# Unraveling the Scale Dependency of SIF-Based Phenology: Amplified Trends and Climate Responses
## 3 Results
### Temporal and Spatial Distribution of Phenology  
#### 3.2.1 Spatial Distribution of Phenology  
Using the phenological information extracted from SIF data at different resolutions, this study determined the pixel mean values of the phenological metrics SOS and EOS for deciduous forests across the Northern Hemisphere's middle and high latitudes during 2008-2017. The spatial distribution of these mean values over the ten-year period is shown in Figures 5-7. It was found that the SOS and EOS of deciduous forests in the middle and high latitudes of the Northern Hemisphere indicate significant spatial heterogeneity. From Figures 5-7, it can be seen that, in general, the phenological changes of deciduous forests are closely related to latitude. With increasing latitude, the SOS of deciduous forests was gradually delayed while the EOS was gradually advanced, a phenomenon consistent with the findings of [66], and this latitudinal dependence may be related to the latitudinal gradient of climatic factors, such as sunshine hours, temperature, and precipitation. In the study area, the multi-year mean SOS of the deciduous forest extracted from the 1-km resolution SIF data ranged from day 61 to day 141, while the multi-year mean EOS ranged from day 215 to day 284. Over the years, the regions with the earliest SOS were centered in the southeastern U.S. (ca. day 70), Eastern Europe (ca. day 90), and the Black Sea coast (ca. day 95), while the regions with the latest SOS were located in the northern U.S. (ca. day 120) and East Asia (ca. day 125). The regions with the earliest EOS were centered in the northeastern central part of the U.S. (ca. day 240), Eastern Europe (ca. day 245), and Central Asia (ca. day 250), while the latest emergence regions are in the southeastern United States (ca. day 270), southern China (ca. day 275), the Korean Peninsula (ca. day 285), and the Japanese islands (ca. day 270). The observed spatial distribution pattern could be influenced by the climatic conditions and soil types prevalent in these regions. For example, the humid subtropical climate in the southeastern United States, where soils are mostly black or red and cool slowly, combined with abundant rainfall and long frost-free periods [67], results in the local deciduous forests having an early onset of the growing season and a late end of the growing season. In the 5-km resolution GOSIF data, the extracted multi-year average SOS of deciduous forests extends from day 62 to day 142, while the multi-year average EOS extends from day 195 to day 283. The spatial distribution pattern of phenology does not  
Figure 4: Comparison of the phenological metrics of three SIF products with different spatial scales. Green line & white circles: Median and mean of SOS, respectively; Purple line & yellow circles: Median and mean of EOS, respectively. differ significantly from that of the phenology features extracted from the 1-km resolution SIF data. However, the range of SOS/EOS values extracted from the 50-km resolution GOME-2 SIF differed significantly from the two previously mentioned datasets. Specifically, its annual mean SOS is distributed from day \(-6\) to day 92, while the multi-year mean EOS is located between day 190 and day 254. This suggests that the vegetation growth cycle revealed by the GOME-2 SIF is more advanced relative to the high-resolution SIF data, such that the SOS sometimes occurs at the end of the previous year. Figure 5: Spatial distribution of mean values of deciduous forest phenology extracted from 1-km SIF data across the Northern Hemisphere’s middle and high latitudes (2008–2017). Figure 6: Spatial distribution of mean values of deciduous forest phenology extracted from 5-km SIF data across the Northern Hemisphere’s middle and high latitudes (2008–2017). In summary, there are differences in the ability to reveal phenological characteristics of deciduous forests between datasets of different resolutions.