OSU Extension: Understanding Douglas-fir Decline

Trees on the Edge

An OSU publication written by Max Bennett and Christopher Adlam

The OSU Extension guide, Trees on the Edge: Understanding Douglas-fir Decline and Mortality in Southwest Oregon, explores why Douglas-fir trees are in trouble, especially in places like the Rogue and Umpqua Valleys. Over 260,000 Douglas-firs have died across nearly 150,000 acres between 2016 and 2019 alone, a stark contrast to the occasional diebacks seen in the past. The problem? A mix of drought, difficult growing conditions, and an insect called the flatheaded fir borer. These factors have combined to create what experts are calling a “decline spiral,” where trees already struggling with low water reserves are even more vulnerable to insects and diseases.

Causes of Douglas-fir decline and mortality

Besides the ecological loss, this dieback has broader impacts: when dead trees pile up, they add a lot of extra fuel to forests, which raises wildfire risk. This deadwood contributes to high-severity fires and can slow firefighter response. There’s also the loss of timber income, damaged forest scenery, and increased costs of removing hazard trees – which can exceed $5,000 per acre in some cases.

OSU’s recommendations emphasize thinning out dense stands of Douglas-fir, especially in dry areas that see less than 35 inches of rain a year. This can help reduce competition for resources like water and sunlight and ease drought stress on the remaining trees. Land managers are also encouraged to add more drought-resistant trees like pines, oaks, and madrones, especially in high-risk areas with hot, dry, south- and west-facing slopes. Douglas-fir struggles to thrive in these exposed spots in a warming climate – for example, the average summer temperatures in southwest Oregon have steadily risen since 2013, with an expected increase of 4.5–6°F by mid-century.

While there are ways to manage and lessen the impact of the dieback, the harsh reality is that without adequate rainfall or frequent low-intensity fires, which historically kept Douglas-fir populations in check, the species will likely lose ground in its lower-elevation ranges. OSU’s ongoing research into tree health and mortality risk aims to guide landowners in making informed choices for preserving forest health. As conditions continue to change, we may see these lower areas transition more permanently to pine-oak woodlands or open savannas.

By understanding the risks and taking proactive steps now, we can give remaining Douglas-firs a chance to survive, while adapting forest landscapes to be more resilient in the face of ongoing climate change.