The curious case of the disappearing maple

By PASSANT RABIE
Bigleaf maple trees in Washington state are on the decline. Researchers are on the hunt for the cause, and climate change is turning into a lead suspect.

Daniel Omdal has driven past the same bigleaf maple tree for decades, often stopping his car to take pictures of its full, expansive crown. In the past few years, however, the tree has started to look more lopsided, with bare branches and patches in its crown with little to no growth.

To Omdal, a forest pathologist, it seemed like an obvious case of an insect infestation. If not, perhaps some kind of disease: a damaging fungus, wilt or a rogue bacterium. Whatever it was, it wasn’t isolated to one tree. The extent of sick bigleaf maples was alarming, and Omdal wasn’t the only one who was worried.

Omdal’s colleagues at the Washington State Department of Natural Resources, where he has worked since 1997, had noticed the same symptoms in many other bigleaf maples. So had many residents of the region, who called the state to report their concerns. The issue had also been occurring nationwide, with reports of sharp declines of urban tree populations in different states, such as the oak tree in Southern California. In Washington, the problem was hard to miss: Bigleafs, also known as Oregon maples, are a staple of the Pacific Northwest landscape.

“These calls became more frequent, I couldn’t so easily dismiss the concerns,” Omdal says. In 2011, he became part of a state-led team investigating the bigleaf die-offs.

The group discovered that about 40 percent of bigleaf maple trees in Washington state are declining, says Jacob Betzen, a graduate student at the University of Washington’s School of Environmental and Forest Sciences, who has been working with the investigative team for the past two years.

The first suspect on their list was Armillaria, a fungus that causes the roots of the tree to rot. But when the team tested hundreds of trees for it, most of their results came back negative. Then, the researchers tested for another fungus called verticillium wilt. Also negative. Often, a few trees would be infected, but it was never widespread enough to be the primary cause of the species’ decline.

Omdal collected soil samples to test in the lab to look for other causes. Every time his team followed a new lead, it didn’t pan out. “We would come to a dead end,” Omdal says.

Patrick Tobin, Betzen’s advisor and a specialist in disturbance ecology, added, “It’s been puzzling, there’s no smoking gun here.”

Then Betzen noticed something curious about the die-offs. They are much more common in developed landscapes and areas that are warmer, drier and closer to roads. That led to a new suspect: climate change. “It seemed probably related to recent weather patterns, it’s getting hotter and drier in Washington in recent years,” Betzen says. The group’s results won’t be published until Betzen concludes his research at the end of the year, but Tobin is confident that the key driver causing bigleaf maple die-offs is, in fact, climate change.

Source: The curious case of the disappearing maple – Scienceline, 2018-12-05