# Unraveling the Scale Dependency of SIF-Based Phenology: Amplified Trends and Climate Responses
## 3 Results
### Temporal and Spatial Distribution of Phenology
#### 3.2.2 Trends in Temporal and Spatial Patterns of Phenology  
To quantify the trend of vegetation phenology, the Sen's slope factor was utilized in this study. The spatial distribution pattern of the Sen's slope factor of the phenological metrics extracted from the 1-km SIF across the study region is provided in Figure 8. It was found that the SOS of mid- to high-latitude deciduous forests in the Northern Hemisphere from 2008 to 2017 had an overall advance trend, with an average Sen's slope factor of \(-0.229\), in which the range of advancement (62.62%) and significant advancement (2.42%) was greater than the range of delay (18.94%) and significant delay (0.17%), respectively. EOS also has an overall advance trend, with an average Sen's slope factor of \(-0.181\), where the range of advancement (55.87%) and significant advancement (4.91%) is greater than the range of delay (16.81%) and significant delay (0.15%), respectively. Although the proportion of significantly advanced areas is relatively small, their wide distribution reflects the response of deciduous forest ecosystems to environmental changes in the context of global warming. Spatially, SOS advancement is predominantly observed in the eastern and northern parts of the U.S., southern Europe, the Black Sea coast, Central Asia, and East Asia. The regions with SOS postponement are mainly located in the northeastern and southern U.S., the northeastern Middle East, and Eastern Europe. The regions with EOS advancement are mainly situated in the northern and southeastern regions of the U.S., the northeastern Middle East, and northeastern China. The areas exhibiting EOS postponement were mainly in the northeastern U.S., Eastern Europe, and Central China. The trend of advancement of phenological metrics in most of the regions may be related to global warming, where a rise in overall temperature could cause plants to start growing earlier, thus entering the beginning of the growing season earlier (SOS), and similarly, this increase in temperature  
Figure 8: Trends in deciduous forests’ phenology extracted from 1-km SIF data across the Northern Hemisphere’s middle and high latitudes (2008–2017). may cause plants to complete the growth cycle earlier, thus ending the growing season earlier (EOS) [68]. Regions showing delays in phenological metrics may be doing so because of complex interactions of localized climatic conditions, soil moisture, or land use changes, such as the combination of changing temperature and precipitation patterns and extreme weather events in the northeastern United States resulting in a delayed phenological period in local deciduous forests [69]. These local variations highlight the spatial heterogeneity of phenological changes and add to the complexity of accurate ecological predictions and the development of adaptive management strategies. In this study, in addition to the 1-km SIF dataset, the GOSIF and GOME-2 SIF datasets were used for the TSM-MK method (Figures 9 and 10). The results show that using the coarser spatial resolution of the SIF data resulted in a more pronounced trend in the advancement of phenology. Among them, the SOS extracted from GOSIF showed an overall advance trend with an average Sen's slope of \(-0.264\), where the proportion of advance (\(62.42\%\)) and significant advance (\(2.36\%\)) was greater than the range of delay (\(18.71\%\)) and significant delay (\(0.10\%\)), respectively. EOS, on the other hand, likewise has an overall advance trend with an average Sen's slope of \(-0.187\), where the range of advancement (\(56.73\%\)) and significant advancement (\(2.93\%\)) is greater than the range of delay (\(20.75\%\)) and significant delay (\(0.20\%\)), respectively. In addition, the GOME-2 SIF extracted SOS and EOS showed an advance trend for all pixels, with an average Sen's slope of \(-8.644\) and \(-8.448\) for SOS and EOS, respectively. Figure 11 shows a comparison of phenological trends extracted from three different scales of SIF.