As its name suggests, the bigleaf maple tree’s massive leaves are perhaps its most distinctive quality. A native to the Pacific Northwest’s wet westside forests, these towering trees can grow leaves up to 1.5 feet across — the largest of any maple.

But since 2011, scientists, concerned hikers and residents have observed more stressed and dying bigleaf maple across urban and suburban neighborhoods as well as in forested areas. Often the leaves are the first to shrivel and die, eventually leaving some trees completely bare. While forest pathologists have ruled out several specific diseases, the overall cause of the tree’s decline has stumped experts for years.

A led by the 91±¬ÁĎ, in collaboration with Washington Department of Natural Resources, has found that bigleaf maple die-off in Washington is linked to hotter, drier summers that predispose this species to decline. These conditions essentially weaken the tree’s immune system, making it easier to succumb to other stressors and diseases. The findings were published Sept. 16 in the journal Forest Ecology and Management.

“These trees can tolerate a lot, but once you start throwing in other factors, particularly severe summer drought as in recent years, it stresses the trees and can lead to their death,” said co-author , associate professor in the 91±¬ÁĎ School of Environmental and Forest Sciences.

In addition to warmer, drier weather, the researchers found that bigleaf maple are more likely to decline near roads and other development — especially in hotter urban areas. Across multiple years and sites in Western Washington, they weren’t able to find any single pest or pathogen responsible for the mass decline; rather, all signs point to climate change and human development as the drivers behind the regional die-off.

“Managing, protecting and utilizing our urban and wild ecosystems in the face of climate change and human population growth is and will continue to be one of the major challenges facing us,” said lead author , a biological technician with the U.S. Forest Service who completed this work as a 91±¬ÁĎ graduate student. “This research investigating bigleaf maple is one small piece of that larger puzzle.”

From field sampling and lab work, the researchers found that bigleaf maple grew less in summers that were hot and dry, both in their overall mass as well as leaf size. One of the signature signs of distress, they found, was significantly smaller leaves. In drought conditions, trees use more energy trying to survive and defend themselves from diseases and other threats.

“These results show that summer heat and drought impact the health of iconic tree species of Washington, like bigleaf maple, even in Western Washington, a region known for abundant precipitation. Health impacts to our forests and tree species are likely to continue as we have increased periods of drought each year,” said co-author , an environmental planner and forest pathologist with Washington DNR.

For this study, the research team revisited a selection of sites around Western Washington where DNR in 2014 and 2015 had taken samples and performed testing on trees in decline. They also chose 36 roadside sites where maples were present. Finally, they randomly selected an additional 59 sites on public land across the region where bigleaf maple are known to exist. Across these randomly chosen sites, they found that nearly a quarter of the bigleaf maple trees showed signs of decline.

From each study site, they collected soil, leaves, stems and tree cores, which they analyzed in the lab. Tree cores allow scientists to learn about the age and growth rate of a tree — as well as weather history at that location — without having to cut it down.

From the analysis of the tree cores, the team found that the growth of bigleaf maple has varied significantly since 2011, and was especially lower in years with hotter, drier summers. They compared this growth to that of Douglas fir trees, which they also cored, and found their annual growth was consistent — meaning that bigleaf maple are especially sensitive to dry, hot weather.

“For us, these analyses were a big piece of the puzzle,” Tobin said. “This helped us determine that their decline is a recent phenomenon that is linked to weather conditions.”

These findings will likely change the way foresters manage bigleaf maple in both urban and wild settings. This might mean planting the trees in different locations, watering more in urban areas or using seed stock better adapted to the projected future conditions of a site, Betzen said. In forests, it might mean a focus on keeping intact landscapes free from more urbanization.

Other co-authors are of the 91±¬ÁĎ and of Washington DNR. This research was funded by the U.S. Department of Agriculture NIFA McIntire-Stennis Cooperative Forestry Program, Washington DNR and the David R.M. Scott Endowed Professorship in Forest Resources.

For more information, contact Tobin at pctobin@uw.edu and Betzen at jacob.betzen@usda.gov.

About 450 nonnative, plant-eating insect species live in North American forests. Most of these critters are harmless, but a handful wreak havoc on their new environment, attacking trees and each year causing more than $70 billion in damage.

