## 6 Conclusions  
Our study used robust bootstrapping regression method to successfully derive monthly net primary production (NPP) of the Okavango, Kwando, and upper Zambezi (OKZ) catchments for the long period 1982-2010 from the third generation Global Inventory Monitoring and Modeling System (GIMMS3g) normalized difference vegetation index (NDVI) data, with the Moderate Resolution Imaging Spectroradiometer (MODIS) NPP product. Then we investigated the variations in NPP over 29 years and disentangled the relationships between NPP and precipitation factor by both savanna types and precipitation intervals. A highly positive correlation between NPP and GIMMS3g NDVI within our study area provides the basic premise for retrieving NPP effectively through relatively simple statistical method. The use of single satellite-derived metric has great appeal for ease of application, assessing the effects of uncertainties caused by input data, and simplifying the exploration of the underlying mechanism [52]. Our findings show that there is a universal decrease in NPP across our study area and NPP of the study area as a whole has reduced significantly at a rate of -2.13 g\(\cdot\)C\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\), corresponding to a decrease of 6.7% over 29 years. The reduction in NPP might decrease the food supply for grazing and browsing herbivores, as the ecosystem is known as their habitats and food sources. It is also a possible indicator of the degradation of savanna ecosystem, but degradation expressed by decreasing vegetation coverage, bush encroachment, changes in community composition, and reduced rain-use efficiency is still worthy of special research. Given the importance of carbon sequestration of savannas, a decline in NPP regionally has significant global implications. It remains to be seen if this multi-decadal trend, resulting from the newly available GIMMS3g dataset, is also found in other regions of the globe.  
Our research teased out the relationships between NPP and precipitation by different savanna ecosystem types. There is a significant positive relationship between mean annual NPP and mean annual precipitation (MAP) over bush and grassland savanna, but no significant relationship is witnessed over tree savanna. This is useful for understanding landscape level changes in vegetation amounts, and for improving the characterization of the impact of climate change on such landscapes. Our study also got some innovative points in terms of the relationship between mean annual NPP and MAP. Notably, the relationship between mean annual NPP and MAP varies with increases in MAP, characterized as a linear relationship that breaks down when MAP exceeding around 850-900 mm. The existence of this tipping point still needs to be confirmed at other areas and broader extents. The use of such 30-year GIMMS3g NDVI time series will greatly improve our ability to determine trends, shifts and potential landscape level changes in vegetation growth, such as land degradation, and to systematically quantify the relationships between key drivers of vegetation growth and the resultant vegetation cover over different temporal and spatial scales [17, 28, 53, 54, 55].