the influence of climate change on SOS in this region may be more pronounced than previously reported. Regarding the trend of EOS, existing research has generally reported a delayed EOS (from 0.19 to 0.45 days yr\({}^{-1}\)) across the Northern Hemisphere [26]. Our analysis reveals divergent changes EOS among vegetation types. The slowed delaying rate of forestlands EOS from 2001 to 2018 aligns with the findings of 0.03 days dec\({}^{-1}\) after 1998 [49]. Regarding spatial distribution, some studies [18; 33; 52] have documented a decline in the advancement of SOS after the early 2000s in Siberia, northwestern America, and Central Europe, while our analysis indicates that this trend is not uniform across all regions. While Central Europe experienced a decline, other regions continued to show substantial  
Figure 8: Relative Incremental Effect (RIE) of LOS extension to GPP changes. Panels (**a**,**b**) present the spatial distribution of RIE across boreal vegetation for periods P1 and P2, respectively. Green indicates a positive contribution of LOS variation to GPP increase, while red signifies a negative contribution. Panel (**c**) displays boxplots of RIES for different vegetation types. Panels (**d**–**g**) show the frequency distributions of RIES for forestlands, shrublands, grasslands, and all vegetation types. advancements in SOS. Additionally, although the spatial distribution of the EOS trend aligns with a previous study [49] from 1982 to 1998, the delayed EOS during P2 was more prevalent in Siberia compared to Europe, reversing the pattern observed in P1. These contrasting findings highlight the uncertainties associated with estimating large-scale changes in vegetation phenology. The magnitude of phenology trends can vary considerably depending on the vegetation index used, the period of investigation, and the spatial scale considered [17; 33]. Most existing studies rely on various vegetation index (VI) products, such as NDVI [53; 54]. However, VIs only represent vegetation greenness and may not be sufficiently sensitive to changes in the photosynthetic process [55]. The nonlinear relationship between VIs and green biomass, along with the numerical saturation problem often encountered in high-vegetation-covered areas and during peak vegetation growth [56], can distort the fitted curve shape of the seasonal growth process, leading to reduced accuracy in extracting phenology metrics [55]. Furthermore, the physiological (GPP/SIF) and structural (VIs) phonologies may differ in certain vegetation types, as the onset of photosynthesis does not necessarily coincide with the beginning of greenness changes [57]. Therefore, employing products such as GPP, which characterize actual photosynthetic activities, can provide more objective results for determining changes in vegetation phonologies [14].