Global tree count has fallen by half during human civilization

Counting the number of trees on Earth may seem like an impossible task. But researchers have taken a crack at the problem in a new study and estimated the total at roughly 3 trillion — about seven times more than previous calculations suggested. The bad news? The planet had almost twice as many trees before human civilization began, the authors say.

In the past, scientists estimated Earth’s tree inventory by examining satellite images of forests. One calculation put the total at about 400 billion, or roughly 61 trees per human.

The current study in Nature relied upon 429,775 tree density measurements taken around the world, in ecosystems ranging from deserts to tropical forests. The team created models to capture the relationships between tree density and factors such as climate, topography, and land development.

Roughly 3.04 trillion trees are present on Earth, or about 422 trees per human, the researchers estimate. Trees are the most tightly packed in boreal and tundra forests. But tropical and subtropical areas hold about 43 percent of the world’s trees.

Not surprisingly, trees tended to flourish in warm, moist environments. But the team found some exceptions. For example, additional moisture wasn’t necessarily good for trees in cold areas because it encouraged the formation of permafrost.

No matter which type of ecosystem the researchers studied, though, human development was always linked to lower tree density. The planet is losing more than 15 billion trees per year due to logging and other disturbances caused by people, the team says. And the number of trees worldwide has plunged by 46 percent during the rise of human civilization.

Taking tree density into account, not just tree cover, can give researchers a deeper understanding of forest dynamics, the authors say. “A dense forest environment is a fundamentally different ecosystem from a sparse one,” they write. Roberta Kwok | 3 September 2015

Source: Crowther, T.W. et al. 2015. Mapping tree density at a global scale. Nature doi: 10.1038/nature14967.

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