Trees living

What it is

In 2015 the journal Nature published a paper by T. W. Crowther and colleagues titled Mapping tree density at a global scale. It gives the first spatially continuous map of forest tree density at a global scale and estimates that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these, approximately 1.30 trillion are in tropical and subtropical forests, 0.74 trillion in boreal regions and 0.66 trillion in temperate regions. The paper appeared on 2 September 2015, and the density map itself is available from Yale University EliScholar repository.

Why it matters

A count of trees is a different way of measuring forests from an area in hectares. The authors report that, based on their projected tree densities, over 15 billion trees are cut down each year and the global number of trees has fallen by approximately 46 percent since the start of human civilization. Their biome-level results show how much climate and topography control tree density at finer scales, and they also show the overwhelming effect of humans across most of the world.

How to read it

The figures come from field measurements linked to a suite of remote sensing and mapping layers, from which regression models were built and applied to every map pixel. A companion paper in Scientific Data, published on 16 August 2016, describes the models. It reports 429,775 independent plot records that each had a location and a tree density in trees per hectare, and pixels of nominally 1 square kilometre. The authors defined trees as those larger than 10 centimetres in diameter at breast height, although minimum sizes for counting a tree vary by country and inventory purpose.

Limits

The 3.04 trillion figure is a modelled estimate from 2015, and the Scientific Data paper says the data offer precise estimates of the number of trees at global and biome scales and advises against using them for local-level estimation. The estimate for the biomes of mangroves and tropical and subtropical coniferous forests uses models from the most similar biomes, because too few plots were available. The density map is published by Yale under a Creative Commons Attribution-NoDerivatives 4.0 licence. The map is linked from the source list and is not reproduced here.

Sources

• Nature: Mapping tree density at a global scale, T. W. Crowther and others (2 September 2015)
• Yale University EliScholar: Global tree density map, dataset (2015)
• Scientific Data: Spatially-explicit models of global tree density, H. B. Glick and others (16 August 2016)
• Wikimedia Commons: Amazon Manaus forest (photo)

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