A comprehensive new study finds that climate change and human activity have led to more deaths of older trees worldwide, with profound implications for global ecosystems.
"Everywhere we look, all the evidence we find suggests that tree mortality is increasing, particularly of larger trees," said Nate McDowell, an earth scientist at the Pacific Northwest National Laboratory, and lead author on the study. "It was an overwhelming amount of information, all pointing in the same direction."
The study, published today in the journal Science, was notable for its breadth. Researchers reviewed more than 150 scientific papers and examined satellite data to look for global trends. They found that increasing levels of carbon dioxide in the atmosphere help younger trees grow quicker, but the effect levels off as trees get older. And those gains in growth are cancelled out by other effects of climate change: increased heat and drought, and more extreme wind and wildfire.
"Plants may grow 10% faster, but if wildfire increases 50%, it’s a net loss," said Michael Dietze, an ecologist at Boston University and co-author of the study.
Wildfires and hurricane-force winds probably won’t be a huge threat to New England forests, according to Dietze. But he said that the region — which was widely clear-cut by the mid-1800s and has experienced substantial regrowth — is now facing a "second wave" of deforestation from which recovery is less likely. That's because modern changes in land use — cutting down forests for new buildings and parking lots — are harder to reverse. And also because climate change is playing an increasingly significant role.
While it’s hard to predict exactly what will happen in New England, Dietze says that a warmer climate will likely bring more drought to the region, and allow invasive insects to move in and breed longer.
"Species can survive longer through the growing season and move farther north than they previously were ... things that used to go through one lifecycle per year are able to squeeze in a second round of reproduction," he said. "I don't think there's any reason to expect that trajectory to change."