Douglas Fir Mortality

While the immediate effects of Douglas-fir mortality can be negative, they also provide opportunities to restore and improve oak habitats in the long term. A primary threat to oak habitats in the Klamath Siskiyou Oak Network geography is the disruption of low to moderate-severity fire regimes generated by both cultural burning and lightning. Disruption of the fire regime allowed the spread and increased density of Douglas-fir trees into areas where they were historically uncommon, including pine-oak woodlands. Douglas-fir trees grow faster and taller than oaks, so, in the absence of fire, crowd and eventually overtop them. Our restorative stewardship aims to reduce conifer encroachment and set the state for the return of beneficial fire to maintain the oak woodlands. The recent die-off of Douglas-fir trees can be viewed as beneficial for oaks, yet the resultant stands with uncharacteristically abundant snags and downed wood pose a severe fire risk and impediments to restoring the beneficial role of fire.

Douglas-fir trees in lower elevations are facing a dramatic die-off, and the mortality rate is expected to rise significantly in the coming years. The Rogue Valley in southwest Oregon is one of the hardest-hit areas, where the sight of declining Douglas-fir trees across the landscape can be quite stark. Specifically, Jackson County has one of the highest rates of tree mortality in Oregon. U.S. Forest Service Forest Health Protection aerial detection surveys revealed that more Douglas-fir trees died between 2015 and 2019 than in the previous four decades combined. Recent surveys in 2022-23 showed over 350,000 acres (over 500 square miles) in southwest Oregon affected by Douglas-fir die-off at varying levels of severity. The die-off of Douglas-fir trees results from forests becoming overcrowded in the absence of low-severity fire, and it is compounded by climate change with prolonged drought and extreme heat waves which cause high stress levels in trees, making them highly susceptible to mortality from wood-boring insects.

Forest restoration projects aim to thin excess Douglas-fir trees to improve conditions for pine and oak trees, which are more drought and fire-tolerant. This thinning allows more sunlight to reach the oaks and improves the condition of understory grasses and forbs. There is still much to learn about how climate change and Douglas-fir mortality will impact oak habitats in both the short and long term.