## 3 Results
### Trends in Vegetation Phenology  
The trend analysis of vegetation phenology indicators in Northeastern China from 2001 to 2020 (Figure 5). The findings revealed that the trends of the SOS\({}_{\text{SIF}}\) and the SOS\({}_{\text{NDVI}}\) in western Hulunbuir, northern Daxinganling, and Changbai Mountain were diametrically opposed, with the former showing a weak delayed trend and the latter showing a weak advanced trend. Both the EOS\({}_{\text{SIF}}\) and the EOS\({}_{\text{NDVI}}\) showed a delayed trend in southern Daxinganling and Liaohe Plain. The vegetation EOS\({}_{\text{SIF}}\) showed an opposite trend to the EOS\({}_{\text{NDVI}}\) in other areas; the former showed a predated trend, and the latter showed a delayed trend. Advance (delay) of the SOS and delay (advance) of the EOS would lead to prolonging (shortening) of the LOS accordingly. Therefore, the trend of the LOS\({}_{\text{SIF}}\) and the LOS\({}_{\text{NDVI}}\) in each region is consistent with that of the EOS. That is, the LOS\({}_{\text{SIF}}\) and the LOS\({}_{\text{NDVI}}\) both showed a delayed trend in southern Daxinganling and Liaohe Plain, while the LOS\({}_{\text{SIF}}\) showed an advanced trend and the LOS\({}_{\text{NDVI}}\) showed a delayed trend in other regions. The POP\({}_{\text{SIF}}\) and the POP\({}_{\text{NDVI}}\) showed a delayed trend in Hulunbeier, southern Daxinganling, and parts of Liaohe Plain, while the POP\({}_{\text{SIF}}\) showed an early trend and the POP\({}_{\text{NDVI}}\) showed a weak delayed trend in other regions.  
By calculating the average of trends in different phenological indicators in the study area (Table 2), the SOS\({}_{\text{SIF}}\) had an overall advanced trend, and the trend of the SOS\({}_{\text{NDVI}}\) was delayed, both of which were contrary. The EOS\({}_{\text{SIF}}\) and the EOS\({}_{\text{NDVI}}\) showed a latency trend. However, the delayed trend of the EOS\({}_{\text{NDVI}}\) was more pronounced. The advancement of the SOS\({}_{\text{SIF}}\) and the delay of the EOS\({}_{\text{SIF}}\) led to an overall trend of prolongation of the LOS\({}_{\text{SIF}}\). The delayed trend of the SOS\({}_{\text{NDVI}}\) was much larger than that of the EOS\({}_{\text{NDVI}}\), so the total trend of the LOS\({}_{\text{NDVI}}\) was shortened. The POP\({}_{\text{SIF}}\) was consistent with the POP\({}_{\text{NDVI}}\), and both indicated a weak trend of advance.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Phenological Parameters** & **Data Type** & **Mean Value/d** & **Change Trend Mean/d** \\ \hline SOS & SIF & 138 & \(-\)0.24 \\  & NDVI & 128 & 2.09 \\ EOS & SIF & 254 & 0.07 \\  & NDVI & 290 & 0.47 \\ LOS & SIF & 116 & 0.30 \\  & NDVI & 162 & \(-\)1.61 \\ POP & SIF & 210 & \(-\)0.13 \\  & NDVI & 214 & \(-\)0.07 \\ \hline \end{tabular}
\end{table}
Table 2: Average values of the phenological parameters and the change trends based on SIF and NDVI in Northeastern China from 2001 to 2020.  
Figure 4: Phenology of different vegetation types in Northeastern China from 2001 to 2020. (**a**) SOS of different vegetation types based on SIF and NDVI; (**b**) EOS of different vegetation types based on SIF and NDVI; (**c**) LOS of different vegetation types based on SIF and NDVI; (**d**) POP of different vegetation types based on SIF and NDVI (CV: cultivated vegetation; GL: grassland; MD: meadow; SC: scrub; SP: steppe; CF: coniferous forest; BF: broad-leaved forest; Mix B-C: conifer-broadleaved mixed forest).  
Figure 5: \(Cont.\)