## 3 Results and Discussion  
### SIF, GPP, and Climatic Factors at the Tower Scale  
The GOME-2 SIF global climatology shows large spatial variability in seasonal cycles congruent with general climate and latitudinal variations, indicating the influence of underlying environmental controls on vegetation phenology and productivity (Figure 2). In the northern hemisphere, a relatively abrupt and persistent SIF increase in early spring indicates the start of the growing season, consistent with a relaxation of cold temperature constraints to growth following winter dormancy [26]. The start and end of the growing season observed by satellite SIF is consistent with local flux carbon tower measurements and also different from the traditional vegetation indices, including MODIS [26]. We also note that the SIF data distinguish northern hemisphere cropland patterns and seasonality that are also different from traditional vegetation indices records [27; 35]. The SIF record also shows a large seasonal amplitude and distinctive growing season over extratropical regions consistent with characteristic seasonal variations in daylength, and suitable temperature and moisture conditions for growth [45]. The SIF record shows a relatively small but distinct seasonality in the tropics, which is larger in sub-tropical savanna and grassland ecosystems, consistent with a regional gradient in water-related environmental constraints to productivity [46; 47]. To investigate and clarify relationships between SIF, GPP, and underlying environmental control factors at the stand level, we compared the SIF monthly climatology with corresponding GPP observations from selected tower sites representing a range of global climate and land cover types (Table 1). The tower site comparisons showed statistically significant correspondence (\(p<0.05\)) between the SIF and GPP observations at most sites, except for two low productivity grassland and savanna sites in Australia where the average GPP was less than 2 g C m\({}^{-2}\) day\({}^{-1}\). On average, the SIF and GPP correspondence was strongest for mixed forests, followed by deciduous broadleaf and evergreen \begin{table}
\begin{tabular}{c c c c c} \hline
**IGBP** & **No. of Sites** & **Mean Annual Temp (\({}^{\circ}\)C)** & **AI (P/PET)** & **SIEGPP (\(R^{2}\), \%)** \\ \hline EBF & 3 & \(17\pm 7\) & \(1.07\pm 0.4\) & \(50\pm 32\) \\ ENF & 11 & \(8\pm 7\) & \(1.13\pm 0.4\) & \(83\pm 17\) \\ DBF & 8 & \(11\pm 6\) & \(0.98\pm 0.2\) & \(87\pm 12\) \\ MF & 4 & \(7\pm 3\) & \(1.18\pm 0.2\) & \(90\pm 16\) \\ OSH & 3 & \(4\pm 16\) & \(0.45\pm 0.2\) & \(59\pm 41\) \\ WSA & 4 & \(22\pm 6\) & \(0.50\pm 0.1\) & \(79\pm 15\) \\ SAV & 6 & \(23\pm 5\) & \(0.33\pm 0.2\) & \(62\pm 37\) \\ GRA & 16 & \(11\pm 7\) & \(1.04\pm 0.8\) & \(76\pm 27\) \\ WET & 4 & \(12\pm 11\) & \(0.85\pm 0.1\) & \(78\pm 14\) \\ CRO & 8 & \(11\pm 3\) & \(0.83\pm 0.3\) & \(82\pm 15\) \\ \hline \end{tabular}
\end{table} 
Table 1: Average and standard deviation (SD) in correspondence (\(R^{2}\)) between GOME-2 SIF and collocated tower GPP observations (SIF.GPP) with annual air temperature and climate aridity (AI) information for 67 tower sites stratified by their IGBP land cover class, including: EBF (Evergreen Broadleaf Forest); ENF (Evergreen Needleleaf Forest); DBF (Deciduous Broadleaf Forest); MF (Mixed Forest); OSH (Open Shrubland); WSA (Woody Savanna); SAV (Savanna); GRA (Grassland); WET (Wetland) and CRO (Cropland). Figure 2: Global monthly SIF climatology derived from the GOME-2 satellite record from 2007–2015. needleleaf forests, and cropland sites (\(R^{2}>80\%\)). SIF and tower GPP correspondence was lower, but still significant for the evergreen broadleaf forest (EBF) sites (\(R^{2}=50\%\)). The weaker SIF and GPP correspondence at lower productivity and tropical EBF sites may reflect greater SIF uncertainty for sparsely vegetated, low productivity sites [48] and tropical rainforest sites characterized by persistent cloud cover [28].