# WUE and CO2 Estimations by Eddy Covariance and Remote Sensing in Different Tropical Biomes
## 4 Discussion
Despite its wide use, our results corroborate previous studies reporting the need to use MODIS GPP data with \begin{table}
\begin{tabular}{c c c} \hline
**ForestTypes** & **WUE (g C kg \({}^{1}\)H\({}_{2}\)O)** & **References** \\ \hline Temperate broad-leaved & 5.0 & [PERSON] et al. \\ deciduous & 5.40 & [PERSON] et al. [66] \\ Douglas-fr & 5.79 & **This study** \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._cution for interannual and intra-annual studies, as suggested by [PERSON] et al. [55]. Our study revealed that humid biomes present lower WUE, and drier biomes present higher WUE, with decreasing values with the onset of the wet season and maximum values in the dry season. This result is consistent with previous studies in other environments ([PERSON] et al., 2017; [PERSON] et al., 2014; [PERSON] et al., 2008) [60; 62; 74]. [PERSON] and [PERSON] [75] claim that forest ecosystems presented WUE values comparable or superior to arid (such as caatinga) and shrub biomes (such as Cerrado and Pantanal), with an intermediate value of 0.6-1.2 g C kg\({}^{-1}\) H\({}_{2}\)O for humid forest biomes. The average WUE for the Pantanal is within this range. WUE responds differently to seasonal water availability in each biome. Overall, more productive biomes present high WUE ([PERSON] et al., 2015) [76]. In sparsely vegetated areas, such as the Cerrado (savannas) and the Pantanal wetlands, WUE varies greatly throughout the year, despite presenting lower values than the Caatinga. From January to April, when precipitation is high in all sites (except Caatinga), the Amazon site featured an increase in WUE daily values, while the Cerrado WUE decreased, and the Pantanal WUE remained unaltered. From May, rainfall triggers a sharp decrease in WUE over these regions until September. Our results reveal important implications for the understanding of climate change effects on carbon and water exchange processes in tropical biomes, because the projected reduction in water availability over these sites due to the increasing number of dry days [77] may lead to an increase in WUE at the ecosystem level. However, increasing temperatures may further increase or reduce ecosystem WUE at the monthly scale. Consequently, changes in ecosystem WUE due to climate change will depend on the relative impact of these changes in precipitation and temperature. Caatinga WUE is highly dependent on water availability, with lower values and variability in the wetter months and higher values and variability in the drier months. The results found here corroborate other studies in other parts of the Caatinga biome [78; 79; 80; 81; 82], and it is important to report that owing to measurement difficulties, few studies have systematically compared global patterns of WUE of terrestrial ecosystems across different biomes or have analyzed the seasonal variability of WUE in relation to weather conditions, because ecosystem WUE is slightly different from plant WUE [61]. Plant physiologists consider WUE at leaf or stand scales and are mainly interested in relations between total or above-ground biomass, stem biomass or net CO\({}_{2}\) uptake to transpiration or evapotranspiration (ET), and although uncertainties associated with site-to-site variation in site quality criteria, flux measurement methods, calculations and data quality control still exist, ongoing standardization and quality assurance efforts enable global integration with other tools [61].