How Hot Is Too Hot?
ON MONDAY, JUNE 28, 2021, forecasts called for yet another day of record-breaking temperatures in the Pacific Northwest. Phil Hunter and his wife decided to beat the heat by leaving their home and Christmas tree farm near Tacoma, Washington, for the higher, cooler elevations offered by nearby Mount Rainier. When they left their fir trees in the early morning, they were green. By the afternoon, Seattle and Portland had reached the unprecedented temperatures of 108°F and 116°F, respectively. That evening, as Hunter drove down the mountain on his way back home, he saw the first signs of what this extreme heat was doing to trees. “We started seeing that the trees on the side of the road were burnt. The trees were really, really red on one side,” Hunter recalls. When he arrived home, the tree farmer saw his firs — green and verdant that morning — were now orange, red, and brown, scorched in the heat. This surprised Hunter, because it wasn’t as if his trees didn’t have access to water. The previous day, he had irrigated them, something most Christmas tree farmers in Washington and Oregon don’t typically need to do. “I don’t know what trees I saved by continuing to irrigate. I only know what I lost,” he says. Hunter estimates that within a matter of hours many of his trees had been irrevocably harmed. “They slowly died after that. There was no way to rehydrate them.” In the end, he lost 1,500 trees to the heatwave, amounting to a 20 percent loss of revenue for the year. What became known as the 2021 “heat dome” lasted from June 25 to July 2, 2021. During those eight days, the Pacific Northwest was enveloped in a high-pressure bubble of extreme temperatures reminiscent of Phoenix or Death Valley. The heat dome killed hundreds of people in Oregon, Washington, and British Columbia. And while the Pacific Northwest’s human residents suffered in the extreme heat, so did its trees. In a matter of days, the green needles and leaves on many of the region’s trees turned shades of red and brown. This widespread “foliage scorch,” as scientists would later name it, affected wild forests, commercial tree plantations, and Christmas tree farms alike. Scientists would later determine that the 2021 heat dome was so rare and extreme for the region that it was “virtually impossible without human-caused climate change.” In fact, they say the heat event was so unlikely that a similar event had the probability of occurring just once in the past 1,000 or even 10,000 years. Climate change has upended all that. In recent decades, heatwaves have increased in their duration, frequency, and intensity, a trend that is expected to persist and worsen as climate change accelerates in the years ahead. This emerging heatwave trend has scientists concerned, not only for human health and wildlife, but also for the health of plants, those key organisms that make up the base of most terrestrial food webs on Earth. Reporter Nathan Gilles digs into the growing threat that heat and drought pose to trees, and why understanding these related phenomena could help protect forests.
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