Done for the season: How do Douglas-fir know when to stop growing?

Abstract

Diameter growth is seasonal in Douglasfir, the evergreen tree found in much of western Washington, Oregon, and northern California. Initiation and cessation of diameter growth are both triggered by environmental cues. The tree responds to these cues to improve its chances of growing under favorable conditions. As environmental conditions change, however, land managers want to know how warmer summers and falls may affect diameter growth in Douglas-fir. Forest Service scientists Connie Harrington, Brad St. Clair, and Kevin Ford conducted a study to examine the environmental cues that drive diametergrowth cessation in Douglas-fir under the current climate. They also modeled seasonal cessation into the future, assuming increasing emissions of greenhouse gasses. In the warmer parts of Oregon and northern California, day length will be the limiting factor, preventing an extended fall growing season. In the cooler parts of Oregon and Washington, growth may extend nearly 4 weeks into fall by 2100 because low temperatures are currently the limiting factor. Data gathered during record-high temperatures in 2015 revealed that Douglasfir appears to stop growing when high temperatures coincide with long daylight hours. Research continues on this summer growth-cessation pathway, which currently may be occurring across 2 percent of the coast Douglas-fir range, but by 2100 may affect more than 30 percent of the area.

Citation

Mazza, Rhonda; Harrington, Connie; St. Clair, Brad; Ford, Kevin. 2018. Done for the season: How do Douglas-fir know when to stop growing? Science Findings 210. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 5 p.