Roosevelt Elk and Forest Structure in Olympic National Park

Roosevelt elk occur along the Pacific Coast of North America from Vancouver Island to northern California. Large populations occupy Washington's Olympic Peninsula (1.38 million hectares); the 5,000 or so elk in Olympic National Park (370,000 hectares) represent the last large population in mostly undisturbed natural habitat, which includes old-growth forests (Houston et al. 1990; Fig. 1).

The elk of Olympic National Park include year-round residents and populations that migrate to high elevations during summer. From 3,000 to 4,000 elk reside entirely in the park along drainages on the north and west sides, including the Elwha, Hoh, Queets, and Quinault rivers. Winter densities are around 6-7 elk per square kilometer. Censuses revealed that subpopulations on the west side of the park remained stable during the 1980's (Houston et al. 1990). Old-growth forests of massive western hemlock, Sitka spruce, western redcedar, and Douglas-fir provide much of the habitat used by these subpopulations.

Scientists study elk-vegetation relations in the park, primarily in forests on the west side, to increase understanding of the long-term effects of native ungulates in forest communities. Elk consume ferns, shrub foilage, and coniferous foilage during fall and winter, and grasses and herbaceous plants during spring and summer. Seasonally important dietary items include western hemlock, sword fern, red alder, and oxalis. Digestible energy in these foods is usually low, indicating that elk densities may be limited by the quality and quantity of winter forage (Leslie et al. 1984).

Twenty-five ungulate exclosures were established in Olympic National Park (23) and the surrounding Olympic National Forest (2) in the 1930's and 1950's. Woodward et al. (1994) recently (1987-1990) resampled vegetation inside and adjacent to the original exclosures. When past vegetation was compared with present vegetation on either side of the exclosures, scientists learned that ungulates, mainly elk, are a powerful force shaping plant communities (Fig. 2). Ungulate herbivory influenced the species composition, morphology, and standing crop of forest vegetation at all structural layers (herbaceous understory, shrubs, lower tree canopy, and overstory canopy). In communities on valley floors initially dominated by grasses, exclusion of ungulates resulted in decreased cover of grasses and usually forbs, decreased species richness of forbs, and sometimes increased height and abundance of ferns. Shrub size and density also increased in the absence of herbivory, and ungulates influenced the recruitment and morphology of vine maple in the lower forest canopy. Ungulates had variable effects on the establishment of overstory species; browsing seemed to affect recruitment of Pacific silver fir and western redcedar on Olympic National Forest after clear-cut logging, but effects on other tree species outside and inside the park were unclear.

The intensity of ungulate herbivory varies in time and space, and the effects on vegetation are complex. As in other recent studies, the Olympic National Park studies show that mammalian herbivores strongly interact with vegetation and are not just passive components of the ecosystems they occupy.