The extent of forest in dryland biomes

TL;DR: An estimate of global forest extent in dryland biomes is reported, based on analyzing more than 210,000 0.5-hectare sample plots through a photo-interpretation approach using large databases of satellite imagery at very high spatial resolution and very high temporal resolution, available through the Google Earth platform.

Abstract: Dryland biomes cover two-fifths of Earth’s land surface, but their forest area is poorly known. Here, we report an estimate of global forest extent in dryland biomes, based on analyzing more than 210,000 0.5-hectare sample plots through a photo-interpretation approach using large databases of satellite imagery at (i) very high spatial resolution and (ii) very high temporal resolution, which are available through the Google Earth platform. We show that in 2015, 1327 million hectares of drylands had more than 10% tree-cover, and 1079 million hectares comprised forest. Our estimate is 40 to 47% higher than previous estimates, corresponding to 467 million hectares of forest that have never been reported before. This increases current estimates of global forest cover by at least 9%.

Summary

Main text

• The most recent climate model simulations, based on contrasted Representative Concentration Pathways (RCPs), i.e. RCP 8.5 and RCP 4.5, show that global climate change could cause dryland biomes to expand by 11% to 23% by the end of the 21 st century (7) .
• The geographical scope of this assessment is framed by the delineation adopted by the United Nations Environment Programme World Conservation Monitoring Centre (1), i.e. lands having an Aridity Index (AI) lower than 0.65.
• To determine the extent of forests and tree cover throughout the worldÕs dryland biomes, the authors assessed a large sample of 0.5 ha plots through visual interpretation of VHR images available from Google Earth.
• Trees were distinguished from shrubs by considering crown shadows, which are related to vegetation height, and by using field-based photographs available from the Web.

Figures

Table 2. Comparison of the estimate in this paper (Global Dryland Assessment) of 203 areas in the drylands in 2015 with forest and ≥10% tree canopy cover (Table 1), with 204 other estimates based on satellite images and following the same definition of dryland 205 (Mha) (1). 206

Figure 2. Comparison of ≥10% tree cover in Africa’s drylands as mapped by the 212 Global Drylands Assessment (GDA) and Hansen et al. (13). Green dots show plots 213 are coloured green where the GDA reports ≥10% tree cover but Hansen et al. reported 214 a lower percentage; blue dots show plots where Hansen et al. reported ≥10% tree cover 215 but the GDA reports a lower percentage; and orange dots show plots where both 216 assessments report ≥10% tree cover. Figures 2b and 2c focus on two regions with large 217 discrepancies between the maps. 218

Table 1. Areas in the world’s drylands in 2015 of forest (as defined by FAO(24)) and 200 land under different percentages of tree canopy cover (Mha). 201

Figure 1. Forest distribution in drylands. Plots with forest are coloured in green, 209 and without forest in yellow. 210

Full text

This is a repository copy of The extent of forest in dryland biomes.
White Rose Research Online URL for this paper:
http://eprints.whiterose.ac.uk/115833/
Version: Accepted Version
Article:
Bastin, JF, Berrahmouni, N, Grainger, A et al. (28 more authors) (2017) The extent of forest
in dryland biomes. Science, 356 (6338). pp. 635-638. ISSN 0036-8075
https://doi.org/10.1126/science.aam6527
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Title: The extent of forest in dryland biomes 1
One sentence summary: 2
Previously unreported forest areas in dryland biomes increase current estimates of the 3
global forest cover by at least 9 %. 4
List of authors: 5
Jean-Francois Bastin
1,2*
, Nora Berrahmouni
1
, Alan Grainger
3
, Danae Maniatis
4
, Danilo 6
Mollicone
1
, Rebecca Moore
5
, Chiara Patriarca
1
, Nicolas Picard
1
, Ben Sparrow
6
, Elena 7
Maria Abraham
7
, Kamel Aloui
8
, Ayhan Atesoglu
9
, Fabio Attore
10
, ‚ağlar BassŸllŸ
11
, 8
Adia Bey
1
, Monica Garzuglia
1
, Luis G. Garc’a-Montero
12
, NikŽe Groot
3
, Greg Guerin
6
, 9
Lars Laestadius
13
, Andrew Lowe
14
, Bako Mamane
15
, Giulio Marchi
1
, Paul Patterson
16
, 10
Marcelo Rezende
1
, Stefano Ricci
1
, Ignacio Salcedo
17
, Alfonso Sanchez-Diaz Paus
1
, 11
Fred Stolle
18
, Venera Surappaeva
19
, Rene Castro
1*
. 12
*corresponding authors 13
1 Food and Agriculture Organization of the United Nations, Vialle delle Terme di 14
Caracalla, 00153 Rome, Italy 15
2 Landscape Ecology and Plant Production Systems Unit, UniversitŽ libre de Bruxelles, 16
CP264-2, B-1050, Bruxelles, Belgium 17
3 School of Geography, University of Leeds, Leeds LS2 9JT, UK 18
4 Environmental Change Institute, School of Geography and the Environment, South 19
Parks Road, Oxford, OX1 3QY, UK 20
5 Google, Inc. Mountain View, CA, USA 21
6 Terrestrial Ecosystem Research Network, School of Biological Sciences, University 22
of Adelaide, South Australia 5005, Adelaide, Australia 23
7 Instituto Argentino de Investigaciones de las Zonas çridas- Consejo Nacional de 24
Investigaciones Cient’ficas y TŽcnicas, Mendoza, Argentina 25

