Abstract
The baobab tree (Adansonia digitata L.) is an integral part of rural livelihoods throughout the African continent. However, the combined effects of climate change and increasing global demand for baobab products are currently exerting pressure on the sustainable utilization of these resources. Here we use sub-metre-resolution satellite imagery to identify the presence of nearly 2.8 million (underestimation bias 27.1%) baobab trees in the Sahel, a dryland region of 2.4 million km2. This achievement is considered an essential step towards an improved management and monitoring system of valuable woody species. Using Senegal as a case country, we find that 94% of rural buildings have at least one baobab tree in their immediate surroundings and that the abundance of baobabs is associated with a higher likelihood of people consuming a highly nutritious food group: dark green leafy vegetables. The generated database showcases the feasibility of mapping the location of single tree species at a sub-continental scale, providing vital information in times when deforestation and climate change cause the extinction of numerous tree species.
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Data availability
Commercial very-high-resolution satellite images were acquired through NASA under the NextView Imagery End User License Agreement. The copyright remains at Maxar, Inc. and redistribution is not possible. The derived products produced by this study are available via Zenodo at https://doi.org/10.5281/zenodo.10934203 (ref. 63). SkySat images were purchased from Planet Labs and cannot be distributed. Tree species data from the Flotrop database can be downloaded at https://doi.org/10.15468/oxunf1. Global Biodiversity Information Facility data are available under https://www.gbif.org. Google building data are available from https://www.sites.research.google/open-buildings/. The GHS-POP dataset can be accessed via https://ghsl.jrc.ec.europa.eu/ghs_pop2023.php. DHS survey data are available from https://www.dhsprogram.com/methodology/survey/survey-display-457.cfm for the dataset in 2014, https://www.dhsprogram.com/methodology/survey/survey-display-489.cfm for the dataset in 2015, https://www.dhsprogram.com/methodology/survey/survey-display-524.cfm for the dataset in 2016 and https://www.dhsprogram.com/methodology/survey/survey-display-581.cfm for the dataset in 2019. CHIRPS rainfall data are available at https://www.chc.ucsb.edu/data/chirps. Temperature data at 0.1° are the ERA5-Land version of land surface temperature, which is available at the Climate Data Store at https://cds.climate.copernicus.eu/. The gridded soil data at 0.05° spatial resolution are available at https://daac.ornl.gov/cgi-bin/dsviewer.pl?ds_id=1247. The human settlement dataset used for the Sahel is the 2.8 arcsec spatial resolution Global Urban Footprint dataset, which is available at https://www.dlr.de/eoc/en/desktopdefault.aspx/tabid-9628/16557_read-40454/. Land-use data are available at https://lcviewer.vito.be/download. The river shapefile provided by the Food and Agriculture Organization (FAO) is available at https://data.apps.fao.org/catalog/iso/b891ca64-4cd4-4efd-a7ca-b386e98d52e8.
Code availability
The tree detection framework based on U-Net is publicly available via Zenodo at https://doi.org/10.5281/zenodo.3978185 (ref. 64). The R script for spatial analysing, boosted regression trees and visualization are available from the corresponding author on reasonable request.
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Acknowledgements
We thank Maxar, Inc. for providing commercial satellite data through the NextView Imagery End User License Agreement. This study was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 947757 TOFDRY) and a DFF Sapere Aude grant (number 9064–00049B) to M.B., the European Union’s Horizon 2020 Research and Innovation Programme (Grant Agreement 853222 FORESTDIET) to L.V.R. and the Villum Fonden through the project Deep Learning and Remote Sensing for Unlocking Global Ecosystem Resource Dynamics (DeReEco, 34306) to R.F. and A.K.
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K.H., M.B. and R.F. conceptualized and designed the study. C.J.T., J.S. and S.S. prepared the satellite data. K.H. and P.H. identified the trees used for training the model. The codes were done by K.H., A.K., S.L., F.R. and M.M. K.H. and M.B. conducted the analyses and performed the visualization. B.d.B. and L.V.R. prepared the DHS data. M.B. and K.H. prepared the paper with support from the other authors.
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Extended data
Extended Data Fig. 1 Examples of baobab trees as seen in Skysat satellite imagery.
All images were captured during the dry season, with data spanning from 2019 to 2023, showcasing baobab trees in various landscapes across the Sahel. Image © 2023 Planet Labs PBC.
Extended Data Fig. 2 Hotspot region of baobab trees from our map and local report in Kayes, Mali.
Local report ref. 37 showing a high density map of baobab trees in Kayes, Mali. This area was also detected by our study as a hotspot region of baobab trees. The basemap was created using ArcGIS® software by Esri.
Extended Data Fig. 3 Baobab densities at country-scale in and outside croplands.
a, Baobab density plot for 8 countries. b, Baobab density for 5 countries with higher densities. The color of each circle represents the proportion of cropland in the country. In Senegal, Niger, Nigeria, and Sudan, more than 1/5 of the land area is cropland. The size of the circle indicates the percentage of baobabs found in cropland. In Senegal, Burkina Faso, Niger, and Nigeria, more than 2% of the baobab trees are found in croplands. Senegal, Mali, Burkina Faso, Niger and Nigeria have relatively higher baobab densities in croplands with > 1 baobab tree per km2.
Extended Data Fig. 4 DHS data for Senegal used in this study.
a, Statistics of the DHS dataset from different years. In total, 183 clusters (4,708 households) were investigated from 2014, 2015, 2016, 2019. For Senegal, 128 clusters (3020 households) are used for our study, the remaining clusters are outside of the studied rainfall zone. Each slice of the inner circle of the donut pie chart represents the number of clusters for each investigated year, which is regarded as the cluster-year record, while the outer circle shows the number of households across those cluster-year records. b, The violin plot shows the distribution of households for each cluster during the investigation period, each cluster includes 2–20 households. Box, interquartile range (IQR); whiskers, 1.5 × IQR; horizontal line within the each violin, median; points, observations for each cluster. c, Spatial distribution of the DHS clusters. The grey points are clusters from the DHS datasets that were available but outside of the 150–750 mm rainfall zone or located in urban areas and thus not used, while the red points are clusters that were used in our study. d, Histogram of household counts per cluster-year. In total, there are 320 cluster-year records that were included, and 3020 household records.
Extended Data Fig. 5 Presence of baobabs and food consumption at two zones based on rainfall.
DHS data on rural household clusters in Senegal were split into two classes with an almost equal number of household clusters, representing clusters with high and low consumption levels of dark green leafy vegetables. Data are presented for two (overlapping) rainfall range zones: a.750 mm > rainfall ≥ 600 mm; b. 750 mm > rainfall >150 mm. The number of baobab trees in the surrounding areas of the surveyed DHS clusters (5 km radius) and also the total number of trees (including all species extracted from ref. 32) was compared across clusters with high vs low consumption of dark green leafy vegetables. The upper horizontal lines represent error bar and the filled bars denote the mean value. Dots are the original observations for each group. Significant differences from the one-sided t-test are marked with * (p < 0.05).
Extended Data Fig. 6 Examples of Google building data.
Buildings from google building open dataset from ref. 53 are shown as yellow polygons. The basemaps are Skysat satellite images. Inconsistencies with Skysat images may be a result of different image acquisition times. Image © 2023 Planet Labs PBC.
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Huang, K., Brandt, M., Hiernaux, P. et al. Mapping every adult baobab (Adansonia digitata L.) across the Sahel and relationships to rural livelihoods. Nat Ecol Evol 8, 1632–1640 (2024). https://doi.org/10.1038/s41559-024-02483-9
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DOI: https://doi.org/10.1038/s41559-024-02483-9