**3. Results**

In general, the multi-year average phenology of natural vegetation from SIF and EVI has consistent spatial patterns in China, with a delaying pattern in SOS and an advanced pattern in EOS from southeast to northwest (Figure 2a–d), which is consistent with previous studies reported by Wang et al. [35]. Furthermore, we found substantial differences between the SIF and the EVI in the derived phenological metrics (Figure 2e,f). Specifically, the SOS derived from SIF is generally later than that from EVI, which accounts for 70% of the total natural vegetated area, except for those areas of evergreen forest in the south. The EOS from SIF is generally earlier than that from EVI, accounting for 87% of the total natural vegetated area.

Then, the differences in phenology derived from SIF and EVI were presented statistically across different climate-limited areas (Figure 3). We found that the differences in SOS generated using SIF and EVI (here denoted as ΔSOS) mainly ranged from 0 to 20 days (Figure 3a), while the differences in EOS generated using SIF and EVI (here denoted asΔEOS) mainly distributed between −30 and −10 days (Figure 3b). This indicates that the difference in EOS generated using SIF and EVI is generally larger than that generated using SOS. For both SOS and EOS, the largest difference of phenology from SIF and EVI (i.e., ΔSOS > 30 days or ΔEOS < −30 days) occurred in temperature-limited areas and water-limited areas, while the difference of phenology from SIF and EVI at no climatic limitations was the smallest.

**Figure 2.** The spatial pattern of multi-year average phenology of natural vegetation from SIF (**<sup>a</sup>**,**<sup>c</sup>**) and EVI (**b**,**d**) and their differences, i.e., SOS/EOS derived from SIF subtracted by that derived from EVI (**<sup>e</sup>**,**f**).

**Figure 3.** The differences in SOS (**a**) or EOS (**b**) derived from SIF and EVI among different climatic limitation areas.

As shown in Figure 4, we found that increasing the climatic limitation index could result in larger differences in SOS and EOS from SIF and EVI for each dominant climatic

limitation area. This finding could explain why the difference in phenology derived from SIF and EVI with no climatic limitations is the smallest in Figure 3. In terms of different climatic limitations, we found that the slope of linear regression of ΔSOS or ΔEOS to water-limitation index was the highest, followed by temperature-limitation index, and then radiation-limitation index, which might be a consequence of divergent responses of SIF and EVI to different climatic or environmental constraints. In addition, the climatic limitation indices were more correlated with ΔSOS than with ΔEOS. A likely cause is that autumn phenology is more complex than spring phenology, as it may be affected by multiple climatic factors, thus weakening the relationship between one dominant climatic limitation index and ΔEOS.

**Figure 4.** Scatterplot of differences of SOS (**first row**) and EOS (**second row**) derived from SIF and EVI versus climatic limitation index at temperature-dominant area (**<sup>a</sup>**,**d**), water-dominant area (**b**,**<sup>e</sup>**) and radiation-dominant area (**<sup>c</sup>**,**f**). The black line shows the linear regression between phenology from SIF or EVI and climatic limitation indices; r is the correlation coefficient of the linear regression.

Table 1 quantified the relative contributions of phenology from SIF and EVI to ΔSOS and ΔEOS under the dominant climatic limitation areas. We found that the phenology extracted using SIF was more correlated with temperature, water and radiation limiting factors than that using EVI, making it the main cause of the difference of phenology from SIF and EVI. Especially in water-limited areas, the contributions of phenology from SIF to ΔSOS or ΔEOS is much larger than those from EVI (SOS: 90.00% vs. 10.00%, EOS: 80.00% vs. 20.00%), as phenology derived from EVI had a low correlation with the water limitation index. These different responses of SIF and EVI to the water limitation index attributed to the differences in phenology from SIF and EVI in the water-limited area, which was also shown in Figure 4.

**Table 1.** The relationships between phenology from SIF or EVI and climatic limitation indices. s: slope of the linear regression between phenology from SIF or EVI and climatic limitation indices. r: correlation coefficient of the linear regression. Cr: contribution ratio of phenology from SIF or EVI to the difference of phenology derived from SIF and EVI under climatic limitations.

