The killing effects of the long, hot drought of 2015 are showing up in dying tree tops, thinning needles, burgeoning beetles and an unprecedented number of dead trees in city parks.
From mountain forests to city parks, trees that suffered terribly in last year’s drought are dying, and burgeoning pests are taking advantage of stressed trees struggling to hang on.
More than 500 dead trees from big leaf maples to cottonwoods, birches and more already have been counted this year in Seattle city parks, and summer’s far from over. A typical year sees 130 trees culled, said Jon Jainga, urban forestry operation manager for Seattle Parks and Recreation.
Statewide, officials are seeing trees with red needles, dead tops, dying branches, dropped needles and other signs of stress.
Implicated in the unfolding treemageddon is the drought of 2015.
“We had that drought all the way through the summer, and we are seeing some of the effects and signs of it,” Jainga said. “Our rate of tree failure is much higher than normal.”
The state experienced a record low snowpack in 2015, below-normal spring and summer precipitation, and record high temperatures for most of the year. By August, Eastern Washington was experiencing an extreme drought that lasted through the end of October.
Other factors also are taking their toll, including an aging urban forest, and more active or severe windstorms that tear up and topple trees, Jainga said.
Seattle Parks and Recreation is alarmed at the rate of tree deaths and has hired a consultant to assess city parks to determine where pruning and removal are needed for safety.
“There is not a heck of a lot we can do, other than make sure it’s safe for park users,” Jainga said. Long term, it may also be time to consider replacement with new tree varieties that are more drought-resistant, Jainga said.
Meanwhile annual aerial surveys by the state Department of Natural Resources of state, federal, tribal and privately owned forests this year reveal not the usual mountainsides cloaked with green, but a lot of red and brown.
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“We are seeing widespread effects,” said Glenn Kohler, a tree-health specialist with DNR. “It’s pretty much statewide, it’s scattered around, but worse in places where you have well-drained, rocky, glacial outwash soils, and it’s worse in young trees with less-developed root systems.”
Douglas firs have dead tops and are flagged with dead branches. Cedars show entire crowns turned brown. Western hemlocks have dropped their needles.
Drought kills a tree when it loses more water through its needles and leaves than it can replace from the soil with its roots. Without water, trees can’t make food by photosynthesis. Their interior plumbing can also cavitate with air bubbles, causing tissues above the damaged tissue to die.
Forest-health experts predict the effects of the killer drought will be with us for some years to come, as trees continue to struggle and even die. It is typical for effects to unfold in the next season, when a tree that used up its resources to get through the previous year has nothing left to defend itself or thrive in the next season.
With so many trees under stress, legions of pests are prospering. Center stage right now is the bronze birch borer, thriving and expanding its range. The beetles detect chemicals emitted by dying and distressed trees and move in for the feast.
“Bronze birch borers love drought-stressed birch trees,” said Nolan Rundquist, arborist for the city of Seattle. “We’ve had several years of really hot, dry weather and now you go into the older neighborhoods and you are seeing big birches, 15, 18 inches in diameter, and the tops are dying. It’s been building, every year you see more.”
At the Washington Park Arboretum, horticulturists last week recorded the first ever attack of bronze birch borer in the collection. An entire grove at the arboretum is affected, with some trees mostly dead from the top down and one perished.
The insect arrives as an adult beetle and lays an egg on the tree’s bark in a groove or crack. The larvae hatch and bore their way inside, to eat the nutritious cambium layer that is the living skin of the tree, right under the bark.
Wood showing where bark dropped off the birches was deeply marked all over with the galleries, or tunnels, chewed by larvae getting bigger and bigger, as the grub fattens on the trees’ vital nutrients. By June, the larvae fledge as a beetle about the size and shape of a firefly, but with a metallic sheen from which the bronze birch borer takes its name.
The bug makes it way out of the tree by chewing a characteristic D-shaped hole, then flies off to prospect for another tree, and the cycle repeats.
At the arboretum, the plan for now is to remove dead wood of affected trees, and monitor the collection for trouble, said Ryan Garrison, a staff horticulturist.
The best prevention is to keep trees in good health: well watered, nourished with compost and coddled with mulch to deter weeds and help retain moisture. Native Western Washington varieties of birch are also less susceptible. At the arboretum, it’s the European black birches in dry sites that are taking a beating.
Similarly at Green Lake Park, the native river birches near the water are doing fine but European birches on drier higher ground are dying as the beetles feast. Insect damage first noticed six years ago is accelerating, said Nicholas Johnson, arboriculturalist with Seattle Parks and Recreation.
“We are seeing all these changes; we don’t get those eight months of drizzle like we used to,” Jainga said. “Things like you normally see in Southern California, we are starting to see them here.”
As Washington’s climate changes, no one should be surprised to see effects on trees. On average, spring is earlier, fall is later and winter is getting squeezed on both ends, with a growing season about five days longer than it used to be, notes Nick Bond, Washington state climatologist and a senior research scientist with the Joint Institute for the Study of Atmosphere and Ocean (JISAO) of the University of Washington.
Average yearly temperatures in the Puget Sound lowlands have warmed 1.4 degrees Fahrenheit since the 1890s, Bond said, while in summer, warming is greater, at 1.7 degrees. Even the nights don’t cool down like they used to. Winters also bow out far earlier.
Typically, going back at least 20 years, the last killing frost was in late March or April. But this year the last frost recorded at Sea-Tac came on Jan. 9.
It adds up to trees needing more water, more violent weather events that beat up trees already brittled by drought, and a bonanza of bugs that warmer winters don’t knock back.
It’s important to remember, Bond said, that last year was way out of range, the hottest summer in state history. Heat and drought also stoked a historic fire season in Washington, with three firefighters killed and another still recovering from terrible burns, and hundreds of thousands of acres of forest burned.
The longer-term picture isn’t as extreme, and there’s plenty of interannual variability. But the overall pattern holds true. “There is that inexorable upward trend,” Bond said.
“As the climate changes we have to adapt to that, maybe we should not put in those trees that need a colder climate.”