IOWA CITY — A University of Iowa researcher says climate change may eventually threaten Iowa’s state tree, the bur oak.

Matt Dannenberg, a professor in the U-I Department of Geographical and Sustainability Sciences, says we’re already seeing the start of what could be a very troublesome weather pattern in the years ahead that could be difficult for several species of trees, especially the bur oak.

“We expect to see an increase in extreme events,” Dannenberg says. “We expect dry years to get drier, wet years to get wetter and for those extremes to be more frequent and more severe.” U-I researchers used tree-ring data from more than 1,000 sites across the country to examine how increases in precipitation extremes affect forest growth. He says many types of trees in the upper Midwest, the southwest and western U.S. would be impacted.

Dannenberg says, “As the climate continues to warm, as extreme events continue to be more frequent, we could see declines in bur oak as well as other species as a result of that change.” The study found many tree species respond more strongly to drought than they do to rainy periods, meaning, that reduced growth during drought is not completely offset by increased growth during rainy years.

This means some species could see their growth stunted by shifting rainfall patterns.  “We’re certainly not saying all of Iowa’s bur oaks are going to die or anything like that,” Dannenberg says. “But, what it does mean is that you increase the probability of having multiple bad years in a row and that is something that puts trees at greater risk of dying.”

While he couldn’t offer tips for saving specific trees that might be threatened, Dannenberg says, “The most important thing I can think of is mitigating the affects of climate change.” The study was published last week in the journal Science Advances.