Woody encroachment and forest degradation in sub-Saharan Africa's woodlands and savannas 1982–2006

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.

Tropical grassy biomes: linking ecology, human use and conservation

“…Yet, across TGBs, rates of land-use and cover change appear to exceed the effects of climate change [1,11]. With increasing global scarcity of lands for agriculture and increasing food demands [71], land-use intensity is only likely to increase.…”

“…The extent of these vast biomes has shifted with glacial -inter-glacial cycles in response to changing atmospheric CO 2 concentrations, rainfall, rainfall seasonality, temperature and fire [19,21]. Given that all of these aspects of our environment are now changing, and generally at rates not previously observed, extensive alterations in the distribution and dynamics of TGBs over the coming century are inevitable and are likely already being observed ( [1,4,22], see the analysis provided by Stevens et al [23]). …”

“…Historically extensive across the global tropics, tropical grassy biomes (TGBs) are now changing rapidly through high rates of land clearance [1], increasing land-use intensity [2,3], woody encroachment [4] and disruption of the disturbance regimes [5,6] that maintain ecosystem function. These biomes were the cradle of human evolution [7], and in our contemporary world, they support the livelihoods and wellbeing of over one billion people [8].…”

Woody encroachment and its consequences on hydrological processes in the savannah

“…Increases in biomass, stem densities or cover of woody plants in arid and semi-arid environments have been documented over the past decades in various regions around the planet [1][2][3][4][5][6], with a number of proposed abiotic and biotic mechanisms. Among the possible factors behind woody encroachment are changes in rainfall regime [2,4], elevation of atmospheric CO 2 concentration [5,7], invasion by exotic trees [8], changes in disturbance regimes such as suppression of fire [3,7,9] and decline in browsers or overgrazing [3,7,9].…”

The past, present and future of Africa's rainforests

“…Rudel [5] explores the social and economic factors that are driving these patterns of change across African rainforest nations, highlighting, in particular, the importance of oil and mineral extraction in shaping economies and lowering deforestation pressure in rainforests. Mitchard & Flintrop [6] turn attention away from forest loss, and instead review and map the phenomenon of woody encroachment, where rainforests appear to be expanding and tree cover seems to be increasing in some mixed tree-grassed systems.…”