Causes of Decline of Inland Douglas-fir

January 29, 2006

A reader from the Iron Peak area asks why a number of good-size Douglas fir have died over the last five or so years?

Jack Marshall, Forest Pathologist at the California Department of Forestry and Fire Protection, corroborated the readers observations reporting an increase in the death of Douglas-fir in the vicinity of Willits north to Laytonville. However, the number of dead trees observed has been declining since its height in 2003. The flat-headed fir borer and the Douglas-fir engraver beetle are the likely mortality agents. The flat-headed fir borer can cause mortality in any sized Douglas-fir tree. The Douglas-fir engraver beetle can cause mortality in trees less than 10" in diameter. Weakened trees are most susceptible to intrusion by insects. Biologic pressures and environmental pressures can cause trees to become weak. Weak trees are destined to be outcompeted by their neighbors and relegated to a suppressed or intermediate crown position. Environmental pressures include drought, the effects of a disrupted fire cycle, soil compaction and increased exposure to the elements. Biologic pressures include disease agents. In this case, the Dermea canker causes dieback of limbs and tops which may invite the entrance of the Douglas-fir engraver beetle.

Another reader from the Blue Rock Creek area asks a series of related questions: Can the bark beetle that is killing the Douglas-fir be stopped? Should we let them continue since they are only taking out the "weak" trees? How should they be cut and removed without spreading the beetle?

Native pests will always be in the area, but you can be active in protecting the non-symptomatic population. The Douglas-fir engraver beetle, a bark beetle, and the flat-headed fir borer are native insects. Through early removal of the symptomatic tree, you can slow the population growth. Once cut, treat the slash by lopping and scattering, piling and burning or chipping. Debarking the logs is also effective in that the habitat where beetles breed and larvae feed is destroyed. If you cut an infected tree into firewood, tarping and sealing the piles of wood with clear plastic is an effective way to prevent the emergence of the beetle from the wood. Go to http://www.fire.ca.gov/ click on resource management, then pest management and finally Tree Notes #3 or go directly to http://ceres.ca.gov/foreststeward/pdf/treenote3.pdf for more information.

A portion of this production was gleaned from a conversation with Jack Marshall, Forest Pathologist at the California Department of Forestry and Fire Protection. If you have a local disease or insect question for Jack Marshall, he can be reached at Howard Forest in Willits at 707-459-7448.

I welcome responses as well as ideas for future Tree Tips. You can also get an email version by contacting me at thembi@mcn.org.

Thembi Borras is a Registered Professional Forester living and working in Mendocino County, CA USA.