Quick, now, how many trees are there on Earth?
Until last week most of us would have had no idea, really. A very few of us, specialists of one kind or another in a forestry-related field, might have known that the generally accepted benchmark figure was 400 billion.
But as it turns out, that estimate was not only wrong but way wrong – order-of-magnitude wrong. In fact, “a recent broad-scale inventory that used 1,170 ground-truthed measurements of tree density [estimated] that there are 390 billion trees in the Amazon basin alone,” with the U.S. tree population ranking not far behind.
This observation is part of a new research paper, published last week in the estimable journal Nature, which uses ground-source measurements in addition to satellite photos and other remote-sensing data to produce “the first spatially continuous map of forest tree density at a global scale.”
And it concludes that the best estimate of the planet’s total tree population is 3.04 trillion, or nearly 10 times as many as the previous benchmark.
To put that in more readily graspable terms, the old estimate held that there were about 61 trees for each person on the planet. The new count says your individual share is 422 trees.
How could science be so wrong, for so long, about something so fundamental?
The main answer seems to be an over-reliance on high-resolution photography and other satellite-gathered data, in conjunction with sophisticated mapping tools, that are really better suited to making other kinds of measurements of the world’s forests – distribution of forested acreage, types of plant communities, density of canopy cover and so on.
When it comes to actually counting trees, it seems, remote sensing needs some serious supplementation with ground-based measurement. You could say that with satellites alone, we can’t see the trees for the forest.
Tree counts matter
And this matters because trees matter to ecosystem health in ways the authors assess, I think without jest, as countless, with water quality, timber stocks and carbon sequestration as top-of-the-list examples.
Also, because significant policy and management decisions about large-scale forests and their long-term fate are constantly being made and remade. Planners often have very good data about the “ecosystem services” delivered by small forest plots, or even individual trees, but strikingly poor data about tree populations on a very large scale:
For example, international afforestation efforts such as the “Billion Trees Campaign” and city-wide projects including the numerous “Million Tree” initiatives around the world have motivated civil society and political leaders to promote environmental stewardship and sustainable land management by planting large numbers of trees. Establishing targets and evaluating the proportional contribution of such projects requires a sound baseline understanding of current and potential tree population numbers at regional and global scales.
For this paper, a team led by Thomas Crowther of the Yale School of Forestry and Environmental Studies drew upon “429,775 ground-sourced measurements of tree density from every continent on Earth except Antarctica,” where there aren’t any.
Crowther’s 37 co-authors also came from every continent on Earth except Antarctica and Australia – although New Zealand was represented – and one of the paper’s more intriguing points is how the world’s tree population is distributed.
Although the Amazon and similarly tropical/subtropical regions have a lot of trees, they account for less than half the global total – about 1.39 trillion. Nearly half as many – 740 billion – are in boreal regions, and the rest – some 610 billion – are in temperate places like Minnesota.
(Some authorities draw the line between temperate and boreal regions at the 45th parallel, which passes through Fridley – and, for that matter, Skunk Hollow – but for their purposes in this paper the authors used a set of 14 biomes as defined by The Nature Conservancy, which puts all of Minnesota and Wisconsin in the temperate band.)
Highest densities in coldest places
Indeed, the paper says that among all the biomes, the highest tree densities are found not in the world’s warmest places but the coldest – the boreal and tundra zones.
In these northern latitudes, limited temperature and moisture lead to the establishment of stress-tolerant coniferous tree species that can reach the highest densities on Earth. However, the tropical regions contain a greater proportion of the world’s forested land.
A total of 42.8% of the planet’s trees exist in tropical and subtropical regions, with another 24.2% and 21.8% in boreal and temperate biomes, respectively.
By now you may be wondering how the researchers decided what to count as a tree and what to count as a shrub or other non-tree thing, perhaps imagining that the total got so big because they included trees that were pretty small.
Not so. The operational definition was “a plant with woody stems larger than 10 cm diameter at breast height,” that height being a standard forester’s measure that varies a bit from country to country but is set at 1.4 meters (about 4 feet 6 inches) in the U.S. and 1.3 meters in Canada, Australia and most of Europe.
There’s less of a divide over the 10 cm diameter standard; it’s used for the “vast majority” of data sources compiled around the world, although the U.S. Forest Service prefers 12.7 cm.
Applying the 10 cm threshold to my own woods, I concluded that about 99 percent of the buckthorn stems would be excluded from the official tree count, along with quite a few young elms, maples, ashes and black cherries.
Trends in tree losses
You may also be wondering if the paper’s authors address the question of ongoing losses to development, deforestation and other forces driving down tree populations in many places.
They do, of course, and a key conclusion is that human activity is the only common factor driving tree densities – usually though not always in a negative direction – across all biomes, contributing to a net decline in global forest area (not tree count) in the neighborhood of 1 percent or a little less per year.
Excerpts from the discussion:
Current rates of global forest cover loss are approximately 192,000 km2 each year. By combining our tree density information with the most recent spatially explicit map of forest cover loss over the past 12 years, we estimate that deforestation, forest management, disturbances and land use change are currently responsible for a gross loss of approximately 15.3 billion trees on an annual basis.
Although these rates of forest loss are currently highest in tropical regions, the scale and consistency of this negative human effect across all forested biomes highlights how historical land use decisions have shaped natural ecosystems on a global scale.
Using the projected maps of current and historic forest cover provided by the United Nations Environment Programme, our map reveals that the global number of trees has fallen by approximately 45.8% since the onset of human civilization.
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Nature typically does not make its contents available to nonsubscribers without a significant fee, but this paper is accessible under a special content-sharing initiative and can be read in its entirety here.