## 4 Discussion  
### Inconsistency in Distinguished GPP Trends and Their Responses to Drought  
The climate over the Middle East was characterized by hot to warm temperatures, short (winter) rainy seasons, and a favorable radiation amount and duration for vegetation growth, which combined together to induce a high annual variability in GPP in the study area. However, variability in vegetation productivity is not always linked to climatic factors alone but is also related to human activity [83; 84; 85]. The GPP for several ecosystems showed an increasing trend in the Middle East. Regardless of the ecosystem type, the annual averages of GPP showed an increasing trend between 1982 and 2016. Spatially, these increasing trends were more prominent in the northwest part of the region. Figure 9: **(a,b)** Spatial distribution of ERIs in the Middle East during 1982–2016 as suggested when using the different GPP products. **(c)** Cross-correlation values among the calculated ERIs of several GPP products. In addition, the results indicated a slight decrease in GPP with a slower rate in the central portion of eastern Iran. There were various prospective catalysts for long-term increments in the terrestrial carbon pool (GPP); the most significant ones were climate change, land-use and land-cover changes, and increasing concentrations of atmospheric CO\({}_{2}\)[86]. The distinguished long-term trend in GPP across the region supported the hypothesis of increased CO\({}_{2}\) concentrations in the atmosphere leading to increased CO\({}_{2}\) uptake [87], and the revised EC-LUE model supported a positive effect of CO\({}_{2}\) fertilization on vegetation productivity [23]. Because FluxCom did not account for the CO\({}_{2}\) fertilization effects, the lack of trends did not contradict but rather confirmed this hypothesis [57]. According to [PERSON] et al. [88], there was a somewhat strong agreement between the predicted GPP and in situ FLUXNET-GPP trends. It was nevertheless difficult to reach conclusive judgments on the occurrence, extent, and causes of long-term trends in GPP. As a result, the worldwide impact of atmospheric CO\({}_{2}\) on vegetation GPP remains ambiguous, but the analyzed GPP models provided insights into the long-term GPP trends and may aid in identifying and quantifying the driving variables to increase the long-term GPP. Based on several GPP products falling into the same cluster, our findings suggested a significant degree of uncertainty in the GPP spatial trends in the Middle East, although the explained trends in various GPP datasets were somewhat similar for some models regardless of the magnitude of the trend. One key explanation for the models' low performance and inconsistency in presenting the inter-annual trends is that the influence of environmental controls on vegetation productivity was not fully included in some models [89]. However, there is little agreement on the relevance of various drivers of spatio-temporal changes in GPP. Ecosystem models continue to diverge in their representation of the relative influences of various environmental controls; a better understanding of these controls is essential if future carbon cycle dynamics are to be reliably anticipated [90; 91]. As such, ecosystem models are heavily influenced by a number of variables such as nitrogen deposition, CO\({}_{2}\) concentration, climatic variables, and land-use/land-cover types [92]. Some earth-system models suggested that soil moisture would also have a significant impact on long-term CO\({}_{2}\) absorption [93]. The availability of nutrients, especially phosphorus (P) and nitrogen (N), which play key roles in controlling vegetation photosynthesis and development, is assumed to be a constraint on GPP [94; 95]. Thus, long-term GPP changes reflect a complex balance of numerous interacting biophysical (e.g., temperature and water) and biogeochemical (e.g., carbon dioxide and nutrients) feedbacks. According to [PERSON] et al.