Human population growth offsets climate-driven increase in woody vegetation in sub-Saharan Africa

Abstract

The rapidly growing human population in sub-Saharan Africa generates increasing demand for agricultural land and forest products, which presumably leads to deforestation. Conversely, a greening of African drylands has been reported, but this has been difficult to associate with changes in woody vegetation. There is thus an incomplete understanding of how woody vegetation responds to socio-economic and environmental change. Here we used a passive microwave Earth observation data set to document two different trends in land area with woody cover for 1992–2011: 36% of the land area (6,870,000 km2) had an increase in woody cover largely in drylands, and 11% had a decrease (2,150,000 km2), mostly in humid zones. Increases in woody cover were associated with low population growth, and were driven by increases in CO2 in the humid zones and by increases in precipitation in drylands, whereas decreases in woody cover were associated with high population growth. The spatially distinct pattern of these opposing trends reflects, first, the natural response of vegetation to precipitation and atmospheric CO2, and second, deforestation in humid areas, minor in size but important for ecosystem services, such as biodiversity and carbon stocks. This nuanced picture of changes in woody cover challenges widely held views of a general and ongoing reduction of the woody vegetation in Africa.

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Figure 1: Changes in woody vegetation and human population over two decades.
Figure 2: Changes in woody cover (VOD) in different humidity zones.
Figure 3: Climatic drivers of changes in woody cover and biomass in sub-Saharan Africa.
Figure 4: Links between changes in woody cover and human population.

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Acknowledgements

M.B. received funding from the European Union’s Horizon 2020 Research and Innovation programme under Marie Sklodowska-Curie grant agreement no. 656564. We thank Y. Y. Liu for providing the VOD data. A.V. and J.P. acknowledge support from the European Research Council Synergy grant ERC-2013-SYG-610028, IMBALANCE-P. R.F. acknowledges funding from the Danish Council for Independent Research (DFF) grant ID: DFF – 6111-00258.

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M.B., R.F., F.T. and A.V. designed the study. M.B. (VOD) and G.S. (ecosystem model) conducted the analyses with support by F.T., J.P., R.F. and J.B.R.P. The paper was drafted by K.R. and M.B. with contributions by all authors.

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Correspondence to Martin Brandt.

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The authors declare no competing financial interests.

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Supplementary Figures 1–6, Supplementary Table 1, Supplementary References. (PDF 2408 kb)

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Brandt, M., Rasmussen, K., Peñuelas, J. et al. Human population growth offsets climate-driven increase in woody vegetation in sub-Saharan Africa. Nat Ecol Evol 1, 0081 (2017). https://doi.org/10.1038/s41559-017-0081

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