Mapping tree density at a global scale

  • A Corrigendum to this article was published on 16 December 2015

Abstract

The global extent and distribution of forest trees is central to our understanding of the terrestrial biosphere. We provide the first spatially continuous map of forest tree density at a global scale. This map reveals that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these trees, approximately 1.30 trillion exist in tropical and subtropical forests, with 0.74 trillion in boreal regions and 0.66 trillion in temperate regions. Biome-level trends in tree density demonstrate the importance of climate and topography in controlling local tree densities at finer scales, as well as the overwhelming effect of humans across most of the world. Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization.

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Figure 1: Map of data points and raw biome-level forest density data.
Figure 2: Heat plots showing the relationships between predicted and measured tree density data.
Figure 3: Validation plots for biome-level predictions.
Figure 4: The global map of tree density at the 1-km2 pixel (30 arc-seconds) scale.
Figure 5: Standardized coefficients for the variables included in final biome-level regression models.

Change history

  • 09 September 2015

    Minor changes were made to the Author Contributions statements.

  • 13 April 2016

    The link for the global tree density map was added to the Author Information section.

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Acknowledgements

We thank P. Peterkins for her support throughout the study. We also thank Plant for the Planet for initial discussions and for collaboration during the study. The main project was funded by grants to T.W.C. from the Yale Climate and Energy Institute and the British Ecological Society. We acknowledge various sources for tree density measurements and estimates: the Canadian National Forest Inventory (https://nfi.nfis.org/index.php), the US Department of Agriculture Forest Service for their National Forest Inventory and Analysis (http://fia.fs.fed.us/), the Taiwan Forestry Bureau (which provided the National Vegetation Database of Taiwan), the DFG (German Research Foundation), BMBF (Federal Ministry of Education and Science of Germany), the Floristic and Forest Inventory of Santa Catarina (IFFSC), the National Vegetation Database of South Africa, and the Chilean research grants FONDECYT no. 1151495. For Europe NFI plot data were brought together with input from J. Rondeux and M. Waterinckx, Belgium, T. Bélouard, France, H. Polley, Germany, W. Daamen and H. Schoonderwoerd, Netherlands, S. Tomter, Norway, J. Villanueva and A. Trasobares, Spain, G. Kempe, Sweden. New Zealand Natural Forest plot data were collected by the LUCAS programme for the Ministry for the Environment (New Zealand) and sourced from the National Vegetation Survey Databank (New Zealand) (http://nvs.landcareresearch.co.nz). We also acknowledge the BCI forest dynamics research project, which was funded by National Science Foundation grants to S. P. Hubbell, support from the Center for Tropical Forest Science, the Smithsonian Tropical Research Institute, the John D. and Catherine T. MacArthur Foundation, the Mellon Foundation, the Small World Institute Fund, numerous private individuals, the Ucross High Plains Stewardship Initiative, and the hard work of hundreds of people from 51 countries over the past two decades. The plot project is part of the Center for Tropical Forest Science, a global network of large-scale demographic tree plots.

Author information

Affiliations

Authors

Contributions

The study was conceived by T.W.C and G.H. and designed by T.W.C., K.R.C. and M.A.B. Statistical analyses were conducted by H.B.G., S.M.T., J.R.S., C.B., D.S.M. and T.W.C. and mapping was conducted by H.B.G. and C.B. The manuscript was written by T.W.C. with input from M.A.B., P.C., D.S.M., H.B.G. and C.B., with comments provided by all other authors. Tree density measurements or geospatial data from all over the world were contributed by K.R.C., S.M.T., M.C.D., G.A., M.N.T., W.J., C.Sa., C.St., D.P., T.T., S.G., G.B., S.J.W., S.K.W., M.O.H., G.M.H., G.J.N., E.T., P.B., C.F.L., L.W.P.,M.F., A.H., J.H., P.C., A.C.V., P.M.U., S.L.P., C.W.R. and M.S.A.

Corresponding author

Correspondence to T. W. Crowther.

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Competing interests

The authors declare no competing financial interests.

Additional information

The global tree density map can be found at http://elischolar.library.yale.edu/yale_fes_data/1/.

Extended data figures and tables

Extended Data Figure 1 Histogram of the collected measurements of forest tree density in each biome around the world (n = 429,775).

The red line and the blue dotted lines indicate the mean and median for the collected data, respectively. Data in each biome fitted a negative binomial error structure.

Extended Data Figure 2 Histogram of the predicted forest tree density values for the locations that density measurements were collected in each biome around the world (n = 429,775).

The red line and the blue dotted lines indicate the mean and median for the collected data, respectively. As our models were based on mean values, the majority of points fall on or close to the mean values in each biome.

Extended Data Figure 3 Histogram of the total predicted forest tree density values for each pixel within each biome around the world (n = 429,775).

This illustrates the spread of pixels throughout each biome, and highlights that our map accounts for the sampling bias in tree density plots (for example, although we had no zero values in our desert plots, the vast majority of desert pixels contain no trees).

Extended Data Figure 4 Comparison between approaches to generate the global tree density map.

The initial map was generated using 14 biome-level models (biomes delineated by The Nature Conservancy http://www.nature.org) to account for broad-scale variations in terrestrial vegetation types. With several thousand plot-level density measurements in most biomes, this approach provided highly accurate estimates at the global scale. However, to improve precision at the local scale, we also generated a map using ecoregion-scale models. Separate models were generated within each of 813 global ecoregions (also delineated by The Nature Conservancy to reflect smaller-scale vegetation types) using exactly the same statistical approach (see Methods). The same 429,775 data points were used to construct each map. Biome-level and ecoregion-level maps provide total tree estimates of 3.041 and 3.253 trillion trees, respectively.

Extended Data Table 1 Estimates of the total tree number for each of the biomes that contain forested land, as delineated by The Nature Conservancy (http://www.nature.org)

Supplementary information

Supplementary Table 1

Summary Table showing the number of plot estimates and total tree numbers (with 95% confidence interval) at the biome and global scale. (XLSX 15 kb)

Supplementary Table 2

This table shows the number of trees and tree densities for countries of the world, as estimated using 2 independent approaches (biome and ecoregion-level models) and the database of Global Administrative Areas, version 2.7 (http://gadm.org/). (XLSX 53 kb)

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Crowther, T., Glick, H., Covey, K. et al. Mapping tree density at a global scale. Nature 525, 201–205 (2015). https://doi.org/10.1038/nature14967

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