## 4 Discussion
### LOS Variation to GPP Increment  
The starting and ending dates of GPP corresponded closely with the spring and autumn GPP [6]. In this study, we directly employed GPP datasets to extract phenology metrics (SOS, EOS, and LOS), providing a more accurate representation of the photosynthetic processes in boreal vegetation. Furthermore, the statistical methods and controlled experiments utilized in this study offered a more robust explanation of the contribution (RIE) of LOS variation to GPP changes, revealing a strong correlation between vegetation phenology and GPP, consistent with findings from other studies in forestlands, grasslands, and other ecosystems [33; 62]. The increased contribution of phenology extension to the boreal terrestrial GPP increment can be attributed to several underlying factors, with boreal vegetation ecosystems being particularly sensitive to temperature changes. As surface temperatures rise, these ecosystems experience extended growing seasons, which can enhance GPP, particularly in regions where temperature was previously a limiting factor. Although the enhanced VPD forcing may reduce the photosynthetic capacity of boreal vegetation in the growing peak season [11; 13], the extended growing season may enhance plant photosynthesis duration [17] and offset these negative effects [1]. As a result, LOS extension became more crucial contributing to the annual GPP increase in boreal terrestrial ecosystems [6; 36]. Apart from this, favorable climatic conditions can also enhance vegetation richness and coverage, thereby positively contributing to the annual productivity of vegetation [41]. The heterogeneous distribution of RIE values reflects the varying levels of vegetation productivity and the adaptability of different ecosystems to climate changes. For instance, tundra exhibits stronger positive RIE due to the simple ecological structure and interactions [10]. So, during the extended LOS, more energy and resources for photosynthesis are available. Moreover, the differences in the spatial distribution of RIE between P1 and P2 suggest a potential shift in ecosystem responses over time. This shift may be driven by ongoing climate change, which is affecting the phenology and productivity of boreal ecosystems [42; 63]. The increase in RIE observed across shrublands and grasslands during P2 indicates a general intensification of LOS's influence on GPP in northern high-latitude regions. However, the concurrent rise in standard deviation suggests that this intensification is accompanied by increased variability, pointing to a more complex and heterogeneous response across different ecosystems. This could be due to varying local environmental conditions, differential adaptability of vegetation types, or region-specific climatic changes [41]. Further research is necessary to disentangle these contributing factors and gain a deeper understanding of the underlying mechanisms driving these observed patterns. The Relative Incremental Effect (RIE) index is particularly effective in quantifying the contribution of phenological extension to annual GPP variations due to its ability to isolate the effect of growing season length (LOS) changes while controlling for variations in maximum photosynthetic capacity (\(GPP_{max}\)). Unlike traditional methods, which often focus on meteorological factors such as temperature or precipitation [38; 64] without considering the specific role of growing season length, RIE emphasizes how changes in LOS directly influence GPP. By calculating the ratio of the trend in GPP variability induced by LOS changes (\(GPP_{LOS}\)) to the overall GPP trend, RIE provides a clear and comparative measure of the phenological impact on productivity, offering a more focused and direct approach to understanding the role of phenology in GPP dynamics. While the RIE index has been applied to the boreal ecosystem in this study, its applicability to other ecosystems remains to be explored.