## 3 Discussion
With the positive effect of spring TA on Vgreenup, increasing spring TA could enhance spring GPP, which had been previously confirmed across 21 FLUXNET sites in temperate and boreal forests [38]. In this study, increasing SW also contributed to shorter Dygreenup and higher Vgreenup, similar to effects of TA (Figure 4a). SW is highly related to the heat requirement for leaf unfolding [39]. Lower solar radiation can significantly delay spring bud development by decreasing the accumulation of growing degree days [40]. Therefore, SW can influence Dygreenup and Vgreenup by altering the temperature sensitivity of SOC and EOG [41]. VPD and SM are commonly used indicators for atmosphere and soil dryness, respectively [42; 43; 44]. A significant effect by drought on vegetation greenup development was observed in this study. By increasing VPD or decreasing SM, Dygreenup and Agreenup tended to become longer and lower, respectively, resulting in the deceleration of Vgreenup for the whole study area (Figure 4a). Previous studies based on eddy covariance observations also demonstrated the reduction in photosynthesis caused by dryness [45; 46]. GPPmax has also been reported to be enhanced by summer PRE, based on data from 24 AmeriFlux sites [47]. However, no significant relationship was found between spring PRE and Vgreenup in this study. This might be caused by the fact that vegetation growth was more directly affected by SM and VPD compared with PRE. Vegetation tends to lower photosynthesis by closing stomata in a high VPD condition [48; 49]. Different from VPD, SM influences vegetation photosynthesis by controlling the absorption of soil nutrients. Increasing SM will facilitate plant nutrient absorption, leading to the stimulation of Vgreenup [50]. Besides, this study found that there were positive effects by wind speed on Agreenup and Vgreenup for the entire study area (Figure 4a). This might be explained by the fact that moderate wind speed would increase the gas exchange rate of leaf stomata, accelerating transpiration and intercellular CO\({}_{2}\) exchange, which results in a higher intercellular CO\({}_{2}\) concentration and improved photosynthesis capacity [51]. In this study, the climatic controls on the spatial variations of Vgreenup were also compared between Europe and North America. The temperature sensitivities of Agreenup and Vgreenup were greater in North America than in Europe (Figure 5). This difference in temperature sensitivity could be caused by the effects of climatic factors other than TA. The spatial variations of Vgreenup were jointly controlled by TA, SW, and VPD in Europe but only affected by TA in North America (Figure 5b,c). Without the limitations of solar radiation and water supply, the vegetation may have developed greater temperature sensitivities to maximize the thermal benefits [52; 53; 54]. ENF sites were included to investigate Vgreenup in this study. It is difficult to extract the phenological metrics from ENF by using NDVI or LAI, due to their narrow seasonal variations in vegetation greenness [55; 56]. In comparison, GPP derived from eddy covariance has a great advantage in extracting the phenological metrics from ENF, which can help improve the estimation of Vgreenup for ENF. Besides, solar-induced chlorophyll fluorescence can reflect vegetation photosynthesis directly [57], which can be used to investigate the phenological metrics and Vgreenup for ENF in the future. VPD and SM are often coupled through land-atmosphere interactions, hindering the ability to predict ecosystem responses to dryness [58; 59]. Several methods have been proposed to distinguish the effects of VPD and SM on vegetation growth [60; 61; 62]. Multi-methods may need to be synthesized for quantifying the relative contributions of VPD and SM to Vgreenup in future work.