Swiss Needle Cast in Northwest Oregon

Since the early 1990s, Swiss needle cast, caused by the native fungus Phaeocryptopus gaeumannii, has severely damaged Douglas-fir trees along the coast range of western Oregon. The primary effect of the pathogen on Douglas-fir trees, which is the only tree species susceptible to the fungus, is premature loss of foliage. This reduces normal tree growth by more than 50 percent in severely damaged stands. Recent reduction in volume growth from Swiss needle cast in northwestern Oregon exceeds 100 million board feet per year.

This map features two methods of measuring the severity of Swiss needle cast. One method is by surveying the Douglas-fir forests of western Oregon with a twin-engine plane to monitor the amount and distribution of damage from Swiss needle cast. The second method is to create needle retention zones by interpolating data from permanent plots.

Flights for the aerial survey method were made in April and May 2004 at 1,500 to 2,000 feet above the terrain, following north–south lines separated by two miles. The surveys covered approximately three million acres of forest each year. Observers looked for patches of Douglas-fir forest with obvious yellow to yellow-brown foliage and sketched them onto 1:100,000-scale topographic maps (prior to 1999) or computer touchscreens (after 1999).

The needle retention zones were interpolated from Swiss needle cast permanent plots. Each plot is measured to determine the needle retention value. This value is the length of time that the branches on the tree hold their needles. A value of 1 means that only the current year's growth is still on the branch. A value of 4 means that the previous four years' growth is still on the tree.

Interpolation was performed using the Inverse Distance Weighted method, with a decay function to the power of 2. The measurement radius is five kilometers, with a minimum of three plots. The cell size is one kilometer.