# Fires Drive Long-Term Environmental Degradation in the Amazon Basin
## 2 Materials and Methods
### Gross Primary Production (GPP)  
Among the various environmental services provided by the Amazon rainforest, the removal of C from the atmosphere via C sequestration and storage with increasing GPP is one of the most important [51; 52; 53]. GPP is related to the total photosynthetic uptake of C by vegetation and plays a key role in the C balance between the atmosphere and biosphere [54; 55]. Thus, it is necessary to accurately and quantitatively estimate the temporal and spatial variations in the GPP to understand ecosystem functions and C estimates in response to climate change [56; 57].  
The MOD17A2 product is a cumulative compound of GPP values based on the efficient use of solar radiation by vegetation (\(\epsilon\)). Primary production is linearly related to photosynthetically active absorbed radiation (APAR). APAR is the product of the incident photosynthetically active radiation (PAR) in the visible spectral range of 0.4-0.7 \(\upmu\)m (assumed to be 45% of the total incident solar radiation) and the fraction of photosynthetically active radiation (FPAR) absorbed by plant cover [51; 58]. GPP is defined as  
\[\text{GPP}=\epsilon\ \times\ \text{PAR}\times\text{FPAR}, \tag{1}\]  
Figure 1: Study area located in the Amazon Basin (South America) with fire foci from January to August 2019 obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) data. MODIS tile centers are marked by a red circle with the corresponding reference code (Terra/Aqua [50]).  
where FPAR is the fraction of PAR (MJ m\({}^{-2}\) d\({}^{-1}\)). PAR can be calculated from the total shortwave solar radiation (SWRad; MJ m\({}^{-2}\) d\({}^{-1}\)) as follows:  
\[\text{PAR = 0.45\times\text{SWRad}.} \tag{2}\]  
One of the major challenges of using such a model is obtaining the magnitude of the light-use efficiency (LUE), or \(\varepsilon\), over large areas because of its dependence on environmental factors and the vegetation itself. One way to overcome this problem is to relate \(\varepsilon\) to its maximum value (\(\varepsilon_{\text{max}}\)) by adding environmental contributions synthesized by the minimum air temperature (\(T_{\text{min}}\)) and the state of the water in the vegetation [59]. The \(\varepsilon\) value (g C m\({}^{-2}\) d\({}^{-1}\) MJ\({}^{-1}\)) can be calculated as follows:  
\[\varepsilon=\varepsilon_{\text{max}}\times\text{T}_{\text{misscalar}}\times \text{VPD}_{\text{scalar}}, \tag{3}\]  
where \(\varepsilon_{\text{max}}\) (g C m\({}^{-2}\) d\({}^{-1}\) MJ\({}^{-1}\)) is the maximum LUE specified by the Biome Properties Look-Up Table based on biome type, and T\({}_{\text{s}}\) and VPD\({}_{\text{s}}\) are the attenuation scalars influenced by the daily minimum temperature (T\({}_{\text{min}}\), \({}^{\circ}\)C) and daytime vapor pressure deficit (VPD\({}_{\text{avg}}\), Pa), respectively [58; 60].  
In this study, the Moderate Resolution Imaging Spectroradiometer (MODIS) GPP version 5.0, as well as the Google Earth Engine platform, were used for the period of 1 January 2001-22 September 2019. The pixel values with reference to the MODIS image digital numbers were converted into biophysical values (kg C m\({}^{-2}\)) via multiplication with a scale factor (0.0001) [58]. [PERSON] et al. [58] showed that MODIS/GPP data correlated well with the GPP records of 12 flow towers (in situ) in North America, showing that MOD17A2 is a reliable data source for this variable. Meanwhile, the GPP values were transformed from the accumulated eight-day values to daily values for the entire extension of the Amazon Basin, and consequently, all the uses that comprise it. To calculate the C absorption for the entire study area, each pixel and its GPP value were calculated for each year of the study period (kg C m\({}^{-2}\) year\({}^{-1}\)).