The world’s collective forests have become shorter and younger overall in the past 50 years, according to a study published in the journal Science on Friday. This means that forests have less capacity to remove carbon from the atmosphere and are less hospitable to the many species that rely on them for shelter. Oh, and it’s going to get worse.
The team of researchers reviewed more than 160 previous studies, analyzed satellite imagery, and created models to examine how forests changed between 1900 and 2015. They found that over that 115 year period, the world has lost 14 percent of its forests to tree harvesting alone. That includes 30 percent of old growth forests, which are home to trees more than 140 years old and are generally tall and biodiverse.
The study doesn’t account for other environmental stressors on trees, such as increased carbon dioxide fertilization due to higher carbon emissions, and more frequent and severe climate-related disruptions such as insect infestations, wildfires, and droughts. Nate McDowell, a scientist at the Pacific Northwest National Laboratory and the study’s lead author, told Earther that means the 30 percent dip in old growth is “a very conservative estimate.”
In North America and Europe, where more detailed data was available, the researchers found that tree mortality doubled over that time, and a higher proportion of those deaths were older trees. Their findings suggest that on average, the world is losing old trees. Due to a lack of data, the researchers weren’t able to make a precise estimate as to how much shorter the forests have gotten.
In different places, this loss is happening at different rates and for different reasons. While wildfires are driving forest loss in Australia and Mongolia. California has seen massive wildfires and beetle infestations. And logging in the Amazon rainforest is increasing. There are some rare exceptions. Tree mortality in parts of the Pacific Northwest, for instance, is decreasing.
But though the changes vary regionally, the impact will be felt globally. Eighty percent of Earth’s terrestrial land-based plant and animal species live in forests. Old growth forests tend to be highly biodiverse and are home to more endangered species. They also store massive amounts of carbon dioxide.
“Old growth forests are... by far the biggest carbon store on the land,” said McDowell. “Small trees don’t store much carbon. Big trees do.”
As the climate crisis becomes more severe, this trend will continue. Temperatures are rising, and climate disturbances like wildfires, droughts, and insect outbreaks are becoming more frequent and severe.
This creates a vicious feedback loop. When old trees die, they not only stop sequestering carbon. They also release all the carbon they’ve sequestered back into the atmosphere. Young forests that replace them are in turn more susceptible to forest fires, which unleash more carbon into the atmosphere. In short, global warming kills trees, and tree loss contributes global warming. There are ways to mitigate these losses, though.
“There are already active efforts in the forest community, such as the sustainable burning and thinning of trees out west, where ecosystems used to experience fires and now they don’t,” McDowell said. “Reintroducing fire in a healthy way can mitigate these big mortality events and these disturbance events. So there are solutions that are already being applied, but they’re not being applied at the scale they’re needed. Scaling that will be a global challenge.”
Perhaps the biggest challenge when it comes to tree loss, though, will be curtailing carbon emissions. Doing so would help protect forests—and protecting forests, would help us manage our global carbon cycle. If we don’t work to preserve our forests, it’s not just old trees that will suffer—all life on Earth will. The scientists are doing more research to determine what all the impacts will be, but it’s clear they won’t be good.