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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References
Stebbing, E. P. The encroaching Sahara: the threat to the West African colonies. Geogr. J. 85, 506–519 (1935).
Aubréville, A. Climats, Forêts et Désertification de l’Afrique Tropicale (Société d’éditions géographiques, maritimes et coloniales, 1949).
Lüdeke, M. K. B., Moldenhauer, O. & Petschel-Held, G. Rural poverty driven soil degradation under climate change: the sensitivity of the disposition towards the Sahel syndrome with respect to climate. Environ. Model. Assess. 4, 315–326 (1999).
Ribot, J. C. A history of fear: imagining deforestation in the West African dryland forests. Glob. Ecol. Biogeogr. 8, 291–300 (1999).
Mbow, C., Mertz, O., Diouf, A., Rasmussen, K. & Reenberg, A. The history of environmental change and adaptation in eastern Saloum-Senegal-driving forces and perceptions. Glob. Planet. Change 64, 210–221 (2008).
Hansfort, S. L. & Mertz, O. Challenging the woodfuel crisis in West African woodlands. Hum. Ecol. 39, 583–595 (2011).
Lykke, A. M., Kristensen, M. K. & Ganaba, S. Valuation of local use and dynamics of 56 woody species in the Sahel. Biodivers. Conserv. 13, 1961–1990 (2004).
Couteron, P. & Kokou, K. Woody vegetation spatial patterns in a semi-arid savanna of Burkina Faso, West Africa. Plant Ecol. 132, 211–227 (1997).
Rasmussen, K., Fog, B. & Madsen, J. E. Desertification in reverse? Observations from northern Burkina Faso. Glob. Environ. Change 11, 271–282 (2001).
Tappan, G., Sall, M., Wood, E. & Cushing, M. Ecoregions and land cover trends in Senegal. J. Arid Environ. 59, 427–462 (2004).
Reij, C., Tappan, G. & Belemvire, A. Changing land management practices and vegetation on the Central Plateau of Burkina Faso (1968–2002). J. Arid Environ. 63, 642–659 (2005).
Mortimore, M. J. & Adams, W. M. Farmer adaptation, change and crisis in the Sahel. Glob. Environ. Change 11, 49–57 (2001).
Fairhead, J. & Leach, M. False forest history, complicit social analysis: rethinking some West African environmental narratives. World Dev. 23, 1023–1035 (1995).
Gonzalez, P. Desertification and a shift of forest species in the West African Sahel. Clim. Res 17, 217–228 (2001).
Rasmussen, K. et al Environmental change in the Sahel: reconciling contrasting evidence and interpretations. Reg. Environ. Change 27, 673–680 (2015).
Tucker, C. J. & Nicholson, S. E. Variations in the size of the Sahara Desert from 1980 to 1997. Ambio 28, 587–591 (1999).
Olsson, L., Eklundh, L. & Ardo, J. A recent greening of the Sahel-trends, patterns and potential causes. J. Arid Environ. 63, 556–566 (2005).
Brandt, M. et al. Ground- and satellite-based evidence of the biophysical mechanisms behind the greening Sahel. Glob. Change Biol. 21, 1610–1620 (2015).
Kaptué, A. T., Prihodko, L. & Hanan, N. P. On regreening and degradation in Sahelian watersheds. Proc. Natl Acad. Sci. USA 112, 12133–12138 (2015).
Hansen, M. C. et al. High-resolution global maps of 21st-century forest cover change. Science 342, 850–853 (2013).
Sexton, J. O. et al. Global, 30-m resolution continuous fields of tree cover: Landsat-based rescaling of MODIS vegetation continuous fields with lidar-based estimates of error. Int. J. Digit. Earth 6, 427–448 (2013).
Good, S. P. & Caylor, K. K. Climatological determinants of woody cover in Africa. Proc. Natl Acad. Sci. USA 108, 4902–4907 (2011).
Browning, D. M. et al. Emerging technological and cultural shifts advancing drylands research and management. Front. Ecol. Environ. 13, 52–60 (2015).
Axelsson, C. R. & Hanan, N. P. Patterns in woody vegetation structure across African savannas. Biogeosciences 14, 3239–3252 (2017).
Hill, M. J. & Hanan, N. P. Ecosystem Function in Savannas: Measurement and Modeling at Landscape to Global Scales (CRC Press, Boca Raton, 2010).
Funk, C. et al. The climate hazards infrared precipitation with stations-a new environmental record for monitoring extremes. Sci. Data 2, 150066 (2015).
Tatem, A. J. WorldPop, open data for spatial demography. Sci. Data 4, 170004 (2017).
