## 1 Introduction
[PERSON] and [PERSON] (1975) carried out the first NPP assessment on a global scale through conducting a regression analysis based on the data related to temperature and simple annual measurement of actual evapotranspiration in millimeters gleaned from around 1000 weather stations. Placing the data in an experimental equation and conducting interpolation, they estimated global NPP, which was equal to 118 million metric tons per year of biomass [26]. Over the past five decades, remote sensing on spatial and temporal scales has played an important role in quantifying carbon flux and estimating biomass reserves [27; 28; 29; 30; 31]. Some studies which have used this method include [32; 33; 34; 35; 36], which examined the impact of global changes on the dynamism of carbon flux in plants. Previous studies have primarily concentrated on the effect of precipitation and temperature on NPP changes in climatic boundaries. In these studies, the study areas have been initially classified in light of climatic features. Then, the influence of changes in climatic elements on NPP has been explored based on geographical locations [37; 35]. Some studies have also examined changes in a particular ecosystem [38; 39; 40]. There is a considerable spatial and temporal variation in NPP across the globe, which is basically influenced by climate, land cover, and land use practices [28]. It is essential to conduct further studies in other areas to draw a more comprehensive picture and make better decisions. The first step in this regard is identifying fluctuations in ecosystem NPP patterns, followed by monitoring and extracting spatiotemporal variation patterns. Ecologically, two major landscapes in Iran are desert and mountain. As one of the largest countries in world with complex topography, distinct climate areas, and massive plant and animal diversity, Iran supports various ecosystems, which often plays an important role in preserving biodiversity throughout the world. For instance, North Forests and coral reefs in south coastal regions have built a lot of ecosystems and great genetic diversity. Additionally, several Iranian wetlands are globally significant, hosting large populations of migratory birds for wintering. Considering the spatial extent, the climatic diversity distribution of NPP is very high in Iran, particularly in the northern and northwestern parts of the country. In contrast, the eastern and southeastern regions have lower NPP values such that the Northern Forests NPP amounts to 1.3, and in the case of central deserts, it is almost 0. During recent years, the major quantitative changes in NPP are caused by the pure effects of climate changes and intensified anthropogenic activities in Iran. Given the spatial extent, climatic diversity, and lack of relevant data in Iran, few studies have focused on NPP in this country. In addition, there is no comprehensive study on spatiotemporal variations in NPP in different ecosystems and the impact of droughts on this index. To partially address this gap, the primary objective of this study was to delineate spatiotemporal variation patterns in terrestrial carbon flux of plants in different ecosystems of Iran using remote sensing and statistical procedures. The study also aimed to examine the impact of carbon flux variations on agricultural droughts. The findings can help authorities make better decisions regarding risk management to reduce fluctuations in carbon flux.