Forests Are Getting Shorter and Younger All Over the World

The loss of the oldest, tallest trees makes forests store less carbon dioxide and diminishes the wildlife they can support

A forest in Koenigshain, Germany.
A forest in Koenigshain, Germany. Photo by Florian Gaertner/Photothek via Getty Images

The world’s forests are losing their big, old trees, rendering those forests shorter and younger on average, according to new research. These old growth trees, which provide vital habitat for wildlife and store more carbon than younger trees, are being destroyed by a host of contributing factors, including rising global temperatures, climate-related disasters such as fire and insect outbreaks, and deforestation, reports Craig Welch for National Geographic.

The study, published last week in the journal Science, draws on more than 160 prior studies to take a comprehensive look at the erasure and degradation of the planet’s forests over the last century.

"It's not a shock but it's very sad," Kristina Anderson-Teixeira, an ecologist and leader of the ForestGEO Ecosystems & Climate Program at the Smithsonian Conservation Biology Institute who worked on the new research, tells Nathan Rott of NPR. "We as a human society are hitting these forests so rapidly with so many different changes that they can't keep up."

The international team of more than 20 researchers from more than a dozen institutions conducted an exhaustive survey of existing studies that looked at forest loss and combined those results with satellite imagery and computer modelling to track forest loss from 1900 to 2015, reports National Geographic.

The team found that over the last 115 years, the world has lost more than one-third of its old-growth forests, reports Damian Carrington of the Guardian. North American and European forests, about which more detailed data exist, saw tree mortality double over the past 40 years, with the majority of those deaths comprised of older trees, according to National Geographic.

The change has not been directly caused by any one factor, though the Guardian reports that scientists estimate 12 percent of the total forest area lost since 1900 has come from human land-clearing. Climate change-related stresses such as drought, wildfire and insect outbreaks are also driving the death of old trees and the loss of forests, and they’re all predicted to get worse in coming decades, Tom Pugh, a scientist at the University of Birmingham and co-author of the study, tells the Guardian.

Because plants use carbon dioxide (CO2) to convert sunlight into food, some thought the grand human experiment of pumping ever-increasing quantities of carbon into the atmosphere might actually accelerate tree growth, according to National Geographic. But the world heats up, trees start to batten down the hatches to avoid drying up which stops them from making use of all that extra CO2.

Nate McDowell, a tree physiologist with Pacific Northwest National Laboratory and the study’s lead author, tells National Geographic it’s akin to “going to an all-you-can-eat buffet with duct tape over their mouths.”

As the world loses its largest, oldest trees, McDowell tells National Geographic the impact on greenhouse gases in the atmosphere is twofold. “When old trees die, they decompose and stop sucking up CO2 and release more of it to the atmosphere,” he says. “It’s like a thermostat gone bad. Warming begets tree loss, then tree loss begets more warming.”

The researchers did find some locales in which the added carbon dioxide in the atmosphere may wind up increasing tree growth, reports NPR, but it’s a tiny fraction and not nearly enough to override the lost capacity of those forests to draw-down and store carbon.

Researchers not involved in the study unscored the gravity of its findings but sought to point out that the worrying trends make the world’s remaining forests that much more important.

Tom Crowther, an environmental scientist at ETH Zurich University in Switzerland who was not involved in the study, tells the Guardian, "if we can protect the forests that we already have, and allow them to grow to maturity, there is a huge potential for them to capture a lot of additional carbon.”

Another researcher who was not involved in the report, Simon Lewis, an environmental scientist at University College London, tells the Guardian, “the world’s forests currently slow climate change, and while future mortality trends could reverse this, the ideas in the new report don’t change what the world needs to do: stabilize the climate by quickly driving fossil fuel emissions to zero and protect the world’s forests.”