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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 new study led by the University of Washington, 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 Patrick Tobin, associate professor in the UW 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 Jacob Betzen, a biological technician with the U.S. Forest Service who completed this work as a UW 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.
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