Scientists say dry air rather than less rain will likely cause more stress for Douglas fir trees

Drier air caused by the changing climate is stressing Douglas fir trees, Oregon State University researchers say. 

PORTLAND, Ore. (KOIN) – Drier air caused by the changing climate is stressing Douglas fir trees, Oregon State University researchers say. 

The stress these trees experience from drier air is more than they experience from less rain, according to researchers.

Douglas fir trees are widespread throughout the Pacific Northwest and have ecological, cultural and economic significance. Scientists at OSU said learning how the trees respond to drought is crucial for understanding how sensitive Pacific Northwest Forests are to a shifting climate. 

In the OSU study, which was published in Agricultural and Forest Meteorology, scientists simulated the response of a 50-year-old stand of Douglas fir trees on the Oregon Cascade Range’s west slope to less rain and high vapor pressure deficit, or VPD. VPD is “basically the atmosphere’s drying power,” OSU said. 

Karla Jarecke, a postdoctoral researcher in the OSU College of Earth, Ocean, and Atmospheric Sciences, led the team of researchers. They wanted to see how the exchanges of water between trees and the atmosphere would respond to decreases in rainfall and increases in VPD. 

Jarecke said what governs carbon fixation and water fluxes in response to increased temperatures and limited water in places with wet winters and dry summers is only partially understood. 

“High VPD and lack of soil moisture can create significant water stress in forests, but dry atmosphere and lack of rainfall are strongly linked, making it difficult to discern their independent effects. They tend to both occur during the summer,” she explained. 

Jareck worked with Kevin Bladon and Linnia Hawkins from OSU’s College of Forestry, along with the U.S. Forest Service’s Steven Wondzell, to use a computer model to disentangle the effects of the two phenomena. 

Researchers created a model that uses a series of equations to illustrate how well Douglas fir trees are equipped to deal with water stress. These equations showed that less spring and summer rain is likely to have a smaller impact on the forest productivity than increased VPD, or drier air. 

“Decreasing spring and summer precipitation did not have much of an effect on Douglas-fir water stress because moisture remained plentiful deep in the soil profile,” Jarecke said. “This demonstrated that the effect of reduced rainfall under future climate change may be minimal but will depend on subsurface water availability.” 

Instead, Jarecke said heat-driven increases in VPD are likely to cause water stress regardless of the amount of moisture in the soil. 

She said there’s still limited knowledge when it comes to knowing how trees will respond to extreme temperatures and VPD anomalies such as the record-breaking temperatures that occurred in the Northwest during the heat dome in 2021. 

Bladon highlighted that this OSU study shows the important role of atmospheric droughts in creating stress conditions for trees. 

“This has potential implications for not only driving substantial tree mortality, but also influencing wildfires, as other studies have shown strong relationships between VPD and forest area burned in the western United States,” he said. 

Douglas fir trees grow from Northern British Columbia to Central California. They also grow in the Rocky Mountains and parts of Northeastern Mexico. In Oregon, they’re found at elevations from sea level to 5,000 feet and can reach a massive size. 

It’s Oregon’s state tree and is a source for softwood products, including boards, railroad ties, plywood veneer and wood fiber. 

Oregon leads all U.S. states in softwood production and OSU said most of that is from Douglas fir. 

Native Americans have also traditionally used Douglas fir wood for fuel and tools. They used its pitch as sealant and many parts of the tree for medicinal purposes.