## 3 Results and Discussion
### SIF, GPP, and Climatic Factors at the Tower Scale
On the other hand, weak vertical mixing of CO\({}_{2}\) during the nighttime, especially in tropical forests [49; 50], may be a source of uncertainty in flux tower GPP derived using model based gap filling methods. Relatively weak regression relationships in wet tropical forest areas may also be an artifact of lower characteristic seasonal climate variability in these areas. Overall, the significant and relatively strong correspondence between the SIF and tower GPP observations representing major land cover types and biomes is consistent with previous studies showing that SIF is proportional to GPP [26; 27; 28; 29; 30; 33]. The partial correlations between the PAR adjusted SIF records (SIF\({}_{\text{P}}\)) and selected MERRA2 climate variables (VPD, Tmin, SM) at the tower site locations are plotted in two-dimensional climate space defined by mean annual temperature and climate acidity (AI) along with temperature and AI conditions for all other grid cells within the global domain (Figure 3). Box plots showing the spatial distributions of tower SIF\({}_{\text{P}}\) and climate correlations by land cover class are also shown. Most of the moisture-limited sites are located in regions with mean annual temperatures above 20 \({}^{\circ}\)C, with low precipitation and high potential evapotranspiration (AI < 0.5); the moisture limitations to SIF\({}_{\text{P}}\) at these sites were generally represented by respective inverse or direct correlations with VPD and SM. However, the SIF\({}_{\text{P}}\) and SM relationships were highly variable among tower sites. Soil moisture was directly correlated with SIF\({}_{\text{P}}\) for semi-arid (0.2 < AI < 0.5) savanna (SAV) and woody savanna (WSA) sites, with average correlations of 0.61 and 0.48, respectively. In contrast, relatively humid climate sites (AI > 0.9), including forest (EBF, DBF, ENF, MF) and cropland (CRO) towers, show predominantly inverse correlations with soil moisture. The weaker correlation between SIF\({}_{\text{P}}\) and soil moisture in forest ecosystems is consistent with reduced water supply restrictions supporting greater vegetation growth in wetter climates. The reduced SM dependence in forests may also reflect characteristic deeper rooting depths for trees [51] and additional water storage capacity in woody biomass beyond root zone soil moisture. The relationship between SIF\({}_{\text{P}}\) and soil moisture is highly variable for open shrubland (OSH), grassland (GRA), and wetland (WET) sites, including both the sign and strength of correlations; these land cover types also showed a large diversity in temperature and moisture conditions. For example, open shrubland covers much of central Australia and is characterized by a warm and dry climate (AI < 0.5) where SIF\({}_{\text{P}}\) is directly proportional to SM. In contrast, a major portion of northern taiga and tundra is also characterized as OSH, but with a much colder climate, where SIF\({}_{\text{P}}\) is more strongly correlated with Tmin. The relatively strong SIF\({}_{\text{P}}\) correlations with SM for arid sites also suggests that soil moisture imposes additional constraints on productivity beyond VPD, and that including soil moisture as an additional control in productivity models may enhance model GPP estimates in arid regions. The SAV and WSA sites showed a negative relationship between SIF\({}_{\text{P}}\) and VPD, but the correlation was 17% weaker than the relationship between SIF\({}_{\text{P}}\) and SM. Tower sites in colder climate areas generally showed a positive correlation with VPD. Among the EBF sites analyzed, the tropical rain forest site showed positive SIF\({}_{\text{P}}\) correlations with both VPD and Tmin, consistent with minimal moisture constraints to plant growth and the near-exponential relationship between temperature and atmosphere moisture holding capacity. Other EBF sites located in more arid regions showed a negative SIF\({}_{\text{P}}\) relationship with VPD and Tmin.