## 2 Results
### Spatial Relationships between Vgreenup and Climatic Factors  
The spatial pattern of Vgreenup was regulated by multiple spring climatic factors across all sites in the whole study area (Figure 4a). Temperature (TA) played a predominant role in controlling the spatial variation in Vgreenup across all sites, with a significantly positive partial correlation (\(R\) = 0.66, \(p\) < 0.001), followed by soil moisture (SM) (\(R\) = 0.45, \(p\) < 0.01), vapor pressure deficit (VPD) (\(R\) = \(-\)0.37, \(p\) < 0.05), shortwave radiation (SW) (\(R\) = 0.35, \(p\) < 0.05), and wind speed (WS) (\(R\) = 0.35, \(p\) < 0.05). However, no significant relationship was found between precipitation (PRE) and Vgreenup. TA was significantly negatively correlated with Dgreenup (\(R\) = \(-\)0.51, \(p\) < 0.01) and was positively correlated with Agreenup (\(R\) = 0.71, \(p\) < 0.001), which indicated that increasing TA would shorten Dgreenup and promote Agreenup. Meanwhile, SM and VPD showed contrasting effects on Dgreenup as well as on Agreenup. SM exhibited a negative correlation with Dgreenup  
Figure 3: The spatial relationships between Vgreenup anomalies and Dgreenup anomalies as well as Agreenup anomalies. (**a,b**) The spatial relationships across all sites in both Europe and North America; (**c,d**) the spatial relationships across all sites in Europe; (**e,f**) the spatial relationships across all sites in North America. (\(R=-0.46\), \(p<0.01\)) and a positive correlation with Agreenup (\(R=0.41\), \(p<0.05\)), while VPD was positively correlated with Dgreenup (\(R=0.41\), \(p<0.05\)) and negatively correlated with Agreenup (\(R=-0.33\), \(p>0.05\)). This indicated that increasing VPD and decreasing SM would lengthen Dgreenup and lower Agreenup. Besides, a negative effect of SW on Dgreenup (\(R=-0.45\), \(p<0.01\)) and a positive effect of WS on Agreenup were also observed (\(R=0.49\), \(p<0.01\)). It could also be observed that PRE had a minor influence on Dgreenup and Vgreenup. In Europe, the effects of spring climatic factors on Vgreenup and its components were similar to those in the whole study area. Except for PRE, all spring climatic factors showed significant correlations with either Dgreenup or Agreenup in Europe (Figure 4b). A significantly positive effect of TA (\(R=0.77\), \(p<0.01\)) and SW (\(R=0.60\), \(p<0.05\)) on Vgreenup and a significantly negative effect of VPD on Vgreenup (\(R=-0.78\), \(p<0.01\)) could be found. However, no significant relationship was found between Vgreenup and spring climatic factors except for TA (\(R=0.51\), \(p<0.05\)) in North America (Figure 4c). TA also showed significant relationships with Dgreenup (\(R=-0.54\), \(p<0.05\)) and Agreenup (\(R=0.56\), \(p<0.05\)) in North America. Besides, SM was significantly and negatively correlated with Dgreenup (\(R=-0.61\), \(p<0.01\)) but exerted no significant influence on Agreenup or Vgreenup in North America (Figure 4c). Figure 4: The partial correlation coefficients between spring climatic factors and Dgreenup, Agreenup, and Vgreenup. **(a)** The partial correlation coefficients across all sites in both Europe and North America; **(b)** the partial correlation coefficients across all sites in Europe; **(c)** the partial correlation coefficients across all sites in North America. TA, SW, PRE, WS, VPD, and SM represent temperature, shortwave radiation, precipitation, wind speed, vapor pressure deficit, and soil moisture, respectively. *, ** and *** indicate significant correlation at \(p<0.05\), \(0.01\) and \(0.001\) levels, respectively. This study also analyzed the temperature sensitivity of Vgreenup and its components (Figure 5). In response to a 1 \({}^{\circ}\)C increase in TA, Dgreenup would shorten by 0.87 days across all sites of the whole study area, while Agreementp and Vgreenup would increase by 0.41 g C m\({}^{-2}\) day\({}^{-1}\) and 0.019 g C m\({}^{-2}\) day\({}^{-2}\), respectively (Figure 5a-c).