# A Modeling Application for GHG Fluxes Estimates in Betel Nuts Plantations in Taiwan
## 3 Results
The need for a spatial interpolation was unavoidable; therefore, approximation of variables, such as temperature and radiation measured in the few stations present on the territory, could strongly affect the results of these models. Furthermore, models always try to represent and simplify complex environmental phenomena, such as the gas exchanges between vegetation and atmosphere; in fact, VPM does not consider the laws of physics behind the turbulent dynamics of the air over the canopy boundary layer. Finally, it should be said that, usually, exponential laws can better describe environmental phenomena, but in this case, an easier linear equation was used. In fact, as mentioned before, the relation between GPP and absorbed solar radiation showed an exponential trend. For these reasons, this study represents the first step of a wide scale C cycle estimation, able to give the magnitude order of C fluxes, starting from the only open-source data available. A similar study was carried out for grapevine in Italy, with different validation data and, obviously, different results. The comparison of the two cultivars in the two countries was interesting albeit limited. Studies that consider tropical and temperate species are often complicated because they compare different growth forms and different climates [28; 29]. These climate differences are reflected also in the vegetation phenology, as growth cycles of temperate vegetation are generally associated with temperature, whereas growth cycles of tropical vegetation tend to be associated with precipitation [30; 31]. The choice to study different months of the year was driven by the radiation efficiency use models applied for the GPP and NEP calculation. The periods, from January to March in Taiwan and from June to July in Italy, represent the most cloudless seasons and the time frame with more solar radiation available even though the light duration differs considerably. Furthermore, NEP represented 45% of GPP in Taiwan, while in Italy, NEP was 36% of GPP (Table 3). This result showed that for vineyards in Italy the respiration rate was much greater than the photosynthesis rate. This could be explained by different plant behaviors due to different moisture conditions. Italian summer is usually characterized by drought, so plants close their stomata during the day, limiting water losses and evapotranspiration but also photosynthesis and C uptake; while at night, they open the stomata, allowing evapotranspiration and respiration [32]. Figure 3: Representation of the GPP variability as a function of EVI (**a**), absorbed radiation (**b**,**c**), and evapotranspiration (**d**). Another difference was the distribution of the cultivars. Usually, betel nuts occupy small areas with high fragmentation patterns and require steep versus, while vineyards are characterized by bigger expansions on gentler slopes [33]. These different patterns can influence the vegetation reflectance detected by a satellite. The Lansat 8 satellite requires a minimal area of 900 m\({}^{2}\) for sufficient data resolution, while betel nuts can occupy also very small areas. The Sentinel 2 satellite's resolution of 10 m turned out to be excellent for vineyards reflectance detection; in fact, in this study, there was no need to detect every single grape plant because calculations were carried on cultivar scale and the aim was to evaluate the gas exchanges of the whole vineyards, which included also the grassy inter-rows. The cultivar management is also different: In vineyards, farmers can choose to leave grassy inter-rows or not, changing the CO\({}_{2}\) absorption pattern. The pesticides usage also distinguishes the wine grapes cultivation.