Three trillion trees, but not for long

Geographer Simon Lewis says 'a plantation of many of the same trees isn't the same as a patch of Amazon rainforest' when measuring the health of an ecosystem. (Delwyn Verasamy, M&G)

Geographer Simon Lewis says 'a plantation of many of the same trees isn't the same as a patch of Amazon rainforest' when measuring the health of an ecosystem. (Delwyn Verasamy, M&G)

Scientists have already calculated how many species of fish there are in the sea (230 000), and how many species there are on the planet (8.7-million). Now they have had a crack at counting all the world’s trees.

Using a combination of satellite and ground measurements, researchers estimate that there are just over three trillion trees on the planet, more than seven times as many as the previous, nonpeer-reviewed reckoning that relied on satellite images alone.

But people are having an “overwhelming” impact on the world’s forests, according to the international study team with representatives from 15 countries.

Today, people are responsible for the loss of about 15-billion trees a year as a result of deforestation and demand for farmland, a figure that the authors said is “considerably higher” than just a century ago.

“The scale of the human impact was astronomical. The number of trees cut down is almost three trillion [or 45.8%] since the start of civilisation,” said Thomas Crowther of Yale University, the lead author of the study, published in the journal Nature on Wednesday.

“I didn’t expect human activity to come out as the strongest control on tree density across all of the biomes. It was one of the dominant regulators of the number of trees in almost all of the world. It really highlights how big an impact humans are having on the Earth at a global scale,” he said.

The densest areas for trees were found in the northern boreal forests of Canada, China and Russia, though the authors said the latter two were some of the sparsest parts of the world for data on the ground.

Tropical and subtropical forests hold most of the world’s trees, with about 1.39-trillion in total, whereas heavily human-impacted temperate regions such as Europe and Asia have just 0.61-trillion.

The earlier estimate of 400-billion trees in the world, or 61 per person, was based on satellite imagery and published in a 2009 book rather than in a scientific journal.

The new study combines satellite data with more than 400 000 ground-based measurements from sources such as governments’ forest inventories.

Simon Lewis, a geographer at University College London and the University of Leeds, said the new work is important but does not show there are more trees than previously thought because the earlier estimate was never peer-reviewed.

“To me, it is more the first robust estimate of the number of trees,” he said. “It’s an important and useful piece of work, but we should remember that the number of trees is not necessarily the best metric for measuring the health of an ecosystem or its importance. A plantation of many of the same trees isn’t the same as a patch of Amazon rainforest.”

He said the study does not change our understanding of the role forests play in slowing man-made climate change because the amount of carbon the world’s trees can store is already well studied.

The gross loss of trees is about 15.3-billion trees each year, but the net loss is probably closer to about 10-billion a year because some five billion new trees are planted a year, the authors said.

The study counted trees that were 10cm in diameter, the only universally used standard, meaning there are likely billions more smaller trees that remain uncounted. – © Guardian News & Media 2015

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