Abstract
We review the literature and find 16 studies from across Africa's savannas and woodlands where woody encroachment dominates. These small-scale studies are supplemented by an analysis of long-term continent-wide satellite data, specifically the Normalized Difference Vegetation Index (NDVI) time series from the Global Inventory Modeling and Mapping Studies (GIMMS) dataset. Using dry-season data to separate the tree and grass signals, we find 4.0% of non-rainforest woody vegetation in sub-Saharan Africa (excluding West Africa) significantly increased in NDVI from 1982 to 2006, whereas 3.52% decreased. The increases in NDVI were found predominantly to the north of the Congo Basin, with decreases concentrated in the Miombo woodland belt. We hypothesize that areas of increasing dry-season NDVI are undergoing woody encroachment, but the coarse resolution of the study and uncertain relationship between NDVI and woody cover mean that the results should be interpreted with caution; certainly, these results do not contradict studies finding widespread deforestation throughout the continent. However, woody encroachment could be widespread, and warrants further investigation as it has important consequences for the global carbon cycle and land–climate interactions.
Highlights
We currently have little certainty about the fluxes of carbon in tropical ecosystems: the error bars on estimates of carbon fluxes to and from the land surface are almost as large as the fluxes themselves [1,2,3,4]
It should be noted that we did not attempt to collate the studies finding deforestation or degradation: the aim of this study is to investigate the location of woody encroachment, not to directly assess its magnitude compared with anthropogenic forest loss
All 16 studies were found on pixels that showed a positive Normalized Difference Vegetation Index (NDVI) trend over the series, and seven of these 16 were found on pixels where this trend was identified as significant using the criteria in §3a (iii). From both the literature review and the Global Inventory Modeling and Mapping Studies (GIMMS) analysis, it is clear that both forest loss and gain are occurring widely throughout Africa. It appears that the area of land undergoing woody encroachment may be comparable or even larger than areas where a significant loss of forest cover is occurring; subsetting the GIMMS analysis most of the increase is in the woodland and savannas of sub-Saharan Africa north of the equator, whereas in the Miombo woodland regions, south of the equator, forest loss appears to be dominating
Summary
We currently have little certainty about the fluxes of carbon in tropical ecosystems: the error bars on estimates of carbon fluxes to and from the land surface are almost as large as the fluxes themselves [1,2,3,4]. While it is likely to be true that forest losses exceed forest gains in the tropics, the uncertainties in all the estimation methods used are high [1,9,13] and may be biased towards the detection of deforestation as opposed to woody encroachment or recovery This bias towards the detection of forest loss is due to three reasons: (i) most monitoring bodies are set up with the purpose of mapping forest losses, so emphasize this in their methods, (ii) the sudden, definite nature of forest loss as opposed to the gradual nature of forest regrowth and (iii) the difficulties of assessing changes in mixed tree – grass systems, where significant increases in canopy cover may not trigger a change in a broad vegetation class. These biases may be exacerbated in Africa, as mixed tree–grass systems dominate (it is the location of two-thirds of the world’s savanna [14]); historical ground data are especially rare; and the capacity of environmental and forestry departments to perform mapping
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
