# Unraveling the Scale Dependency of SIF-Based Phenology: Amplified Trends and Climate Responses
## 4 Discussion
### Response of Phenology to Climatic Factors  
We compared how phenological metrics respond to climatic factors at 1-km and 5-km spatial resolution to assess the impact of scale on phenological responses to climate. The results indicate that scale effects did not significantly influence the analysis. Although the finer scale of 1 km allowed us to obtain more detailed geographic information, the coarser scale of 5 km may have smoothed local features [74], resulting in different representations of climatic factors at the two scales, and the response mechanism between climatic factors and phenological metrics remained consistent across both scales, suggesting that climate change has a generalized effect on phenology. As shown in Figures 12 and 13, it is noteworthy that at the 1-km scale, the importance of T2M in SOS exceeds that of TS3, and the importance of D2M in EOS exceeds that of SWC4, which demonstrates that the importance of climatic factors varies at different scales. In addition, the consistency of the direction of influence is also confirmed, e.g., the negative influence of TS4, T2M, etc., on SOS and the positive influence of VPD on EOS remain consistent across both scales. Supporting our findings, [PERSON] and [PERSON] (2020) showed that, despite potential spatial smoothing at coarser resolutions, the underlying climatic drivers of phenological changes produced consistent responses across regions [75]. Therefore, we conclude that the phenological metrics extracted from SIF data at different spatial resolutions responded to them to a similar extent when using preseason climatic factors calculated with monthly time steps, and the observed differences fall within acceptable limits. This study enhances our comprehension of how climatic factors influence phenological metrics at varying scales and establishes a scientific foundation for ecological monitoring and the development of climate change adaptation strategies. Environmental factors interact and jointly influence the developmental rhythm of deciduous forests. Earlier studies have indicated that phenological metrics (e.g., SOS and EOS) are primarily regulated by temperature [76; 77; 78; 79]. Furthermore, precipitation and solar radiation are key factors affecting EOS in some ecosystems [80]. In the case of our study, air and soil temperatures influenced the phenology metrics more than other environmental factors, and the dominant effect of soil temperature on phenology aligns with the previous findings of [44]. The temporal and spatial trends of each climatic variable for the period of 2008-2017 are plotted in Figure S2 in the Supporting Information. Among the many factors affecting phenology, soil and air temperature are the most critical. The rise in soil and air temperatures in spring accelerates heat accumulation, especially in the soil layer where deciduous forest roots are located, to satisfy the need for temperature accumulation and promote the advancement of phenology [81]. High-temperature soil enhances root uptake, accelerates growth, and delays fall phenology [81]. Differences in soil moisture affect soil temperature changes, and reducing moisture accelerates soil temperature rise [82], promoting the early spring development of deciduous forests. Dry soils increase root heat uptake and accelerate plant awakening and growth, but reduced moisture reserves may limit plant growth [83]. Low humidity may also advance fall phenology, as plants accelerate dormancy and defoliation in response to drought [84]. Increased winter precipitation alters the heat balance at the surface and the process of heat accumulation in the spring, leading to a delay in spring phenology [85]. Increased precipitation during the late summer may lead to slower surface heat dissipation, thus delaying fall phenology in deciduous forests [86]. In spring, when VPD is low and air humidity is high, elevated VPD enhances transpiration and promotes growth. However, when the VPD level is too high, plants tend to close their stomata, which in turn slows down their growth [87].