Forest extent and deforestation in tropical Africa since 1900

Abstract

Accurate estimates of historical forest extent and associated deforestation rates are crucial for quantifying tropical carbon cycles and formulating conservation policy. In Africa, data-driven estimates of historical closed-canopy forest extent and deforestation at the continental scale are lacking, and existing modelled estimates diverge substantially. Here, we synthesize available palaeo-proxies and historical maps to reconstruct forest extent in tropical Africa around 1900, when European colonization accelerated markedly, and compare these historical estimates with modern forest extent to estimate deforestation. We find that forests were less extensive in 1900 than bioclimatic models predict. Resultantly, across tropical Africa, ~ 21.7% of forests have been deforested, yielding substantially slower deforestation than previous estimates (35–55%). However, deforestation was heterogeneous: West and East African forests have undergone almost complete decline (~ 83.3 and 93.0%, respectively), while Central African forests have expanded at the expense of savannahs (~ 1.4% net forest expansion, with ~ 135,270 km2 of savannahs encroached). These results suggest that climate alone does not determine savannah and forest distributions and that many savannahs hitherto considered to be degraded forests are instead relatively old. These data-driven reconstructions of historical biome distributions will inform tropical carbon cycle estimates, carbon mitigation initiatives and conservation planning in both forest and savannah systems.

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Fig. 1: Palaeo-proxy sites and locations of historical maps used as inputs for modelling the forest extent in the year ad 1900.
Fig. 2: Modelled distribution of forest and savannah for ad 1900 and ad 2000.

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Acknowledgements

Financial support for this study was provided by a National Science Foundation grant (DMS-1615531) to A.C.S., by the Sessel Fund and by another anonymous donor to Yale University for A.C.S. and J.C.A., and by the Yale Climate and Energy Institute to M.A.J. The authors thank C. Favier and J. Kaplan for early discussions and G. Aleman for help with the map extraction and processing.

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Authors

Contributions

J.C.A. and A.C.S. designed the study and analyses. J.C.A. assembled and calibrated the palaeo-data. M.A.J. developed and ran the models. J.C.A. and A.C.S. co-wrote the paper, with feedback and methods contributions from M.A.J.

Corresponding author

Correspondence to Julie C. Aleman.

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The authors declare no competing financial interests.

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Supplementary information

Supplementary Information

Supplementary Tables 1–3, Supplementary Results, Supplementary Figures 1–7, Supplementary References.

Life Sciences Reporting Summary

Supplementary Data 1

All palaeo-proxy records used for past forest extent reconstructions. This Excel file contains the palaeo-proxy records that were used in this study, with information on the site name, location, type of proxy, value of proxy, reconstructed biome and the publication.

Supplementary Data 2

Modern phytolith records used in biome calibration. This Excel file contains the modern phytolith records that were used for the calibration procedure, with information on the sample name, location, D/P value, the ecosystem type and the publication.

Supplementary Data 3

Modern δ13C of soil organic matter records used in biome calibration. This Excel file contains the modern δ13C of soil organic matter records that were used for the calibration procedure, with information on the sample name, location, δ13C value, the ecosystem type and the publication.

Supplementary Data 4

Rates of forest change per country for a tree cover threshold of 65%, based on a probability threshold of 0.5. This Excel file contains the data about the median forest area in 1900 and 2000 per country modelled for a tree cover threshold of 65% and based on a probability threshold of 0.5, and the 2.5 and 97.5 quantile confidence intervals of the estimates of relative and absolute forest changes resulting from the bootstrapping procedure.

Supplementary Data 5

Rates of forest change per country for a tree cover threshold of 70%, based on a probability threshold of 0.5. This Excel file contains the data about the median forest area in 1900 and 2000 per country modelled for a tree cover threshold of 70% and based on a probability threshold of 0.5, and the 2.5 and 97.5 quantile confidence intervals of the estimates of relative and absolute forest changes resulting from the bootstrapping procedure.

Supplementary Data 6

Rates of forest change per country for a tree cover threshold of 75%, based on a probability threshold of 0.5. This Excel file contains the data about the median forest area in 1900 and 2000 per country modelled for a tree cover threshold of 75% and based on a probability threshold of 0.5, and the 2.5 and 97.5 quantile confidence intervals of the estimates of relative and absolute forest changes resulting from the bootstrapping procedure.

Supplementary Data 7

This R file contains the code for running the models that compute the probabilities of forest presence in 1900 and 2000, for assigning the presence of forest or savannah to each grid cell based on a probability threshold of 0.5, and for quantifying forest change for each country.

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Aleman, J.C., Jarzyna, M.A. & Staver, A.C. Forest extent and deforestation in tropical Africa since 1900. Nat Ecol Evol 2, 26–33 (2018). https://doi.org/10.1038/s41559-017-0406-1

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