Brady Smith/U.S. Forest Service

Ponderosa pine forest in northern Arizona.

Current climate change models assume that trees recover swiftly after a drought ends. That’s not true, according to a new study.

Researchers examined tree-ring data from more than 1,300 sites around the world. By comparing the rings to rainfall records, they could track tree growth before, during and after droughts.

They found most trees grow slower than normal for one to four years following a drought. It’s called a “legacy effect,” and it hasn’t been included in climate change models. 

George Koch, of the Center for Ecosystem Science and Society in Flagstaff, helped design the study.

“If models overestimate the strength of forests as carbon sinks,” he Koch. “That means they underestimate the warming that will occur.” 

Forests usually act like sponges, absorbing CO2 from the atmosphere. The new study shows that drought cripples that ability, even after the rainfall returns.

The finding is a big deal for the American Southwest, where droughts are expected to occur more frequently.

Christopher Schwalm is a computer modeler involved with the study.

“If we have a drying trend, then we’re not going to be a situation where an individual tree can recover,” he Schwalm. “Because in the middle of its one- to four-year recovery window, another drought will come.” 

The research was published last month in Science.