Hengl, T. et al. Mapping soil properties of Africa at 250 m resolution: random forests significantly improve current predictions. PLoS ONE 10, e0125814 (2015).
Lambert, M.-J., Waldner, F. & Defourny, P. Cropland mapping over Sahelian and Sudanian agrosystems: a knowledge-based approach using PROBA-V time series at 100-m. Remote Sens. 8, 232 (2016).
Bayala, J., Sanou, J., Teklehaimanot, Z., Kalinganire, A. & Ouédraogo, S. Parklands for buffering climate risk and sustaining agricultural production in the Sahel of West Africa. Curr. Opin. Environ. Sustain. 6, 28–34 (2014).
Mertz, O. et al. Climate factors play a limited role for past adaptation strategies in West Africa. Ecol. Soc. 15, 25 (2010).
Bucini, G. & Hanan, N. P. A continental-scale analysis of tree cover in African savannas. Glob. Ecol. Biogeogr. 16, 593–605 (2007).
Devine, A. P., McDonald, R. A., Quaife, T. & Maclean, I. M. D. Determinants of woody encroachment and cover in African savannas. Oecologia 183, 939–951 (2017).
Kulmatiski, A. & Beard, K. H. Woody plant encroachment facilitated by increased precipitation intensity. Nat. Clim. Change 3, 833–837 (2013).
Brandt, M. et al. Human population growth offsets climate driven woody vegetation increase in sub-Saharan Africa. Nat. Ecol. Evol. 1, 0081 (2017).
Mertz, O., Lykke, A. & Reenberg, A. Importance and seasonality of vegetable consumption and marketing in Burkina Faso. Econ. Bot. 55, 276–289 (2001).
Sankaran, M. et al. Determinants of woody cover in African savannas. Nature 438, 846–849 (2005).
Tiffen, M., Mortimore, M. & Gichuki, F. More People, Less Erosion: Environmental Recovery in Kenya (Wiley, Chichester, 1994).
Bucini, G., Saatchi, S., Hanan, N., Boone, R. B. & Smit, I. Woody cover and heterogeneity in the Savannas of the Kruger National Park, South Africa. In 2009 IEEE International Geoscience and Remote Sensing Symp. 4 IV-334–IV-337 (IEEE, 2009).
Herrmann, S., Wickhorst, A. & Marsh, S. Estimation of tree cover in an agricultural parkland of Senegal using rule-based regression tree modeling. Remote Sens. 5, 4900–4918 (2013).
Roerink, G. J., Menenti, M. & Verhoef, W. Reconstructing cloudfree NDVI composites using Fourier analysis of time series. Int. J. Remote Sens. 21, 1911–1917 (2000).
Jonsson, P. & Eklundh, L. TIMESAT—a program for analyzing time-series of satellite sensor data. Comput. Geosci. 30, 833–845 (2004).
Breman, H. & Kessler, J.-J. Woody Plants in Agro-Ecosystems of Semi-Arid Regions: with an Emphasis on the Sahelian Countries (Springer, Berlin, 1995).
Hiernaux, P. et al. Woody plant population dynamics in response to climate changes from 1984 to 2006 in Sahel (Gourma, Mali). J. Hydrol. 375, 103–113 (2009).
Hiernaux, P. & Ayantunde, A. The Fakara: a Semi-Arid Agro-Ecosystem Under Stress (ILRI, 2004).
Qi, J., Chehbouni, A., Huete, A. R., Kerr, Y. H. & Sorooshian, S. A modified soil adjusted vegetation index. Remote Sens. Environ. 48, 119–126 (1994).
Haralick, R. M., Shanmugam, K. & Dinstein, I. Textural features for image classification. IEEE Trans. Syst. Man Cyb. SMC-3, 610–621 (1973).
Shimada, M. et al. New global forest/non-forest maps from ALOS PALSAR data (2007–2010). Remote Sens. Environ. 155, 13–31 (2014).
Dierckx, W. et al. PROBA-V mission for global vegetation monitoring: standard products and image quality. Int. J. Remote Sens. 35, 2589–2614 (2014).
Breiman, L. Arcing the Edge (Statistics Department, University of California, Berkeley, 1997).
Brandt, M. et al. Woody plant cover estimation in drylands from Earth Observation based seasonal metrics. Remote Sens. Environ. 172, 28–38 (2016).
Chen, J. et al. Global land cover mapping at 30 m resolution: A POK-based operational approach. ISPRS J. Photogramm. Remote Sens. 103, 7–27 (2015).
World Database on Protected Areas v.2007 (World Conservation Union and UNEP-World Conservation Monitoring Centre, 2007).
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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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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DOI: https://doi.org/10.1038/s41561-018-0092-x
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