## 3 Discussion  
Many previous studies have reported the distribution patterns along latitude, longitude, and altitude for vegetation phenology and productivity in adaptation to climate changes [27; 28; 29]. This study revealed the spatial pattern of Vgreenup across PFTs in the northern temperate and boreal regions. A strong dependence on latitude was observed for Vgreenup across all sites in both Europe and North America (Figure 2a), with a decreasing  
Figure 5: The spatial relationships between temperature anomalies and Dgreenup anomalies, Agreenup anomalies, and Vgreenup anomalies. (**a–c**) The spatial relationships across all sites in both Europe and North America; (**d–f**) the spatial relationships across all sites in Europe; (**g–i**) the spatial relationships across all sites in North America. trend with rising latitude. This means there was more rapid canopy development in lower latitudes than in higher latitudes. However, Vgreenup demonstrated no clear patterns with longitude and altitude (Figure 2b,c). Furthermore, the above patterns were also confirmed when only considering sites in Europe or North America (Figure 2d-i). A previous study based on remote sensing observation found the latitude dependence of Vgreenup on both continents, but an opposite trend with latitude was observed in this study [21]. The discrepancies might be caused by the difference in the target scale between the satellite and field observations. The spatial variation of Vgreenup was associated with the decoupling of the spatial patterns of its two components [19; 22]. In this study, both Dygreenup and Agreenup were significantly correlated with Vgreenup along a spatial dimension, indicating that the spatial variations of Vgreenup were jointly regulated by Dygreenup and Agreenup (Figure 3a,b). In the whole study area, Vgreenup was more correlated with Agreenup (\(R^{2}\) = 0.83) than with Dygreenup (\(R^{2}\) = 0.61), which implies a more predominant effect of Agreenup variability compared with Dygreenup variability. This result is in line with the finding of a previous satellite-based study at a global scale [19]. Nevertheless, the variations of Dygreenup and Agreenup made different contributions to Vgreenup variation between continents. According to the correlation coefficients between Vgreenup and its components (Figure 3c-f), the spatial pattern of Vgreenup was more determined by Dygreenup in Europe but by Agreenup in North America. This study further investigated the controls of the main spring climatic factors on the distributions of Vgreenup and its components (Dgreenup and Agreenup), by using the partial correlation. TA showed significant effects on Dygreenup (\(R\) = \(-\)0.51, \(p\) < 0.01), Agreenup (\(R\) = 0.71, \(p\) < 0.001) and Vgreenup (\(R\) = 0.66, \(p\) < 0.001) in the whole study area (Figure 4a). Specifically, increasing TA could shorten Dygreenup and promote Agreenup simultaneously, leading to an acceleration of Vgreenup. TA was reported to be the major driver for spring phenology based on long-term FLUXNET measurements [30]. Advanced SOC and the peak of the growing season (closing to EOG) in response to climate warming had been widely observed [10; 31; 32; 33], but their non-uniform response magnitude would alter Agreenup. Under global warming, global average Agreenup decreased by 1.4 days during 1982-2015, according to the satellite LAI product [19]. Besides, TA had a stronger link with Agreenup (\(R\) = 0.71, \(p\) < 0.001) than with Dygreenup (\(R\) = \(-\)0.51, \(p\) < 0.01) in this study. The boosting effect of TA on Agreenup was consistent with the global greening and increasing productivity in recent decades due to climate warming [17; 34; 35]. [PERSON] et al., also reported the positive effect of increasing TA on the seasonal maximal gross primary productivity (GPPmax) based on eddy covariance at an alpine meadow site [36]. Rising TA can promote plant growth by enhancing enzymatic activity when TA is lower than the optimum temperature [37].