## 5 Discussion
Our results found a universal decline in NPP across the study area from 1982 to 2010. These conclusions are in accordance with previous studies which have reported that NPP reduces during both periods 1982-1999 and 2000-2009 in this region of the world [4, 10]. Our research highlights the NPP change of land-cover classes in the longer temporal extent (1982-2010) as this new dataset has become available and given the large scale variability, both intra and inter annually within most of these trends a longer time series approach is most definitely required. Almost half of our landscape revealed significant trends in declining NPP amount for bush and grassland savanna, and tree savannas showed declines in about a third of its extent. These trends become more significant in the more southerly areas. There are also a limited number of areas representing increases in NPP though these most often relate to the presence of water bodies and are linked more to flood regime patterns. The overall decline in NPP across this region is \(-2.13\) g\(\cdot\)C\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\) (\(p<0.1\)), corresponding to a decrease of 6.7% over 29 years (1982-2010). Although multiple mechanisms (e.g., nitrogen deposition, CO\({}_{2}\) fertilization, climate change) have contributed to the change in NPP, NPP variability has been primarily attributed to water availability in arid and semi-arid areas where water is a limiting resource [12]. Precipitation as the critical importance of water source for plant use is well recognized for savanna ecosystems. Although soil moisture, precipitation--evapotranspiration influence directly plant water availability, it seems to be more useful to disentangle the NPP-precipitation relationship as precipitation is more easily and directly measured at different geographical scales and more frequently as the input for process-based and diagnostic models. Our research found that the NPP of tree savanna is less influenced by MAP, where the NPP of bush and grassland savanna has a significantly positive relationship with MAP. This is consistent with previous studies which have reported that the relationship between NPP and precipitation varies with land cover [14, 46]. Savanna ecosystems are characterized by the co-dominance of two contrasting plant life forms, trees and grasses, which are manifest by their relative representation across savanna types. [PERSON] _et al._[47, 48] suggests that MAP is the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover shows a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors are accounted for. [PERSON] _et al._[49] find that the impact of a suite of environmental covariates on NDVI varies from grass-dominated regions with MAP \(<750\) mm to tree-dominated regions with MAP \(>950\) mm. Our study also found that the relationship between NPP and MAP varies with the increase in MAP. The linear relationship is observed when MAP is less than 750-850 mm, while the linear increase in NPP with MAP levels off when MAP exceeds the range. Whether this range is applicable to other savanna ecosystems still needs further research. For example, [PERSON] and [PERSON] [50] states that the proportionality breaks down at a precipitation of 300 mm\(\cdot\)yr\({}^{-1}\). Estimating the interannual variations in NPP quantitatively is important to support policy development for food security and resource conservation as the degradation of the vegetation cover in savanna ecosystems is increasing. Our study identified the pixels that have experienced a reduction in vegetation production, which may provide an early warning of land degradation. However, our results cannot disentangle the reduction in NPP caused by a number of factors, including climate variations (mainly precipitation), natural disturbances (mainly fire), and human activities (grazing, agriculture, _etc._).