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8 Ministry of Agriculture, General Directorate of Forests-Inventory service, Tunis-26
Tunisia 27
9 Bartõn University, Faculty of Forestry, Department of Forest Engineering, Bartõn, 28
Turkey 29
10 Department of Environmental Biology, Sapienza University of Rome, Rome, Italy 30
11 UN Food and Agriculture Organization, Subregional Office for Central Asia, 31
Ankara, Turkey 32
12 Technical University of Madrid (UPM), Department of Forest and Environmental 33
Engineering and Management, E.T.S.I. Montes, Ciudad Universitaria, Madrid 28040, 34
Spain 35
13 Laestadius Consulting, LLC, Silver Spring, MD 20901, USA 36
14 Environment Institute and School of Biological Sciences, University of Adelaide, 37
North terrace, Adelaide, South Australia 5005, Australia 38
15 Centre RŽgional AGRHYMET. Niamey BP 11011, Niger 39
16 Interior West- Forest Inventory and Analysis, Forest Service, US Department of 40
Agriculture, Fort Collins, USA 41
17 Instituto Nacional di Semiarido, 10067, Bairro Serrot‹o, Brazil 42
18 World Resources Institute, 10 G street, NE, Washington DC 20002, USA 43
19 Department of Forest and Hunting Inventory of Kyrgyztan, Bishkek, Kyrgyztan 44

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Abstract 45
Dryland biomes cover two fifths of the EarthÕs land surface but their forest area is 46
poorly known. Here, we report an estimate of global forest extent in dryland biomes, 47
based on analysing more than 210,000 0.5 ha sample plots through a photo-48
interpretation approach using large databases of satellite imagery at (i) very high spatial 49
resolution and (ii) very high temporal resolution which are available through the Google 50
Earth platform. We show that, in 2015, 1,327 million ha of drylands had more than 10% 51
tree-cover, and 1,079 million ha comprised forest. Our estimate is 40-47 % higher than 52
previous estimates, corresponding to 467 million ha of forest that have never been 53
reported before. This increases current estimates of global forest cover by at least 9 %. 54

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Main text 55
Dryland biomes cover about 41.5 % of the EarthÕs land surface (1). They contain some 56
of the most threatened, yet disregarded, ecosystems (2, 3), including seven of the twenty 57
five biodiversity hotspots (4), while facing pressure from climate change and human 58
activity (5, 6). The most recent climate model simulations, based on contrasted 59
Representative Concentration Pathways (RCPs), i.e. RCP 8.5 and RCP 4.5, show that 60
global climate change could cause dryland biomes to expand by 11% to 23% by the end 61
of the 21
st
century (7). If this occurs, dryland biomes could cover more than half of the 62
global land surface (7). Climate change will lead to extended droughts, regional 63
warming (8, 9) and, combined with a growing human population, to an increased risk 64
of land degradation and desertification in the drylands (7). Such changes will 65
particularly affect developing countries, where most dryland expansion is expected to 66
occur (7, 10) and where woody resources provide key goods and services to support 67
human livelihoods (11). 68
69
However, our current knowledge of the extent of tree cover and forests in drylands is 70
limited. This is illustrated by significant spatial disagreements between recent satellite-71
based global forest maps (12Ð14) and by the scarcity of large-scale studies of dryland 72
biomes (3). The most recent estimates of tropical dry forest extent based on remote 73
sensing surveys vary greatly, from 105 Mha for the year 2000, derived from a wall-to-74
wall map at coarse resolution (5) to 542 Mha for the year 2010 derived from a global 75
sample of medium resolution images (15). This disparity can partly be explained by 76
differences in satellite data characteristics (e.g. spatial resolution), mapping approaches 77
(e.g. mapping unit) and forest definitions (e.g. tree cover thresholds). It has led to major 78

Frequently asked questions

Q1. What are the contributions in this paper?

This is indicated by the licence information on the White Rose Research Online record for the item. 

Q2. how many savannas are affected by drought?

The darkening effect in 260 drought affected savanna woodland environments relative to soil reflectance in 261 Landsat and SPOT wavebands. 

Q3. What is the percentage of open forests in Africa and Oceania?

156 Open forests cover 355 Mha and are dominant in Africa and Oceania, where they 157 account for 52% and 74% of all dry forest, respectively. 

Q4. How many of the 57 five biodiversity hotspots are in the dryland bio?

They contain some 56 of the most threatened, yet disregarded, ecosystems (2, 3), including seven of the twenty 57 five biodiversity hotspots (4), while facing pressure from climate change and human 58 activity (5, 6). 

Q5. What is the impact of climate change on dryland biomes?

Their findings could also lead to the development of innovative 195 conservation and land restoration actions in dryland biomes, i.e. in regions with low 196 opportunity cost, to mitigate climate change, combat desertification, and support the 197 conservation of biodiversity and ecosystem services that underpin human livelihoods 198 (31). 

Q6. how much carbon is in the woody savannas?

190 191 Using numbers on the carbon pools of woody savannas (28), further research could use 192 their publicly available data to increase estimates of global forest carbon stocks by 15 to 193 158.3 GtC, or by 2 to 20 % (29), thereby helping to reduce uncertainty about the global 194 carbon budget (30). 

Q7. How many trees are covered by a closed canopy?

Their results show that in 2015 there were 1,327 (±98) Mha of dryland where tree canopy 134 cover percentage is over 10%, of which 777 Mha (57%) present a closed canopy (Table 135 1, Table S1), i.e. with a tree canopy cover over 40% (24). 

Q8. What is the average area of dryland forest in the world?

Of the total area of 1,079 Mha 158 of dryland forest, 523 Mha are located in the tropics, of which 203 Mha (37%) are open 159 forest and 320 Mha (63%) are closed forest (Supplementary Table 2). 

Q9. How many trees and trees are in the dryland?

To determine the extent of forests and tree cover throughout the world’s dryland 108 biomes, the authors assessed a large sample of 0.5 ha plots through visual interpretation of VHR 109 images available from Google Earth.