PORTLAND, Ore. (KOIN) – The scorch that was visible on trees across the Pacific Northwest after the 2021 heat dome was more a result of the triple-digit heat than drought conditions, researchers at Oregon State University said. 

Scientists from the OSU College of Forestry published a paper explaining that the leaf discoloration and the damage trees sustained were caused by direct exposure to solar radiation during the hottest afternoons during multiple days of record-setting, triple-digit temperatures in June 2021. 

The research, which was led by Christopher Still from the College of Forestry, was in response to an article published in Tree Physiology that concluded the trees’ problems were a result of drought and a failure in the trees’ hydraulic system. This system helps foliage stay cool as water evaporates. 

“While we think the drought/hydraulic hypothesis is partly true, we argue that multiple lines of evidence suggest the main issue was in fact direct heat damage,” Still said. 

The heat dome was one of the most extreme heat events ever recorded anywhere in the world and researchers point out that heat waves are not just associated with droughts. Heat waves are becoming more frequent during both wet and dry conditions. 

Researchers said they studied coastal Douglas fir and western hemlock forests in Western Oregon and Washington and found the greatest impacts of the heat dome were in areas that were experiencing comparatively low levels of drought. 

In some parts of the Willamette Valley and parts of the western slopes of the Cascade Range that were experiencing severe to exceptional drought during the heat dome, researchers said the trees showed less damage to their foliage.

“It’s also important to remember that conifer needles can discolor for many reasons besides being dried out,” Still said.

Another indication that the foliage damage was caused by the extreme heat was the fact that much of it resembled the damage that is caused by heat generated by wildfires, Still said. 

Trees on south- and west-facing slopes and in exposed areas near roads generally showed more scorch than other areas. These trees also did not show as much damage on their other sides, further proving that drought and hydraulic failure were not the likely causes of leaf damage, Still said. 

“The scorching that did occur happened fast, within days and sometimes hours, much faster than would typically be associated with a malfunction of the trees’ water moving capabilities,” he said. 

The scientists clarified that they are not saying hydraulics didn’t play any role in the leaf damage or eventual death of some trees, but said they need to explore connections between hydraulic properties and heat tolerance. 

“What happened during the heat dome argues for a renewed emphasis on understanding the underlying physiological and biophysical mechanisms that can lead to heat resilience,” Still said. 

OSU College of Forestry research associate Adam Sibley co-authored the commentary, along with scientists from the U.S. Forest Service and from the Australian organizations Biodiversity and Conservation Science and the Commonwealth Scientific and Industrial Research Organization.