Abstract
In recent decades, investigations of NPP (net primary production) or proxies here of (normalized difference vegetation index, NDVI) and land degradation in Sahelian West Africa have yielded inconsistent and sometimes contradicting results. Large-scale, long-term investigations using remote sensing have shown greening and an increase in NPP in locations and periods where specific, small scale field studies have documented environmental degradation. Our purpose is to cast some light on the reasons for this phenomenon. This investigation focuses on the south of Ouagadougou, Burkina Faso, a city undergoing rapid growth and urban sprawl. We combine long-term MODIS (moderate resolution imaging spectroradiometer) image analysis of NDVI between 2002 and 2009, and by using high resolution satellite images for the same area and a field study, we compare trends of NDVI to trends of change in different categories of land cover for a selected number of MODIS pixels. Our results indicate a strong, positive association between changes in tree cover vegetation and trends of NDVI and moderate association between man-made constructions and trends of NDVI. The observed changes are discussed in relation to the unique processes of urban sprawl characterizing Ouagadougou and relative to their spatiotemporal scale.
Highlights
IntroductionThe past decade has seen a number of studies investigating long-term trends in vegetation productivity for various parts of the world using satellite remote sensing data [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]
Several studies have established an empirical, as well as theoretical, relationship between NDVI and the vegetation productivity of a given area, and it has been shown that the temporal integral of NDVI throughout the growing season in semi-arid parts of the world correlates to the net primary production (NPP) [19,20,21,22,23,24,25,26,27]
An interesting pattern of decreasing vegetation productivity is found in the peri-urban areas, whereas the center is characterized by a general greening
Summary
The past decade has seen a number of studies investigating long-term trends in vegetation productivity for various parts of the world using satellite remote sensing data [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] Common among these studies are the methods applied and the data types upon which the analysis has been built. This relationship has been utilized in several of the above-mentioned studies of long-term trends in vegetation productivity, where NDVI is calculated for large areas for many years of data as a proxy for vegetation productivity
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