High and dry for a NW icon?

If May and June continue to be hotter and drier than average⁠—like this May’s 90-degree weather in Seattle⁠—and the next four or five summers follow the same pattern, more of the Northwest’s iconic western red cedars are likely to die back.

Dieback is characterized by dead tops, brown canopy, and thinning foliage and can be followed by tree mortality.

Robbie Andrus, a postdoctoral researcher with the Washington State University School of the Environment, is the lead author on the first peer-reviewed study of western red cedar (WRC) dieback, released early this year. The study is titled “Canary in the Forest?” because WRCs are more sensitive to drought than other conifers and may be an indicator of more forest damage to come.

Robbie Andrus (Courtesy Researchgate)

Andrus and his colleagues took 30,000 core samples (tree rings) from 280 WRCs in 11 sites from coastal and interior Washington and Oregon. They measured radial growth⁠—increase in the girth of the trunks.

“We found that May and June climate is the most important for increasing tree growth,” Andrus says. WRCs are resilient, living up to 1,000 years and sometimes growing more than 200 feet high. “The tree is well conditioned to weather our hot and dry summers and early fall if May and June are wetter and cooler.”

WRC trees that died experienced a period of declining tree growth for four to five years before death. Most of the mortality in the coastal areas occurred in 2017–2018, which coincided with exceedingly hot temperatures and the longest regionally dry period for May to September in 1970–2020. The early summers of 2019 and 2020 were cooler and wetter, which may give trees a chance to recover.

WRCs older than 150 years with more extensive root systems fared better than younger trees. Trees in eastern Washington and Idaho, where conditions are drier, dealt with extremes better than those west of the Cascades⁠—perhaps because they have longer taproots to reach the water table.

Henry Adams, assistant professor in the School of the Environment, is one of the study’s coauthors and has studied trees around the United States for 22 years. Trees pull water from the soil, he explains, and they draw in moisture from the air. Sunlight turns the nutrients in water to sugar and sap, which nourishes new growth.

If both soil and air are dry because of drought, trees must “pull harder” to get enough water.

“One water molecule pulls on the next like a chain. If it pulls too hard under tension, air bubbles get in there and release pressure⁠—like a rubber band being stretched so far that it breaks,” he says. This sudden hydraulic failure prevents the flow of water to leaves or needles. They dry out and turn brown.

This isn’t just ugly-looking. Trees limit water loss by closing their stomata or pores, according to a 2022 study published in Nature Communications. They reduce or stop photosynthesis, draw less carbon dioxide out of the air, and release less oxygen.

Adams contributed to the international study on the death of local trees such as the WRC. “So far we can’t find any insect, fungus, or other disease attacking this tree, so our research is showing it appears to be the effect of drought and heat alone,” he wrote.

Some of his earlier research indicates that long-time drought is more stressful than extreme heat, like the 2021 “heat dome” over western Washington and Oregon, but the two probably interact. “If the amount of rain and snow stay the same, the water won’t last as long if it’s too hot.”

Wildfires are dramatic, but drought and heat affect landscapes even more, Adams says. “They are like a massively spread-out, slow-burning wildfire.”