Forests are losing their oldest and tallest trees, study says. Here’s why

The world’s forests are losing their oldest, tallest trees ⁠— making them collectively shorter and younger, a trend that’s expected to continue, according to a new study.

As old-growth trees die off, more carbon dioxide will find its way into the atmosphere instead of being absorbed ⁠— exacerbating the effects of climate change, NPR reported. Losing those trees also impacts endangered species that rely on them for shelter, according to Gizmodo.

The journal Science published a study titled Pervasive shifts in forest dynamics in a changing world, which analyzed more than 160 previous studies about tree mortality, current satellite data and global models to form a comprehensive report about the changes in Earth’s forests, NPR reported. The researchers found between 1900 and 2015, the world lost 14% of its forests to deforestation, according to Gizmodo.

“Old-growth forests” are home to trees more than 140 years old and over a 115-year period, the world has lost 30% of them to deforestation alone, Gizmodo reported.

“A future planet with fewer large, old forests will be very different than what we have grown accustomed to,” Nate McDowell, the study’s lead author and a scientist at Pacific Northwest National Laboratory, told CNBC. “Older forests often host much higher biodiversity than young forests and they store more carbon than young forests.”

Why is this happening?

There are multiple factors that explain the decimation of the world’s forests, the study says. All are human-caused, including rising global temperatures, increasing carbon dioxide levels, frequent wildfires, severe droughts and land-use changes, according to the study.

“Increasing rates of tree mortality driven by climate and land-use change ⁠⁠— combined with uncertainty in the mix of species that will form the next generation ⁠— pose big challenges for conservationists and forest managers alike,” Tom Pugh, an author of the study and a scientist at the Birmingham Institute of Forest Research, told CNBC.

Climate change has caused trees to die at an accelerated rate, while also stunting their growth, CNBC reported. Younger, shorter forests lack the capacity to store the same amount of carbon as older forests, creating a cycle where climate change is both the cause and the effect, according to the study.

“We as a human society are hitting these forests so rapidly with so many different changes that they can’t keep up,” Kristina Anderson-Teixeira, an author of the study and leader of the ForestGEO Ecosystems & Climate Program at the Smithsonian Conservation Biology Institute, told NPR.