Solving the Mystery of the Washington Bigleaf Maple Mortality

Bigleaf maple (Acer macrophyllum) is an iconic deciduous tree of the Pacific Northwest. Even if you’ve never had the pleasure of standing in the dappled shade of this largest North American maple, you’ve probably seen its namesake leaf. It’s become something of a meme to post photos of the huge leaves, often more than a foot wide, with a relatively puny human hand next to it for scale. Among the hardwoods of the northwest, it is second in abundance to the ubiquitous red alder. But this widespread and ecologically important tree species is in serious long-term decline. A new article in the journal Forest Ecology and Management explore why.

The first signs of trouble appeared in 2010. Calls began coming in to the Washington Department of Natural Resources (DNR) reporting the decline of maple trees on private and public lands in the area – whole trees were becoming bare and many trees , especially the younger ones, were dying. Amy Ramsey, a forest pathologist with the department, began investigating these early reports with a few other people in the office.

“It was really just a few people prioritizing bigleaf maple,” Ramsey says. “We really had to scrap everything at the beginning, without too much budget.”

The DNR team began collecting samples and partnered with local labs to begin weeding out suspects. From the start, they considered all sorts of causes – fungal infections, root diseases, even damage from squirrel chewing. One by one, these were brushed aside, and as the true culprit remained elusive, the extent of the damage only worsened.

Ramsey says 2016 was a particularly deadly year for young maples, and that may have helped the team narrow down possible causes. That’s because 2015 was, at the time, a record drought year in Washington. Although the winter had seen about average total precipitation, it had been very warm so almost everything had fallen as rain, not snow. The precious little accumulation of snow that accumulated melted away early and the following summer saw some of the hottest and driest conditions on record in the state. Signs pointed to climate-related causes for the fate of the trees.

Then something else happened that moved the investigation forward: a University of Washington graduate student named Jacob Betzen decided to base his master’s research on the question of bigleaf maple decline. . In 2014 he started collecting samples.

“Jake’s a pretty humble guy,” Ramsey says, “but we really wouldn’t have gotten the data that we got or been able to figure out what we’re doing without him focusing his grad work on that. .” His master’s thesis served as the basis for the recent article, and the observations he made, combined with data already collected at DNR and other agencies, pointed to a clear, albeit indirect, culprit: the heat.

Betzen’s research focused on bigleaf maples at sites in western Washington, examining not only the trees themselves, but also the soil and leaf litter surrounding them. Nearly a quarter of the randomly sampled maples showed signs of decline. Although the team found no clear signs of disease or other biological threats, they observed that more trees were damaged in areas with higher summer temperatures. Tree core samples, which allowed them to study stem growth by year, also showed clear signs of damage in years following severe drought summers, particularly after the Great Drought of 2015. This damage begins with premature leaf wilting, then entire branches becoming bare, weakening the tree to the point that it can no longer fight disease or other stressors, and it dies.

The decline of such a large and common tree species will certainly have a long list of downstream effects. “This tree is quite important, economically, culturally and ecologically as well. It is used by many people and is also very common in urban areas,” explains Betzen. Historically, Bigleaf Maples have been a source of timber and medicine for the Indigenous peoples of the Northwest. Bigleaf wood is used today especially in musical instruments, especially guitars and piano frames.

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