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Reduction of tree cover in West African woodlands and promotion in semi-arid farmlands

Abstract

Woody vegetation in farmland acts as a carbon sink and provides ecosystem services for local people, but no macroscale assessments of the impact of management and climate on woody cover exist for drylands. Here we make use of very high spatial resolution satellite imagery to derive wall-to-wall woody cover patterns in tropical West African drylands. Our study reveals that mean woody cover in farmlands along all semi-arid and sub-humid rainfall zones is 16%, on average only 6% lower than in savannahs. In semi-arid Sahel, farmland management promotes woody cover around villages (11%), while neighbouring savannahs had on average less woody cover. However, farmlands in sub-humid zones have a greatly reduced woody cover (21%) as compared with savannahs (33%). In the region as a whole, rainfall, terrain and soil are the most important (80%) determinants of woody cover, while management factors play a smaller (20%) role. We conclude that agricultural expansion causes a considerable reduction of trees in woodlands, but observations in Sahel indicate that villagers safeguard trees on nearby farmlands which contradicts simplistic ideas of a high negative correlation between population density and woody cover.

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Fig. 1: High-resolution woody cover mapping and validation with field data.
Fig. 2: Predicting woody cover.
Fig. 3: Determinants and patterns of woody cover.
Fig. 4: Land management impacts on woody cover.

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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. R.F. and M.B. acknowledge funding from the Danish Council for Independent Research (DFF) grant ID: DFF–6111-00258. We thank M.-J. Lambert for providing the farmland mask. We thank the Centre de Suivi Ecologique for providing field data from Senegal. We thank DigitalGlobe for providing commercial satellite data within the NextView license program.

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M.B., R.F., S.H., P.H. and K.R. designed the study. M.B., X.T. and F.T. conducted the analyses with support by L.K., O.M., K.R., R.F., S.H., M.D. and P.H. The data were provided by C.T., J.D., K.M., M.D., L.K., C.V. and P.H.; K.R. and M.B. drafted the manuscript with contributions by all authors.

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Correspondence to Martin Brandt.

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Brandt, M., Rasmussen, K., Hiernaux, P. et al. Reduction of tree cover in West African woodlands and promotion in semi-arid farmlands. Nature Geosci 11, 328–333 (2018). https://doi.org/10.1038/s41561-018-0092-x

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