An unexpectedly large count of trees in the West African Sahara and Sahel

Martin Brandt (), Compton J. Tucker (), Ankit Kariryaa, Kjeld Rasmussen, Christin Abel, Jennifer Small, Jerome Chave, Laura Vang Rasmussen, Pierre Hiernaux, Abdoul Aziz Diouf, Laurent Kergoat, Ole Mertz, Christian Igel, Fabian Gieseke, Johannes Schöning, Sizhuo Li, Katherine Melocik, Jesse Meyer, Scott Sinno, Eric Romero, Erin Glennie, Amandine Montagu, Morgane Dendoncker and Rasmus Fensholt
Additional contact information
Martin Brandt: University of Copenhagen
Compton J. Tucker: NASA Goddard Space Flight Center
Ankit Kariryaa: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Kjeld Rasmussen: University of Copenhagen
Christin Abel: University of Copenhagen
Jennifer Small: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Jerome Chave: Université Paul Sabatier
Laura Vang Rasmussen: University of Copenhagen
Pierre Hiernaux: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Abdoul Aziz Diouf: Centre de Suivi Ecologique
Laurent Kergoat: Geosciences Environnement Toulouse (GET), Observatoire Midi-Pyrénées, UMR 5563 (CNRS/UPS/IRD/CNES)
Ole Mertz: University of Copenhagen
Christian Igel: University of Copenhagen
Fabian Gieseke: University of Copenhagen
Johannes Schöning: HCI Group, University of Bremen
Sizhuo Li: University of Copenhagen
Katherine Melocik: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Jesse Meyer: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Scott Sinno: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Eric Romero: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Erin Glennie: Science Systems and Applications Inc., NASA Goddard Space Flight Center
Amandine Montagu: Ecole Normale Supérieure
Morgane Dendoncker: Université Catholique de Louvain
Rasmus Fensholt: University of Copenhagen

Nature, 2020, vol. 587, issue 7832, 78-82

Abstract: Abstract A large proportion of dryland trees and shrubs (hereafter referred to collectively as trees) grow in isolation, without canopy closure. These non-forest trees have a crucial role in biodiversity, and provide ecosystem services such as carbon storage, food resources and shelter for humans and animals1,2. However, most public interest relating to trees is devoted to forests, and trees outside of forests are not well-documented3. Here we map the crown size of each tree more than 3 m2 in size over a land area that spans 1.3 million km2 in the West African Sahara, Sahel and sub-humid zone, using submetre-resolution satellite imagery and deep learning4. We detected over 1.8 billion individual trees (13.4 trees per hectare), with a median crown size of 12 m2, along a rainfall gradient from 0 to 1,000 mm per year. The canopy cover increases from 0.1% (0.7 trees per hectare) in hyper-arid areas, through 1.6% (9.9 trees per hectare) in arid and 5.6% (30.1 trees per hectare) in semi-arid zones, to 13.3% (47 trees per hectare) in sub-humid areas. Although the overall canopy cover is low, the relatively high density of isolated trees challenges prevailing narratives about dryland desertification5–7, and even the desert shows a surprisingly high tree density. Our assessment suggests a way to monitor trees outside of forests globally, and to explore their role in mitigating degradation, climate change and poverty.

Date: 2020
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DOI: 10.1038/s41586-020-2824-5

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