In the days following a record-breaking heat wave in the Pacific Northwest in 2021, state and federal foresters heard reports of damaged and dying trees in Oregon and the state of Washington. Willamette Valley Christmas tree growers had lost up to 60% of their popular noble fir trees, while caretakers at Portland’s Hoyt Arboretum said Douglas firs, their state tree, had dropped more needles than ever before. Timber plantations reported massive losses among their youngest trees, with some losing nearly all of that year’s plantations.
The damage was obvious even to those not tasked with looking for it. Drivers, homeowners and tree experts phoned or sent pictures of damaged western redcedar, hemlock and spruce, especially in coastal forests. Whole swaths of the landscape were so scorched it looked like a wildfire had spread.
Some farmers and landowners had been trying to prepare, dumping water on their orchards and yards before and during the heat wave. Either way, many branches, leaves and whole trees were lost. “There’s a misconception that a lot of people think that if things get just enough water, they can get through these events,” said Chris Still, a tree ecologist at Oregon State University and an expert in heat physiology. trees. “But the hot spells that we’re talking about, like the heat dome, are so intense that I don’t think that’s really a tenable hypothesis anymore.”
The simple act of watering trees during extreme heat makes intuitive and practical sense, but much of this idea relies on knowledge of droughts. After all, almost all research on climate-related stress in trees has focused solely on the impact of a lack of water. But it turns out that trees react very differently to extreme heat than to prolonged drought. his own research, including a new study on the heating dome, is part of a growing body of work aimed at disentangling the effects of the two conditions. As extreme heat and drought are becoming more common and intense – and won’t always coincide – foresters and arborists will need tools to prepare for each.
The common thread of human-caused global warming for Northwest forests was evident long before the 2021 heat dome: the most common coniferous species in Oregon and Washington are all dying in alarming numbers, many because of the drought. Starting in 2015, state foresters began warning that western hemlocks, a particularly drought-sensitive species common to the Coast Range and Cascades, were succumb to parasites and fungi that infested already stressed trees. More recently, foresters have witnessed the massive mortality of western white cedar and Douglas firs. Aerial surveys in 2022 documented what foresters have dubbed “firmageddon– the sudden death of 1.2 million acres of “true firs” (which include tall and noble firs, but not Douglas firs), mostly in Oregon.
“All of our trees are drought stressed,” Oregon State Entomologist Christine Buhl told High Country News last July. “They can’t protect themselves from other agents” in their weakened state. Even common pests and native parasites that don’t normally kill trees are now proving deadly.
When the 2021 heat wave hit, foresters weren’t sure what new chaos it might bring. Drought affects tree stems and structures that move water and nutrients, but heat destroys needles and leaves. When these tender green structures heat up – and they often reach temperatures far above those of the air around them – they quickly lose water. The tissues inside break down and they turn red or brown as their chlorophyll breaks down.
“Just like our skin, when (exposure to the sun) tears those cells apart and we get blisters and sunburns, it does exactly the same to those needles and leaves,” said Danny DePinte, a forest health specialist who conducts annual aerial surveys for the United States Forest Service in Washington and Oregon. The 2021 heat dome offered a rare glimpse of large-scale results: When DePinte flew over the region later that year, he saw entire landscapes of scorched trees on their south and west sides, where temperatures would have been the hottest. The worst damage occurred on prolonged exposed southern slopes and in coastal forests adapted to much cooler temperatures.
DePinte’s survey found that at least 229,000 acres of forest had been damaged by the heat wave – a figure state researchers say only begins to capture the total area damaged, which was likely much more big. Research like Still’s, which relied in part on DePinte’s data, has clearly shown that heat stress causes more immediate and acute damage than drought. Its long-term impacts are far less understood, however, as events like the 2021 heat dome are still unusual.
During his 2022 survey flights, DePinte found that the most obvious damage appeared to have been temporary: Damaged areas are again mostly green with new growth. Additional research, led by Still’s team and others, will investigate possible lingering health effects, including if trees become more susceptible to pests, disease and death.
Researchers will also examine how foresters and arborists might respond as extreme heat waves become more common. Adaptations could include planting certain species together to shade the most vulnerable trees, determining which native trees are most tolerant of extreme heat, and planting species on farms or after wildfires that are already adapted to the warmer conditions further south.
“We have to be smart about the trees we plant so that we have forests in the same places,” DePinte said. “We have to think hundreds of years in the future: what will this area look like? And then plan accordingly.