## 4 Discussion  
### Stability Comparison between DBFs and DNFs  
We found no significant difference between the temporal stability of DBFs and DNFs (Figure 4); the range and distribution were similar. However, when the correlations among the SPEI, TMP, and RAD with TS were disentangled, the results suggested that the TS of the two forest types responded to the water balance and other climate factors differently (Figure 5 and Table 11). For DNFs, water balance was an important factor. The smaller the variation of the interannual SPEI was, the more stable the DNFs would be (Table 11). In contrast, for DBFs, temperature was the limiting factor. High temperatures stressed DBFs' growth. However, shortwave radiation was similarly important for both DNFs and DBFs. Hence, it is possible that ecological factors connected with latitude (e.g., solar radiation and temperature) are impact factors for the temporal stability of both forest biomes, and water may only influence certain forest types. Research based on global remote sensing data and random forest model simulations support this hypothesis. Previous results have illustrated that the mean annual shortwave radiation, mean annual temperature, and precipitation contribute the most to ecosystem stability [42].  
Interestingly, even though the region between 40\({}^{\circ}\) N and 50\({}^{\circ}\) N of North China experienced extreme droughts, the DBFs between the two latitudes still maintained a relatively high level of temporal stability (Figures 11 and 12). This could be explained by the optimal temperature threshold, because the optimal photosynthesis temperature for DBFs was generally higher than the observed max air temperature [43; 44], and droughts caused by high temperatures in this area did not significantly decrease the photosynthesis rate.  
Furthermore, the EVI is a spectral index that reflects the pigment and cell structure characteristics of leaves at the canopy scale. The GPP is the gross photosynthesis rate of an  
Figure 6: The histograms of the legacy length of the SPEI on (**a**) the growing season EVI of all constant pixels, (**b**) the growing season GPP of all constant pixels, (**c**) the growing season EVI for significantly correlated pixels only (\(p<0.05\)), and (**d**) the growing season GPP for significantly correlated pixels only (\(p<0.05\)) during 2001–2018.  
ecosystem. The two datasets stand for different perspectives of vegetation growth, and thus the essential differences between optical and physiological characteristics could be used to explain the discrepancy between the temporal stability from the EVI and GPP. Previous research has shown that the canopy structure of a broadleaf forest was not as sensitive to drought as the xylem formation [42; 45]. Experimental manipulations also have demonstrated that several forest species could acclimate to warming by reducing their respiration at a year-to-decades time scale [46], which partly explains the unbelievable stability of DBFs during extreme droughs and the weak positive correlation between the temporal stability and SD of the SPEI. Our future work will further explore the contributions of temperature, radiation, and water to ecosystem stability through droughs.