CORVALLIS – Douglas-fir trees will likely experience more stress from drier air as the climate changes than they will from less rain, computer modeling by Oregon State University scientists shows.

The research is important because Douglas-fir are widespread throughout the Pacific Northwest, an iconic species with ecological, cultural, and economic significance, and learning how the trees respond to drought is crucial for understanding forest sensitivity to a shifting climate.

Douglas-fir grow in a range that stretches from northern British Columbia to central California, and also includes the Rocky Mountains and northeastern Mexico. In Oregon, Douglas-fir are found in a variety of mixed conifer and hardwood forests, from sea level to 5,000 feet, and can reach a massive size; a tree on Bureau of Land Management land in Coos County is more than 300 feet tall and greater than 11 feet in diameter.

Native Americans traditionally used the wood of Douglas-fir, Oregon’s official state tree since 1936, for fuel and for tools, its pitch as a sealant, and many parts of the tree for medicinal purposes.

A versatile timber tree, Douglas-fir is a source of softwood products including boards, railroad ties, plywood veneer and wood fiber. Oregon leads all U.S. states in softwood production and most of that is Douglas-fir.

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

A team led by Karla Jarecke, a postdoctoral researcher in the OSU College of Earth, Ocean, and Atmospheric Sciences, sought to look at how the mechanisms behind carbon fixation and water “fluxes” – exchanges of water between trees and the atmosphere – would respond to decreases in rainfall and increases in VPD.

Douglas-fir, like other plants, create food for themselves using sunlight, carbon dioxide, and water during photosynthesis. The process pulls CO2, a greenhouse gas, from the air, releases oxygen, and results in the long-term storage of carbon in the wood and roots.