Extreme heat affects the forests of the Pacific Northwest

This article originally appeared on High Country News.

In the days after a record heat wave baked the Pacific Northwest in 2021, state and federal foresters heard reports of damaged and dying trees across Oregon and Washington. Willamette Valley Christmas tree farmers lost up to 60% of their popular noble firs, and rangers at Portland’s Hoyt Arboretum said the Douglas fir, their state tree, dropped more needles than ever before. Timber plantations reported massive losses among their youngest trees, with some losing nearly all of that year’s plantings.

The damage was obvious even to those who weren’t tasked with looking for it. Motorists, homeowners and tree experts alike called or sent photos of damaged reds, willows and spruces, especially in coastal forests. Parts of the landscape were so desolate it looked as if a wildfire had torn through it.

Some farmers and home owners had tried to prepare water, dumping it on their orchards and yards before and during the heat wave. Lots of branches, leaves and whole tree lost anyway. “There’s a misconception out there that a lot of people, if things are watered just enough, they can get through these events,” said Chris Still, a tree ecologist from Oregon State University and an expert in tree heat physiology. “But the heat spells we’re talking about, like the heat dome, are so intense that I don’t think that’s really a sustainable assumption.” Only watering trees during extreme heat makes intuitive and practical sense, but that idea is based largely on knowledge about drought. After all, almost all research on climate-related stress in trees has focused on the impact of insufficient water. But it turns out that trees respond completely differently to extreme heat compared to prolonged drought. Fós’ own research, including a new study of the heat dome, is part of a growing body of work aimed at unraveling the effects of both conditions. As extreme heat and drought become more common and more severe — and will not always coincide — foresters and tree farmers will need tools to prepare for each.

“The heat spells we’re talking about, like the heat dome, are so intense that I don’t think that’s a detectable subsite anymore.”

Even the cool forests of the Pacific Northwest are at extreme risk of heat
The US Forest Service’s annual aerial survey in Oregon and Washington found that at least 229,000 acres of forest had been damaged by the 2021 heat wave. The worst damage was on steep southern slopes, which gets the most exposure to the sun. Courtesy of Daniel DePinte/US Forest Service

The threat posed by human-caused global warming to Northwest forests was evident long before the 2021 heat dome: the most common conifer species in Oregon and Washington are dying in alarming numbers, many due to drought. Beginning in 2015, state foresters began warning that western hemlock, a particularly drought-sensitive species native to the Coast Range and Cascades, was succumbing to pests and fungi that infested the strained trees. already. Recently, foresters have seen widespread die-offs of western hemlock and Douglas fir. Aerial surveys in 2022 documented what foresters called “firmageddon” – the sudden death of 1.2 million acres of “real fir” (including grand and noble firs, but not Douglas fir), mostly in Oregon .

“All of our trees are drought-stricken,” said Oregon state entomologist Christine Buhl HCN Last July. “They cannot protect themselves against other agents” in their weakened state. Even common pests and native parasites that don’t normally kill trees are now deadly.

When the 2021 heat wave hit, foresters weren’t sure what new chaos it might bring. Drought affects tree stems and the structures that move water and nutrients around, but heat destroys needles and leaves. When those tender green structures heat up – and they often reach temperatures much higher than the air around them – they lose water quickly. The tissues inside them fall apart, and turn red or brown as their chlorophyll breaks down.

“Just like our skin, when (sun exposure) breaks those cells apart and we have blisters and sunburn, it does the exact same thing to those needles and leaves,” said Danny DePinte, a forest health specialist who flies annual aerial surveys for the public. US Forest Service in Washington and Oregon. The 2021 heat dome provided a rare glimpse of the results on a large scale: When DePinte flew over the region later that year, he saw entire landscapes of dead trees on its southern and western sides, where temperatures would be warmer. The worst damage occurred on southern slopes with long exposure and in coastal forests adapted to much cooler temperatures. DePinte’s survey found that the heat wave had damaged at least 229,000 acres of forest — a figure state researchers say only begins to capture the total area damaged, which was likely much larger. .

Research like Still’s, which drew in part on DePinte’s data, has shown that heat stress causes more immediate and severe damage than drought. However, its long-term impacts are much less understood, as events like the 2021 heat dome remain unusual.

On his 2022 survey flights, DePinte found that the most obvious damage appears to be temporary: Damaged areas are mostly green again with new growth. Additional research by Still’s team and others will investigate the potential long-term health effects, including whether the trees will be more susceptible to pests, disease and death.

Researchers will also consider how foresters and tree farmers might respond, as extreme heat waves become more common. Adaptations may include combining certain species to shade more vulnerable trees, deciding which native trees are most tolerant of extreme heat, and planting species on farms or after wildfires that have already adapted itself for warmer conditions further south.

“We have to be smart about the trees we’re planting so we have forests in the same places,” DePinte said. “We have to think hundreds of years into the future: What will this area look like? And then plan accordingly.”

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