The problem is, scientists often don’t know which insect will emerge as the next harmful invader.

A team led by the 91±¬ÁĎ, drawing largely on the evolutionary history of insect-plant interactions, has developed a way to understand how nonnative insects might behave in their new environments. The team’s model, described in a appearing Oct. 17 in the journal Ecology and Evolution, could help foresters predict which insect invasions will be problematic, and help managers decide where to allocate resources to avoid widespread tree death.

“What makes the bad invaders so special? That has been the million-dollar question, for decades,” said , an associate professor in the 91±¬ÁĎ School of Environmental and Forest Sciences and one of the project leaders. “This has the potential to profoundly change how we predict the impact of nonnative species and prioritize limited resources used to mitigate these impacts.”

The new model can quickly evaluate whether a newcomer insect, even before it gets here, has a high probability of killing a population of North American trees. To use the model, all that’s needed is information about the insect’s feeding method (wood, sap or leaf feeder, for example) and what trees it feeds on in its native range. The model will then determine whether any North American trees are at risk of dying from it.

The research team focused on nonnative insects that utilize North American conifers — cone-producing trees such as pine, cedar, fir and spruce. They identified nearly 60 of these conifer-specialist insects, then built an exhaustive database of information about each one, including life-history traits and characteristics of the trees they attack. Six insects emerged as “high impact,” meaning they have killed large swaths of otherwise healthy native trees.

For example, the — a wingless, sap-sucking insect from Europe that infests and kills fir trees — has left more than 100,000 acres of dead trees across the Pacific Northwest. Another, the from Asia, has decimated New England forests by sucking on the thin inner bark of trees.

So what causes a select few nonnative insects to become the most destructive invaders?

“In the past, research has focused on aspects of the insects themselves, but we realized that wasn’t the case at all,” said lead author , who completed this work as a postdoctoral researcher at the 91±¬ÁĎ.

Whether a nonnative insect takes hold and becomes destructive has more to do with the evolutionary history between the new (North American) host tree and the insect’s native host tree from its home region, Mech explained. Molecular tools that allow scientists to construct comprehensive phylogenies (or maps) of how tree species evolved was key to the team’s breakthrough.

For example, if a pine tree in Asia and another in North America diverged tens of millions of years ago, the North American pine likely wouldn’t have retained defenses against an insect that only lives with the pine in Asia. Alternatively, two pines on both continents that share more evolutionary history and diverged more recently might still share similar defenses.

The new model helps identify the evolutionary “perfect storm” for conifers, where the invasive insect still recognizes the new tree as a food source, but the tree hasn’t retained adequate defenses to keep the invader in check.

“What we did in just two years is what could have taken one person their career to answer, but to have 15 people with insights and expertise sharing, that’s what really led to what we were able to achieve,” said Mech, who will continue this work as an assistant professor at the University of Maine.

The researchers are building a similar database and model for nonnative insects that utilize hardwood trees, such as maple, oak and ash. Both the conifer and hardwood tree databases will be publicly available for other scientists to use.

They are also partnering with the Davey Tree Expert Company to develop a mobile app that a forester could use to determine potential insect threats if a species of tree is planted in a specific location.

Co-authors include Kathryn Thomas of U.S. Geological Survey; Travis Marsico and Ashley Schulz of Arkansas State University; Daniel Herms of the Davey Tree Expert Company; Craig Allen and Daniel Uden of University of Nebraska-Lincoln; Matthew Ayres of Dartmouth College; Kamal Gandhi of University of Georgia; Jessica Gurevitch of Stony Brook University; Nathan Havill and Andrew Liebhold of the U.S. Forest Service; Ruth Hufbauer of Colorado State University; and Kenneth Raffa of University of Wisconsin, Madison.

This research was funded by the U.S. Geological Survey’s John Wesley Powell Center for Analysis and Synthesis, the Nebraska Cooperative Fish and Wildlife Research Unit, the 91±¬ÁĎ, U.S. Forest Service, National Science Foundation and the National Institute of Food and Agriculture.

For more information, contact Mech at angmech23@gmail.com and Tobin at pctobin@uw.edu.