Climate change is the culprit behind the increasing deaths of bigleaf maples in Washington and across the Pacific Northwest. That's the conclusion of University of Washington researchers and government agencies who began studying the phenomenon in 2011.
Researchers looked at bugs, diseases, fungus and every other enemy on the list of things that usually attack and kill trees.
In a paper released earlier this month, lead researchers Jacob Betzen and Patrick Tobin concluded that the varied effects of urbanization and climate change were leading to the decline and death of bigleaf maples across their range from British Columbia to California.
Bigleaf's big role
Bigleaf maples are Washington's biggest broadleaf tree. The tree's green leaves turn yellow in fall, revealing how much of the forest canopy is comprised of the tree.
The trees don't have nearly the same timber value that Douglas fir, cedar, alder and other species do, but they are veritable metropolises of forest life. Their moss-covered limbs and spring flowers support a zoo's worth of other lifeforms.
Licorice ferns grow on the same branches that house bird nests. Salamanders hide in its leaf litter. Pollinators thrive on its spring flowers, and chipmunks eat its seeds. The huge leaves provide shade for salmon-bearing streams.
So it was with increasing alarm in 2011 when state foresters began noticing the trees were dying, branch by branch, up and down the Pacific Northwest. Trees were producing small, scorched-looking leaves or none at all.
When the state Department of Natural Resources surveyed state forests in 2011, it found the problem was already widespread.
Betzen, began studying the problem as a graduate student at the University of Washington's School of Environmental and Forest Sciences. Tobin was his entomology professor in the field of disturbance ecology. The pair worked with DNR scientists. They looked at urban and forest trees all over the state.
Betzen completed most of his fieldwork in 2017. He is now employed with the U.S. Forest Service in Coeur d'Alene, Idaho, where he looks for and studies insects and diseases that harm trees. He and his colleagues didn't find any of those usual suspects when they studied the bigleafs.
Instead, they concluded that increasingly frequent and longer lasting periods of hot, dry weather combined with growing urbanization and less contiguous forest land are making the bigleafs sick, Betzen said.
"The overarching thing is probably just the changing climate recently is stressing these trees out with hotter, drier, more frequent droughts in the summertime," he said.
The evidence, he said, is the larger number and severity of sick and dying bigleafs located next to streets, buildings and other developed areas. It might not be surprising that a tree next to a busy road isn't going to do as well as one deep in a forest but, Betzen said, something has changed.
"Some of these trees next to freeways had been doing fine for the last 50 years, but now they're starting to die," he said. The die back has affected bigleafs of all sizes and ages.
Betzen and Tobin took core samples from bigleafs to get a look at the trees' rings. Trees produce a new ring every year and their thickness, especially when compared with other trees, is a good gauge for a tree's health history. They also took samples from nearby Douglas firs to compare with species with each other.
What they saw in those rings was a decline beginning in 2011 which corresponded with the first reports from the field. They also saw a major setback in 2016 and 2017.
"Those years of reduced growth were highly correlated with drought conditions and also heat conditions," Tobin said.
The researchers tested and ruled out pathogens in leaves, roots, bark and every other part of the tree using polymerase chain reaction testing similar to the kind used in COVID-19 testing.
"We were keeping the diagnostic lab busy," Tobin said.
Grand dame of the forest
Acer macrophyllum, the tree's Latin name, translates as "maple large leaf." The leaves are the biggest in the maple world, reaching a foot in width.
The tree can grow 100 feet tall and have trunks with a diameter of five feet.
It's also prolific. It's the second most abundant hardwood tree — after red alder — in the Pacific Northwest.
But if climate change is affecting bigleafs, could the region's timber trees be next? Red alder is a hardwood valuable to the furniture industry and an important nitrogen fixer for recently logged timber lands in Washington.
While a homeowner might be able to give their stressed bigleaf some extra water during a dry spell, the vast majority of trees are left to fend for themselves in the state's public and private forest lands. Timberland is not irrigated in Washington.
Saving bigleafs from further decline is going to take solutions at the macro level. And they are a tree worth saving, Betzen said. He grew an appreciation for the big, green, shady trees during his study.
"Anywhere they're growing, they're a nice place to hang out — out on the Olympic Peninsula or even in someone's yard," he said.
