## 2 Materials and Methods  
### Study Area and Biome Classification  
The study area covers land areas (\(\sim\)41 million km\({}^{2}\)) located north of 30\({}^{\circ}\)N, encompassing several land cover types from the Arctic/subarctic tundra in the high latitudes to temperate forests and grasslands in the south. Regions located north of 30\({}^{\circ}\)N have manifested a strong seasonality for climatic thermal conditions and vegetation productivity in the growing season. To make our results comparable to many other studies, the seasons were defined as: spring (March, April, and May), summer (June, July, and August) and autumn (September, October, and November). The biome classification used in this study was based on the global 0.05\({}^{\circ}\) land cover data set from the Terra and Aqua Combined Moderate Resolution Imaging Spectroradiometer (MODIS) Land Cover Climate Modeling Grid (MCD12C1) Version 6 Yearly L3 Product [26] and the Koppen-Geiger climate map (1 \(\times\) 1 km\({}^{2}\)) of the world [27; 28] for the year 2011. The MODIS land cover data set uses the biome classification scheme of the International Geosphere-Biosphere Programme (IGBP), which was to generate 17 land classes to meet the needs of the IGBP core science projects relevant to climate, carbon cycle, and others [29]. To separate alpine and high arctic tundra from low-laying temperate grasslands or shrublands, the MODIS grasslands were divided into temperate and arctic grasslands and the MODIS shrublands were divided into temperate and arctic and boreal shrublands. Overall, there are 12 terrestrial biomes that were included in the study area: evergreen needleleaf forests (ENF), evergreen broadleaf forests (EBF), deciduous needleleaf forests (DNF), deciduous broadleaf forests (DBF), mixed forests (MF), arctic and boreal shrublands (ABS), temperate shrublands (TS), savanna (SV), arctic grasslands (AG), temperate grasslands (TG), permanent wetlands (PW), and croplands (CRO) (Figure A1). A more thorough description of each biome type is given in Table 1.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Short Name** & **Description** \\ \hline \multirow{3}{*}{Evergreen needleleaf forests} & \multirow{3}{*}{ENF} & Dominated by Evergreen conifer trees, (canopy \(>\) 2 m). \\  & & Tree cover \(>\) 60\% \\  & & Dominated by evergreen \\ \multirow{3}{*}{Evergreen broadleaf forests} & \multirow{3}{*}{EBF} & broadleaf and palmate trees \\  & & (canopy \(>\) 2 m). Tree cover \(>\) 60\% \\ \multirow{3}{*}{Deciduous needleleaf forests} & \multirow{3}{*}{DNF} & Dominated by deciduous needleleaf \\  & & (lach) trees (canopy \(>\) 2 m). \\ \multirow{3}{*}{Deciduous broadleaf forests} & \multirow{3}{*}{DBF} & Tree cover \(>\) 60\% \\  & & Dominated by deciduous broadleaf \\ \multirow{3}{*}{Mixed forests} & \multirow{3}{*}{MF} & Mixed between deciduous and \\  & & evergreen (40–60\% of each \\ \multirow{3}{*}{Arctic and boreal shrublands} & \multirow{3}{*}{ABS} & Tree cover \(>\) 20\% \\  & & Dominated by woody perennials \\ \multirow{3}{*}{Temperate shrublands} & \multirow{3}{*}{TS} & (1–2 m height) including \\  & & Dominated by woody perennials \\ \multirow{3}{*}{Savanna Arctic grasslands} & \multirow{3}{*}{AG} & (1–2 m height) including \\  & & Dominated by woodens annuals (\(<\)2 m). \\ \multirow{3}{*}{Temperate grasslands} & \multirow{3}{*}{TG} & Dominated by herbaceous annuals (\(<\)2 m). \\ \multirow{3}{*}{Permanent Wetlands} & \multirow{3}{*}{PW} & Permanently inundated lands with 30–60\% \\ \multirow{3}{*}{Corplands} & \multirow{3}{*}{CRO} & water cover and \(>\)10\% vegetated cover. \\ \multirow{3}{*}{
\begin{tabular}{c} Croplands \\ \end{tabular} } & \multirow{3}{*}{CRO} & At least 60\% of area is cultivated cropland. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Definitions of the 12 terrestrial biomes were developed by [26]. Grass- and shrublands were further divided in Arctic/boreal and temperate groups according to [27; 28].