## 3 Results
### Thermal Time to Peak and Peak Height NDVI and VOD Phenometrics
#### 3.2.1 The CxQ Model Derived NDVI Phenometrics  
The NDVI CxQ model fits for all sites are high with 0.78 < R\({}^{2}\) < 0.99. Sample fits appear in Figure 6. Sites at lower latitudes that can support double growing season attain their first peaks at the earliest (<1000 AGDD, e.g., Figure 6d, Cherkessk, RU, 44\({}^{\circ}\)); moving farther north, the seasonal pattern shifts from bimodal to unimodal, and the TTP\({}_{\text{NDVI}}\) is pushed up further to the growing season (between 1000 and 2000 AGDD, e.g., Figure 6c, Zaporiyhzhya 1, UA, 48\({}^{\circ}\) and Figure 6b, Kokshetau 1, KZ, 53\({}^{\circ}\)). Moving towards the northern study sites, the TTP starts to decrease to 1000 AGDD due to the thermal limitation and shorter growing season (e.g., Figure 6a, Cheboksary, RU, 56\({}^{\circ}\)). Figure 7 presented the general relationship of TTP\({}_{\text{NDVI}}\) as a function of latitude for all sites. Note also the NDVI CxQ fit intercept in the different latitudes in Figure 6 that the growing season in the lower latitudes (e.g., Figure 6d) starts a bit earlier than the time we considered in this study (DOY 89 = March 30 (March 29 in leap years)). Figure 6: Average NDVI (2003–2010) as a function of AGDD for cropland sites that encompass the whole latitudinal range: (**a**) 48, RU; (**b**) 28, KZ; (**c**) 11, UA; and (**d**) 1, RU. Coefficients of determination (R\({}^{2}\)) for these sites ranged from 0.88 to 0.97. Average (2003-2010) PH\({}_{\rm NDVI}\) phenology metric in our study croplands ranged between 0.58 and 0.84 (not shown here). NDVI at half-TTP (NAHT) as a function of PH\({}_{\rm NDVI}\) displayed a distinct trend indicating that NDVI in these sites is driven by local climate (Figure 8a). Generally, lower latitude study sites have gentler NDVI gradient time series with longer growing season compared to the steeper NDVI gradient of the shorter summer growing season at higher latitudes. Thus, the difference between PH\({}_{\rm NDVI}\) and NDVI at half-TTP increased as latitude increases, yielding a strong linear fit with R\({}^{2}\) = 0.81 (Figure 8a). However, the PH\({}_{\rm NDVI}\) as a function of latitude did not show a clear trend (Figure 8b). Temperature dominates crop growth from spring to summer, while other variables (such as rainfall) play roles that are more important in crop production. The NAHT as a function of PH\({}_{\rm NDVI}\), for selected sites that were more impacted by the 2003, 2007, and/or 2010 heatwaves, showed a positive correspondence (Figure 9b). The two phenometrics for the mixed forest in the most northern study site Mari El, Russia (site 50 appears as the outlying green circles in upper right corner) displayed strong correspondence with R\({}^{2}\) = 0.94. In contrast, the correspondence between these phenometrics for croplands exhibited more interannual variability. due to the fact that crops are annual vegetation and due to possible crop and land rotation annually or every few years. Moreover, croplands are more vulnerable to climatic variability compared to  
Figure 8: Scatterplot for: PH\({}_{\rm NDVI}\) minus NDVI at half-TTP (NAHT) as a function of latitude (**a**); and PH\({}_{\rm NDVI}\) as a function of latitude (**b**). Both are fitted with linear trend line yielding strong correspondence for the former (R\({}^{2}\) = 0.81), but no clear correspondence for the latter (R\({}^{2}\) = 0.07). Figure 7: Scatterplot for TTP\({}_{\rm NDVI}\) as a function of latitude fitted with a convex quadratic model. There is a significant positive relationship (R\({}^{2}\) = 0.50): as latitude increased, TTP\({}_{\rm NDVI}\) increased in lower latitudes to certain limit and then declined in the northernmost study sites (54–56\({}^{\circ}\)N latitude). the perennial mixed forests. The southern study sites exhibited smaller NAHT and PH\({}_{\text{NDVI}}\) in 2003 due to minimal effect of the European heatwave on croplands in the study area [52].