## 4 Results
### Spatial Pattern of Net Primary Production (NPP) Variability from 1982 to 2010  
Generally, NPP decreases from north to south across the study area (Figure 7b). The NPP of tree savanna is higher than that of bush and grassland savanna (Table 1). NPP in the upper reaches of the Zambezi River is lower than that in the same latitudes as this region is covered by floodplain grasses which have lower NPP (Figure 7b). The northern portion is characterized by a small coefficient of variation (CV), whereas the southern section is characterized by a large CV (Figure 11) which suggests that NPP in the south has experienced higher variability. Bush savanna has a higher variability of NPP and tree savanna has the smallest, which is indicated by the mean CV values of each land cover class (Table 1).  
\begin{table}
\begin{tabular}{c c c c} \hline  & **Tree** & **Bush** & **Grassland** \\ \hline Mean predicted NPP (g\(\cdot\)C\(\cdot\)m\({}^{-2}\)\(\cdot\)yr\({}^{-1}\)) & 1,142.3 & 858.2 & 804.2 \\ Mean MODIS NPP (g\(\cdot\)C\(\cdot\)m\({}^{-2}\)\(\cdot\)yr\({}^{-1}\)) & 985.6 & 743.5 & 711.3 \\ CV & 0.11 & 0.12 & 0.12 \\ Increase (\%) & 9.85 & 7.57 & 9.17 \\ Decrease (\%) & 33.53 & 54.40 & 44.33 \\ Not significant (\%) & 56.63 & 38.02 & 46.50 \\ \hline \end{tabular}
\end{table}
Table 1: The statistical measures of net primary production (NPP) of different savanna types.  
Figure 10: Scatter plot of annual aggregated net primary production (NPP) of model estimates with annual Moderate-resolution Imaging Spectroradiometer (MODIS) NPP. These points were randomly selected across our study area.  
There is a general trend of decreasing NPP across the study region as indicated by the universal significant and negative \(\tau\) values (Figure 12). About 9% of the study area shows a significant and increasing trend, there is about 40% showing a significant and decreasing trend, and the remaining 48% shows no significant trend at all (Bare lands and water areas are excluded from analysis, so the total is slightly less than 100%). The percentage of increase, decrease, and insignificant pixels for tree savanna is 10%, 33%, and 57%, respectively; the percentage of increase, decrease, and insignificant pixels for bush savanna is 8%, 54%, and 38% respectively; and the percentage of increase, decrease, and insignificant pixels for grassland savanna is 9%, 44%, and 47%, respectively. So in terms of multi-decadal trends, overall the regional NPP has declined across this thirty year period, and when broken down into savanna vegetation types, the decline in tree savanna NPP is the least, the decline in bush savanna NPP is the highest of all the classes and the decline in grassland savanna NPP is between the other two. This suggests that the bush and grassland savanna is potentially representative of a more degraded ecosystem over time, as represented by the NPP decrease during the period 1982-2010, which would also be reflected in the literature, with declining rates of open grasslands and increasing bush encroachment across this region, specifically in the more southern regions dominated by these savanna ecosystems.  
Figure 11: Spatial pattern of the coefficient of variation (CV) of annual net primary production (NPP) (for water year from September through August) from 1982 to 2010.