The Douglas fir historic plant association occurs in the southwestern corner of the park, at the lowest and warmest elevations (see fig. 7, page 103). Douglas fir is the historic climax tree species. Other tree species are sugar pine and white fir. Some Shasta red fir is in the stands at the higher elevations.
Because of its thick bark, Douglas fir can withstand low- and moderate-intensity ground fires that would kill other tree species. Ground fires create openings and clear away vegetation, exposing the mineral soil material and allowing Douglas fir to regenerate. In the mixed evergreen forests of southern Oregon and northern California, fires occurred every 5 to 25 years (Lotan and others, 1981). The interval between the fires and the resistance of Douglas fir to fire allows it to occupy the stand until the next stand-replacing fire. If fire is excluded for a long period of time, white fir, which is a shade-tolerant species, will eventually invade and become the dominant overstory tree species. Even though the life span of Douglas fir trees is longer than that of white fir, the inability of Douglas fir to regenerate under heavy shade allows white fir to become dominant in the stand.
The ecological sites in this plant association are Douglas-fir/cascade Oregongrape/prince’s pine (Pseudotsuga menziesii/Mahonia nervosa/Chimaphila umbellata) on steep, south-facing slopes and Douglas-fir/blue huckleberry/prince’s pine (Pseudotsuga menziesii/vaccinium membranaceum/Chimaphila umbellata) on west-facing slopes.
Much of the western edge of the park is a transition zone from Douglas fir to Shasta red fir to mountain hemlock plant communities. The elevation ranges from approximately 4,800 to 5,200 feet. Below an elevation of about 5,000 feet, the stand is a mixture of Shasta red fir, Douglas fir, white fir, and ponderosa pine. Above an elevation of 5,000 feet, Shasta red fir is more dominant and the abundance of mountain hemlock is higher. The Douglas fir association extends to elevations of as high as 5,300 feet on the warmer, west- and south-facing slopes.