## 4 Discussion  
There is limited (if any) access to ground data on NPP measurement in Iran. Therefore, it is difficult to conduct spatial matching and validate NPP estimates obtained through remote sensing. Previous researches (e.g., [49; 64; 65; 59]) indicate that MODIS data have acceptable accuracy. MODIS sensor-based NPP data are available for only 46% of the areas of Iran's ecosystems. There are no such data for Central Persian desert basins and South Iran Nubo-Sindian desert and semi-desert, which are characterized by a lack of vegetation. In these locations, the degree of carbon sequestration can be accessed via experimental models or ground observations. The average share of terrestrial carbon sequestration of plants in Iran was estimated to be 1.38 kg*C/m\({}^{2}\)/year. This estimate is larger than the one proposed by the authors of [35], who studied a time period ending in 2014 and used average pixels. Unlike some previous research, the median of estimated image values was used in the current study. The numerical values of the pixels in the images are affected by cloud conditions, various land cover conditions, or radiometric errors. Therefore, a mean score may yield a value that is far from the actual focal point. Conversely, the median is less susceptible to the lack of data in a particular time or climate extremes. [PERSON] et al. [40], [PERSON] et al. [66] claimed that forests play a significant role in carbon sequestration through photosynthesis. In a similar vein, the findings of this research showed that the forest ecosystems in the north and northwest of Iran, with broad-leaved evergreen trees, account for a large amount of carbon sequestration. In contrast, the ecosystems containing deserts and salt marshes in central and southern Iran recorded the lowest portion in carbon sequestration due to lack of vegetation. The results of trend analysis revealed no statistically significant trend in more than two-thirds of the available pixels studied in this research. Eastern Anatolian Mountain steppe and the Zagros Mountains forest steppe registered the highest positive trend despite the fact that these ecosystems are undergoing declining rainfall and increasing temperature. The upward trend in these ecosystems, therefore, cannot be connected to climate factors; rather, as pointed out by [PERSON] et al. [67] and [PERSON] et al. [68], the growing trend in carbon sequestration may be attributed to human activities and expanding agriculture area, which play a substantial role in increased vegetation. Our findings in this regard are in conflict with those of [PERSON] and [PERSON]'s [1]. The impact of human activities on the NPP rate depends on the location where the activities occur; thus, destruction of vegetation in a particular place may result in reduced NPP, while it may lead to higher NPP in another spot. The range of NPP variations in Iran is estimated to be about 135 g/m\({}^{2}\)/year, representing severe changes and photosynthesis instability. The highest standard deviation for change in carbon sequestration was observed in the Hyncanianiferous forest ecosystem, which cannot be explained solely in light of climatic variables. Indeed, destructive human activities in this ecosystem (e.g., deforestation) have led to substantial decline in carbon sequestration. On the other hand, climate change has resulted in rising photosynthesis and NPP rate. Increasing trends can be attributed to climate change and human activities. In contrast, radical decline trends in the short term can only be justified in light of deforestation caused by human activities. At the global scale, greenhouse gas emissions (CH\({}_{4}\) and CO\({}_{2}\) in particular) are largely responsible for the Earth's warming [69; 70]. Terrestrial habitats are generally a sink for atmospheric carbon sequestration and greenhouse gas flux modulation. [PERSON] et al.