![]() ![]() The tissues inside them fall apart, and they turn red or brown as their chlorophyll breaks down. When those tender green structures heat up - and they often reach temperatures far higher than the air around them - they lose water fast. ![]() Drought affects tree stems and the structures that move water and nutrients around, but heat destroys needles and leaves. When the 2021 heat wave hit, foresters weren’t certain what new chaos it might bring. Even common pests and native parasites that don’t normally kill trees are now proving lethal. “They can’t protect themselves against other agents” in their weakened state. “All of our trees are drought-stressed,” Oregon state entomologist Christine Buhl told HCN last July. ![]() Aerial surveys in 2022 documented what foresters have dubbed “ firmageddon” - the sudden death of 1.2 million acres of “true firs” (which include grand and noble firs, but not Douglas firs), mostly in Oregon. More recently, foresters have seen widespread die-offs of western redcedar and Douglas firs. Starting in 2015, state foresters began warning that western hemlocks, a particularly drought-sensitive species common to the Coast Range and Cascades, were succumbing to pests and fungi that infested the already-stressed trees. The threat human-caused global warming poses to the Northwest’s forests was evident long before the 2021 heat dome: Oregon and Washington’s most common conifer species are all dying in alarming numbers, many because of drought. Damage was the worst on steep south-facing slopes, which get the most sun exposure.Courtesy of Daniel DePinte/U.S. Forest Service’s annual aerial survey in Oregon and Washington found that at least 229,000 acres of forest had been damaged by 2021’s heat wave. “The heat spells we’re talking about, like the heat dome, are so intense that I don’t think that’s really a tenable assumption anymore.” Given that extreme heat and drought are both becoming more common and intense - and won’t always coincide - foresters and tree farmers will need tools to prepare for each. Still’s own research, including a new study on the heat dome, is part of a growing body of work focused on untangling the effects of both conditions. But it turns out that trees respond quite differently to extreme heat versus prolonged drought. After all, nearly all of the research on climate-related stress in trees has focused only on the impact of insufficient water. “But the heat spells we’re talking about, like the heat dome, are so intense that I don’t think that’s really a tenable assumption anymore.” Simply watering trees during extreme heat makes intuitive and practical sense, but that idea is based largely on knowledge about droughts. “There’s a misconception out there that a lot of people have that, if things are just watered enough, they can get through these events,” said Chris Still, an Oregon State University tree ecologist and expert in tree heat physiology. ![]() Many lost branches, leaves and entire trees anyway. Some farmers and homeowners had tried to prepare, dumping water on their orchards and yards before and during the heat wave. ![]()
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