Gaze across the mountains of the Northwest these days and you may notice an unusual number of dead firs, pines and other conifer trees scattered among the green ones. Drought is usually considered the prime culprit. But recent research suggests the damage that has historically been done to conifer forests by routine dry spells is being compounded by climate change.
On a clear sunny morning, I join Matt Krunglevich on the side of Mount Ashland. He points toward the hillside to the west.
“The biggest key you can see is these pockets of dead and dying trees,” he says. “The red needles, and then some of the ones that were probably from two or three years ago, that are gray now.”
Krunglevich is with the Oregon Department of Forestry. He says that it’s easy to miss, but once you start tuning your eye to notice the rust red and gray amidst the green, you notice those dead trees all over the place. So, what’s going on?
“Basically, that’s where the trees are starting to succumb to insects and pathogens,” he says. “In the last several years, the combination of the drought, the longer summers and maybe a couple of fires that we’ve had have caused the insect and disease populations to increase, and we’re really starting to see the effects of that.”
Krunglevich takes me further up Mount Ashland for a closer look at the dying trees. We scramble down a hillside to look at a few trees that have been cut down and left.
Krunglevich takes out a hatchet and chips away some of the dead tree’s bark to reveal the insect damage underneath…
“What these insects go after is the cambium layer,” he says. “So they drill through the bark and they get to the cambium layer. I think we’re looking at turpentine beetles here. So this is an itty-bitty little guy, about the size of a piece of pencil lead, maybe … He’s gonna try to go out and eat some more trees.”
What foresters call “drought related conifer mortality” is reaching record levels throughout much of the Pacific Northwest. A 2009 study by researchers from the University of Washington, Oregon State University and federal agencies found the rate of tree death in the region had doubled over the preceding 17 years. Max Bennett -- an Oregon State University Extension agent in the Rogue Valley -- says he’s seen the situation getting worse.
“The level of mortality, of tree die-back, this year is the most that I’ve seen in the 16 years since I’ve been down here,” he says.
Bennett took note of the weather data during the most recent drought in southern Oregon, from 2013 to 2015.
“In Medford, for that 36 month period, 26 of the 36 months had below-average rainfall and 31 of the 36 months had above-average temperatures.”
Bennett says this has historically been a cyclical phenomenon; as drought cycles have come and gone, conifer die-off has fluctuated with them.
But recent research at the Los Alamos National Labs in New Mexico suggests that, as global warming increases, drought may be less of a factor than hotter temperatures.
Plant ecologist Sean Michaletz contributed to that research. He says high temperatures put trees under stress not only by increasing demand for water, but by speeding up the tree’s internal processes, such as photosynthesis and respiration.
“The rate of chemical reactions increases exponentially with temperature,” he says, “and if temperature becomes too high, these physical processes can basically crash and stop working.”
While more frequent droughts are one likely result of climate change in the Northwest, the Los Alamos findings suggest that rising temperatures mean more tree death, even if there’s plenty of rainfall. That’s because warmer air pulls moisture out of trees and the soil. Los Alamos researcher Turin Dickman sums it up.
“So, whether or not precipitation regimes change,” she says, “as temperature increases with CO2, trees are going to be experiencing dryer conditions.”
Dickman and her colleagues project that unless trees adapt quickly to the changing climate, pine and fir-based forests in much of the West could see massive die-off by 2050. That would likely lead to structural changes in Northwest forests, with new species of plants replacing the conifers.
Meaning the forest landscapes our grandchildren grow up with could look very different from the one’s we’ve enjoyed for generations.