Trees can slow the warming of our planet by taking carbon out of the air, but only if they’re healthy. And hotter, longer summers could soon be weakening trees beyond repair, devastating whole forests across North America and turning them into sources of carbon, in some cases as soon as 2050.
That’s the upshot of an ominous new study published Wednesday in the journal Ecology Letters. "It's like a thermostat gone bad," said Margaret Evans, an assistant research professor in the University of Arizona’s Laboratory of Tree-Ring Research (LTRR) and a lead author on the study. "Forests act as a carbon sink by taking carbon dioxide out of atmosphere, but the more the climate is warming, the slower the trees are growing, the less carbon they suck up, the faster the climate is changing."
Evans and her colleagues used more than 2 million historical tree-ring records from across North America, a metric used to understand how much a tree grew in any given year, which they then cross-referenced with annual climate conditions that year to make projections about how the planet’s future climate would correspond to tree growth in coming years.
It turns out that while warmer temperatures can be good for tree growth in the short term, there is a tipping point beyond which more heat is detrimental. In the southwestern United States, as well as the interior of Canada and Alaska, for example, forests could see up to a 75 percent slower growth rate thanks to higher summer temperatures, according to the researchers. That could eventually spell death for the trees.
"There is a critical and potentially detrimental feedback loop going on here," Noah Charney, a researcher at the University of Arizona and another author on the study, said in a statement. "When the growth rate of trees slows down in response to environmental stressors such as cold or drought, they can get by for a few years, but over time they deplete their resources and are much more susceptible to additional stressors, such as damage by fire or a big drought or insect outbreaks.”
Eventually, the heat will stress trees to their breaking point. For example, the vast boreal forests of Alaska have been projected in major climate models to flourish and become greener as the planet warms, Valerie Trouet, an associate professor in the LTRR and another author on the study, explained in a statement. "Instead, we see browning," Trouet said.
We’ve counted on those forests acting as important carbon sinks to stave off the worst of climate change in the future. Yet when the team ran their simulations, they found the opposite could be true.
"The positive influence warmer temperatures are believed to have on boreal forests—we don't see that at all."