## 2 Materials and Methods  
### Study Sites  
The climate of North America east of 110\({}^{\circ}\) W can be generally divided into tropical monsoon, humid subtropical, and humid continental climates from south to north, while it is mainly a semiarid steppe climate west of 110\({}^{\circ}\) W. Forest types are significantly affected by climate; evergreen coniferous forests are mainly distributed in humid subtropical and semiarid steppe climates, while deciduous broadleaf forests are mainly distributed in humid continental climates.  
The National Ecological Observatory Network (NEON) is a nationwide network established to observe the environment at the regional and continental scales [24]. The network composed of 20 regional networks covers most major forests and water bodies to observe biodiversity and ecosystem functions and plays important roles in studying environmental changes and predicting their trends. From the 20 regional networks, 26 field sites with observations between 2017 and 2019 were considered. The stations are evenly distributed in deciduous broadleaf (DB) forests and evergreen needleleaf (EN) forests and cover as wide longitude and latitude spans as possible to observe the influence of climate on forest phenology. The locations and forest types of the selected field sites were given in NEON [25], as shown in Table 1, and marked in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Domain Number**} & \multirow{2}{*}{**Site Name**} & \multirow{2}{*}{**Latitude**} & \multirow{2}{*}{**Longitude**} & \multirow{2}{*}{**Data**} & \multicolumn{2}{c}{**Satellite**} \\  & & & & & **Cycle (Day)** \\ \cline{5-6}  & & & & & **S1** & **S2** \\ \hline \multicolumn{6}{l}{Deciduous Broadleaf (DB)} \\ \hline D01 & BART & 44.06387 & -71.28738 & 2017–2019 & 6 & 5 \\ D01 & HARV & 42.53691 & -72.17265 & 2017–2019 & 12 & 5 \\ D02 & SCBI & 38.89293 & -78.13949 & 2017–2019 & 6 & 5 \\ D02 & SERC & 38.89008 & -76.56001 & 2017–2019 & 6 & 5 \\ D05 & STEI & 45.50894 & -89.58637 & 2018–2019 & 12 & 5 \\ D05 & TREE & 45.49373 & -89.58572 & 2017–2019 & 12 & 5 \\ D05 & UNDE & 46.23391 & -89.53725 & 2017–2019 & 12 & 5 \\ D06 & UKFS & 39.04043 & -95.19215 & 2019 & 6 & 5 \\ D07 & GRSM & 35.68896 & -83.50195 & 2017–2019 & 6 & 5 \\ D07 & MLBS & 37.37831 & -80.52485 & 2017–2019 & 6 & 5 \\ D07 & ORNL & 35.96413 & -84.82859 & 2017–2019 & 6 & 5 \\ D08 & DELA & 32.54173 & -87.80388 & 2017–2019 & 12 & 5 \\ D08 & LENO & 31.85388 & -88.16122 & 2017–2019 & 12 & 5 \\ D11 & CLBJ & 33.40123 & -97.57000 & 2017–2019 & 6 & 5 \\ \hline \multicolumn{6}{l}{Evergreen Needleleaf (EN)} \\ \hline D02 & BLAN & 39.03370 & -78.04179 & 2017–2019 & 6 & 5 \\ D03 & JERC & 31.19484 & -84.46862 & 2017 & 6 & 5 \\ D03 & OSSB & 29.69828 & -81.99343 & 2017–2019 & 12 & 5 \\ D08 & TALL & 32.95047 & -87.93326 & 2017–2019 & 6 & 5 \\ D10 & RMNP & 40.27590 & -105.54596 & 2017–2019 & 6 & 5 \\ D12 & YELL & 44.95348 & -110.53914 & 2019 & 6 & 5 \\ D16 & ABBY & 45.756243 & -122.33033 & 2018 & 12 & 5 \\ D16 & WREF & 45.82049 & -121.95191 & 2019 & 12 & 5 \\ D17 & SJER & 37.10878 & -119.73228 & 2019 & 6 & 5 \\ D17 & SOAP & 37.03337 & -119.26219 & 2018–2019 & 6 & 5 \\ D19 & BONA & 65.15401 & -147.50258 & 2019 & 12 & 5 \\ D19 & DEU & 63.88112 & -145.75136 & 2017–2019 & 12 & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Information of selected field sites (14 deciduous broadleaf sites and 12 evergreen needleleaf sites) and the corresponding data availability.