## 3 Results  
### Spatial Distribution of Vegetation Phenology  
We retrieved vegetation phenology parameters from 2001 to 2020 in Northeastern China using SIF and MODIS NDVI, then estimated the multi-annual mean of each pixel for spatial analysis (Figure 3). The results showed that there were significant differences between the phenology results based on SIF and NDVI data. The SOS\({}_{\text{SIF}}\) mainly concentrated on days 113-170 (Figure 3a), with an area share of 97.9%; the SOS\({}_{\text{NDVI}}\) mainly concentrated on days 110-157 (Figure 3b), with an area of 93.4%. The SOS\({}_{\text{SIF}}\) and the SOS\({}_{\text{NDVI}}\) of the vegetation were higher in the central plain and the eastern Sanjiang plain than in other areas. The EOS\({}_{\text{SIF}}\) was mainly concentrated on days 240-265 (Figure 3c), with an area of 96.6%, and the EOS\({}_{\text{SIF}}\) were predated in the coniferous forests in northern Daxinganling and the broad-leaved forests in Xiaoxinganling than in other areas. The EOS\({}_{\text{NDVI}}\) was primarily focused on days 278-299 (Figure 3d), with an area of 91.3%, and the EOS of Hulunbuir was the latest. The LOS\({}_{\text{SIF}}\) was mainly concentrated on days 83-149 (Figure 3e), accounting for 96.6% of the total area. The LOS\({}_{\text{NDVI}}\) was mainly concentrated on days 130-191 (Figure 3f), with an area of 92.4%. They first occurred in Songliao and Sanjiang Plain. The POP\({}_{\text{SIF}}\) was mainly concentrated on days 190-226 (Figure 3g), accounting for 97.7% of the total area. The POP\({}_{\text{NDVI}}\) was primarily concentrated on days 202-228 (Figure 3h), accounting \begin{table}
\begin{tabular}{c c c c c} \hline
**Trend/Sustainability** & **SAS** & **WAS** & **WS** & **SS** \\ \hline
**AT** & AT-SAS & AT-WAS & AT-WS & AT-SS \\
**DT** & DT-SAS & DT-WAS & DT-WS & DT-SS \\ \hline \end{tabular}
\end{table} 
Table 1: Classification of future trends of vegetation phenology in Northeastern China. for 94.0% of the total area, with the POP\({}_{\text{SIF}}\) and the POP\({}_{\text{NDVI}}\) occurring most recently in southern Daxinganling, Songliao, and Sanjiang plains. Figure 3: Spatial distribution of multi-year mean vegetation phenology based on SIF and NDVI in Northeastern China from 2001 to 2020. (**a**) SOS (start of season) extracted by SIF(SOS\({}_{\text{SIF}}\)); (**b**) SOS extracted by NDVI(SOS\({}_{\text{NVDI}}\)); (**c**) EOS (end of season) extracted by SIF(EOS\({}_{\text{SIF}}\)); (**d**) SOS extracted by NDVI(EOS\({}_{\text{NVDI}}\)); (**e**) LOS extracted by SIF(LOS\({}_{\text{SIF}}\)); (**f**) LOS (length of season) extracted by NDVI(LOS\({}_{\text{NVDI}}\)); (**g**) POP (position of pop) extracted by SIF(POP\({}_{\text{SIF}}\)); (**h**) POP extracted by NDVI(POP\({}_{\text{NVDI}}\)). By calculating the average of the phenological indicators in the study area (Table 2). The results showed that the SOS\({}_{\text{SIF}}\) was closer to the SOS\({}_{\text{NDVI}}\), which was about 10 days later than the latter. The mean value of the EOS\({}_{\text{SIF}}\) predated the EOS\({}_{\text{NDVI}}\) by about 40 days. The LOS\({}_{\text{SIF}}\) was generally shorter than the LOS\({}_{\text{NDVI}}\) because the EOS\({}_{\text{SIF}}\) was generally more advanced than the EOS\({}_{\text{NDVI}}\). The POP\({}_{\text{SIF}}\) was nearly consistent with the POP\({}_{\text{NDVI}}\). The multi-year phenological mean values of different vegetation types in Northeastern China from 2001 to 2020 were analyzed (Figure 4). It can be seen that the SOS\({}_{\text{SIF}}\) of Grassland (GL) was more advanced than the SOS\({}_{\text{NDVI}}\), while the SOS\({}_{\text{SIF}}\) of other vegetation was later than the SOS\({}_{\text{NDVI}}\). Meadow (MD) SOS\({}_{\text{SIF}}\) and SOS\({}_{\text{NDVI}}\) had the largest difference, 17 days in advance, while the difference between Scrub (SC) SOS\({}_{\text{SIF}}\) and SOS\({}_{\text{NDVI}}\) was the smallest and advanced by only 1 day.