May 16, 2017, by Tim Radford
African cattle among trees and shrubs in the drylands of Burkina Faso.
New worldwide study identifies areas of dryland forest whose combined size is equivalent to the landmass of all 28 European Union states.
LONDON, 16 May, 2017 – An international partnership of scientists has just discovered an area of forest bigger than the European Union landmass.
They have been examining the world’s drylands – which cover about 40% of the terrestrial planet – and counting the trees. And their new estimate increases the area of global forest by 9%, and the area of dryland covered by forest by at least 40% and perhaps 47%.
They report in Science journal that this corresponds to an additional 467 million hectares of forest that had never been reported or counted before. This is 4.67 million square kilometres, which is equivalent to an area bigger than all of India, and bigger than the 28 member states of the European Union.
The new figure was arrived at by taking a closer look at satellite data, and studying evidence from 213,795 half-hectare plots around the planet. Satellite monitors can easily identify rain forest, or the great conifer and birch forests of the north, but it is much harder to be sure of small plots of foliage in grassland.
Classed as forest
The scientists, led by researchers from the UN Food and Agriculture Organisation in Rome, are sure that some 1,327 million hectares of dryland had more than 10% tree cover in 2015, and 1,079 million hectares could be classed as forest.
The new estimate is important for two main reasons. One is that it provides statistical substance and reduces uncertainty for those climatologists who have to think about carbon sinks and sources in relation to what happens to atmospheric carbon in a rapidly warming world.
But the new data will also matter powerfully for conservation scientists concerned with the preservation of habitat for the wild things in a world in which, in one lifetime, the human population has grown threefold and is likely to hit the 9 billion mark in mid-century.
The world’s forests are essentially the sink for much of the carbon dioxide released by humans as they burn fossil fuels to drive global warming and climate change.
In addition, photosynthesis is the source not only of the world’s oxygen but the world’s food and fabric, and forests not only provide shelter and habitat for myriad other species but also help manage the flow of the world’s water.
It has therefore become important to know more, not just about forests, but about the planet’s tree population.
One group of researchers estimated in 2015 that the planet was host to more than 3 trillion trees, but humans were felling, burning or otherwise destroying them at the rate of 15 billion a year.
“Ground survey plot data indicates that a lot
of dryland vegetation assumed to be thicket,
shrubland, etc, is actually forest”
Another group surveyed 650,000 trees worldwide and came to a counter-intuitive conclusion that, when it came to the carbon budget, big old trees may matter more than young saplings − that is, the old stable forests were more important than plantations not just as habitat, but as helpmeets to humanity.
A third team measured the way forests actually moderated regional temperatures, at least in the northern hemisphere.
And, only this year, researchers from Botanic Gardens Conservation International (BGCI) published a count of the world’s trees species: 60,065 known so far, on all the world’s continents except Antarctica.
The new study extends knowledge of the world’s trees, but it also highlights a problem of definition, according to Paul Smith, secretary general of BGCI. The next problem is whether a satellite can actually see certain types of tree cover.
Smith told Climate News Network: “A widely-used definition in Africa is ‘a continuous stand of trees at least 10 metres tall, their crowns interlocking’. This physiognomic definition applies to dry forest or rainforest.
“The problem is that you have other native dryland vegetation types that can look pretty similar from the air to dry forest, such as thicket [a closed stand of bushes and climbers usually 3-7 metres tall] or shrubland [an open or closed stand of shrubs up to 2 metres tall].
“The bottom line is that you need to be on the ground to see these differences, hence the use of ground survey plot data in this study, which indicates that a lot of dryland vegetation assumed to be thicket, shrubland, etc, is actually forest.
“Of course, it isn’t just forests that sequester carbon – natural vegetation in all of its forms plays this critical role. Yet another reason to conserve it!”
Drylands are now home to 38% of the planet’s human population. Ominously, climate change will widen their extent, and by 2100 at least half of the terrestrial planet will be classified as dryland − that is, terrain on which rainfall is balanced by evaporation and transpiration.
Since, by definition, agriculture in arid regions is precarious and pastoral farming can degrade the land, the poorest nations will be increasingly hit hard by climate change.
Paradoxically, according to another study, those trees that once made some of the great Californian urban landscape attractive are being lost.
Green canopy is vanishing as home owners chop down trees to make room for extensions, scientists report in the Urban Forestry and Urban Greening journal. And they are doing so in ways that see 20% or even 55% of urban forest loss in some suburbs of Los Angeles.
The paradox is that not only does tree cover in Californian cities help reduce the costs of air conditioning, but it also adds value to properties, according to a separate study in the same journal.
In some parts of California, dryland forest counts as amenity. But in parts of Africa, Asia, Australia and South America, the separated stands of trees could hardly be more important: they offer shade, shelter and sustenance for the wild things, resources for humans, and they help prevent wind erosion and stabilise soil loss during sustained drought.
The new study, its authors say, offers an opportunity to think about new approaches to combat climate change, resist the advance of the deserts, and at the same time support the biodiversity and ecosystem services that, ultimately, underpin human survival. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.