## 2 Method and Material
### Evaluation of Remotely Sensed Phenological Metrics  
We used six VI-based methods to extract onset and offset dates. A brief description of each method is listed in Table 1. Although some previously published methods used EVI (e.g., [25]), and other methods used NDVI (e.g., [19]), we tested all methods with both VIs. NDVI and EVI were calculated following Equations (1) and (2), respectively, using the eight-day 500 m MODIS surface reflectance product (code: MOD09A1) acquired from the USGS website [40]. \[NDVI=\frac{\rho_{\mathit{NIR}}-\rho_{\mathit{RED}}}{\rho_{\mathit{NIR}}+\rho_{ \mathit{RED}}} \tag{1}\]  
\[EVI=G\times\frac{\rho_{\mathit{NIR}}-\rho_{\mathit{RED}}}{\rho_{\mathit{NIR}}+ C_{1}\times\rho_{\mathit{RED}}-C_{2}\times\rho_{\mathit{BLUE}}+L} \tag{2}\]  
where \(\rho_{\mathit{NIR}}\), \(\rho_{\mathit{RED}}\) and \(\rho_{\mathit{BLUE}}\) are surface reflectance in the near-infrared, red, and blue bands, respectively, and \(L=1\), \(C_{1}=6\), \(C_{2}=7.5\), and \(G\) (gain factor) = 2.5. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Abbreviation** & **Description** & **Example** & **Source** \\ \hline \multirow{6}{*}{MIDOPOINT} & VI is normalized to a range of & 0.7 & \\  & 0–1. Onset is defined as the & 0.6 & \\  & DOY when normalized VI & 0.5 & \\  & exceeds 0.5 in the spring. Offset & \\  & is defined as the DOY when & \\  & normalized VI decreases below & \\  & 0.5 in the autumn. & \\ \hline \multirow{6}{*}{LOGISTIC1} & VI time series is fitted using & \\  & logistic function. Then, the rate & \\  & of change in curvature of fitted & \\  & function is calculated. Onset is & \\  & defined as the DOY when the & \\  & rate of change in curvature & \\  & rate of change in curvature & \\  & in the spring. Offset is defined & \\  & as the DOY when the rate of & \\  & change in curvature reaches & \\  & the first local minimum & \\  & in the autumn. & \\ \hline \multirow{6}{*}{LOGISTIC2} & VI time series is fitted using & \\  & logistic function. Onset is & \\  & defined as the DOY when fitted & \\  & VI exceeds 50\% amplitude & \\  & between the minimum and & \\  & maximum in the spring. Offset & \\  & is defined as the DOY when & \\  & fitted VI decreases & \\  & below 50\% amplitude between & \\  & the minimum and maximum in & \\  & the autumn. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of the remote sensing methods for retrieving phenology. Because phenology derived from different data products could produce different results even if the same method were applied [28], we compared phenology derived from the MOD09A1 product with that derived from the 16-day vegetation indices product (code: MOD13A1; Figure S1 and Tables S1 and S2 in Supplementary Material) and eight-day nadir BRDF-adjusted reflectance (NBAR) product  
\begin{table}
\begin{tabular}{l l l} \hline
(code: MCD43A4; Figure S2 and Tables S3 and S4 in Supplementary Material), both also having a spatial resolution of 500 m. MOD09A1 showed the best performance (_i.e._, with inter-annual variability, which is consistent with [28]), therefore we only show results from MOD09A1 for the remainder of this paper. We included in our comparison the land surface dynamic product (code: MCD12Q2), which was derived using one of the methods evaluated here (_i.e._, LOGISTIC1, Table 1), but with NBAR EVI as input, in order to show how different data sources may affect the results. We did not consider the quadratic model [21], which involves temperature, because it is fundamentally different from the nonlinear fitting methods purely based on VI and sometimes fails to capture the offset. Methods based on arbitrary thresholds such as NDVI0.2 and NDVI0.3 [27] (_i.e._, 0.2 and 0.3 were used as NDVI threshold to determine onset) were also excluded, because the difference between sensors could yield large discrepancies in the range of VIs. Our preliminary investigation showed that the MODIS NDVI at our site is sometimes larger than 0.3 over the entire course of a year, which makes it impossible to determine the phenological dates.