Eastern Cascades Forests

• Category: Forest Biomes.
• Geographic Location: Northwestern United States.
• Summary: The biodiversity of the eastern slopes of the Cascade Mountains is endangered by climate change and the ongoing impact of various land-use projects. The area features a high turnover in species along environmental gradients, commonly referred to as high beta diversity.

Underlain by Cenozoic volcanic rock and associated deposits, the Eastern Cascades Forests biome is defined by the eastern slopes of the Cascade Mountains in Oregon and Washington that owe their existence to the subduction of the Juan de Fuca Plate under the North American Plate, some four million years ago. Numerous dormant and active volcanoes are part of an extensive volcanic arc that rims the larger Pacific region. Cinder cones and fields of pumice are evidence of past volcanic activity in eastern Oregon; the 1980 eruption of Mt. St. Helens in southwest Washington is a more recent reminder. To the north is Mt. Rainier, also known as Tacoma or Tahoma, the tallest peak in the region at 14,409 feet (4,393 meters), and the most extensively glaciated mountain in the lower fourty-eight states. Pleistocene glaciations created numerous alpine lakes; however, most glaciers are restricted to small alpine areas.

The largest river in the Pacific Northwest (and fourth-largest in the continental United States), the Columbia, flows out of the Rockies of British Columbia, heads northwest and then south into Washington, and marks the boundary between the states of Oregon and Washington before reaching its terminus at the Pacific Ocean. Fourteen hydroelectric dams block the mainstem and are part of ongoing controversy over the management of wild salmon runs, many of which are threatened with extinction.

The southern extent of these forests is marked just across the Oregon border in northern California by the Klamath River, which was once the third-largest producer of salmon in the continental United States. Numerous instream diversions, hydroelectric dams, logging, and livestock grazing once jeopardized wild salmon runs on the Klamath; however, an agreement was reached to remove the four mainstem dams by 2020, restoring flow and upstream access for fish. In 2024, the project to remove the dams was completed and it marked the largest dam removal project in U.S history, restoring 400 miles of habitat for salmon.

The climate in the area varies depending on one's position. A moist temperate climate currently exists west of the Cascades; a drier, warmer continental climate is to the east.

Ecoregional and Vegetation Types

Vegetation types in these forests are adapted to the prevailing dry climate and periodic fires of varied severity. Proximity to the southern Cascades (especially their western moist slopes), as well as the Klamath-Siskiyou, Sierra-Nevada, and Snake-Columbia shrub steppe ecoregions, allows for mixing of plant communities and numerous “ecotones” (zones of overlap). Natural vegetation is a complex and largely fire-generated mosaic of shrublands, grasslands, and coniferous forests; seven different forest zones and numerous plant associations have been recognized.

Dry forests include ponderosa (Pinus ponderosa) and mixed conifer forests dominated by Douglas fir (Pseudotsuga menziesii), grand (Abies grandis) and white (A. concolor) fir, western larch (Larix occidentalis), lodgepole pine (P. contorta), sugar pine (P. lambertiana), incense cedar (Calocedrus decurrens), and western juniper (Juniperus occidentalis). Topographic moisture gradients such as sheltered valleys or north-facing slopes provide microclimatic conditions for mesic (moderately moisture-dependent) plants to coexist in this dry region. High-elevation areas include Shasta red fir (A. magnifica var. shastensis) and subalpine fir (A. lasiocarpa).

Role of Fire

Fire is a predominant disturbance factor in this region, and land managers and researchers have widely debated its historical versus contemporary influences. Many have assumed that fires in dry mixed conifer forests were historically low-severity (burning primarily along the ground, removing shrubs and small trees), creating open parklike conditions altered by livestock grazing, high-grade logging, and fire suppression. This scenario has created forests that today are more dense and likely to burn severely, which is uncharacteristic for the area.

However, recent studies using surveys from the late 1800s and historic stand reconstructions challenge prevailing views, proving that historic fire regimes varied greatly in extent, frequency, and severity and that parklike conditions were rare. Whether contemporary fires will become more severe due to climate change remains to be determined. Studies show a recent uptick in fire extent; however, climate projections are uncertain for future precipitation patterns. Nonetheless, fire prevention through suppression and forest thinning has dominated land management concerns.

Biodiversity Importance

The Eastern Cascades Forests ecoregion is considered bioregionally outstanding by the World Wildlife Fund, with intermediate levels of taxa richness and endemism. Beta-diversity is particularly high due to numerous ecotones along the Cascade Crest (e.g., portions of the Okanogan-Wenatchee National Forest) and the overlap of plant communities from surrounding ecoregions. Biodiversity of the region was ranked as Endangered due to high numbers of taxa with conservation status (state or federal listed), and to high rates of conversion due primarily to logging, livestock grazing, invasive species, and hydroelectric development. Logging has significantly depleted old-growth forests, particularly large sugar pines, and most such habitat zones, outside inventoried roadless areas, are highly fragmented.

Wildlife in the region is diverse; the mountains, forests, and clean water attract many species. Marmot, black bear, mountain goat, several species of deer, golden mantle ground squirrel, chipmunk, and many species of birds are native to the area. The list of threatened, endangered, and candidate species within the biome is long, and includes the bald eagle, common loon, northern spotted owl, and pileated woodpecker, among others.

Many animal populations have been adversely affected by the encroachment of human development at the same time that similar animal and fish populations are thriving in some areas. The Skagit River, flowing toward Puget Sound, is a major waterway for rainbow, cutthroat, and Dolly Varden trout (Salvelinus malma malma) and several types of salmon. The Upper Snake River region is home to fourteen species of fish. Reaches of the Columbia River below the Snake River are home to as many as thirty-five species of fish and high concentrations of freshwater mollusks.

Climate Change

Downscaled global circulation models for the central Oregon portion of the Eastern Cascades Forests ecoregion predict an increase in annual average temperature of 5 to 9 degrees F (2.8 to 5 degrees C) by the late 21st century; summer projections show the greatest degree of warming. Projections for annual average precipitation vary from 7 percent less to 22 percent more by the late century. Future winters are likely to be somewhat wetter, but all other seasons have variable projections for precipitation trends.

Increasing temperatures suggest snowpack levels will continue to decline despite wetter winters; this could affect seasonal salmon migrations. Vegetation model results indicate climatic conditions may favor mixed conifer forests dominated by pine, at the expense of mixed conifer dominated by fir and subalpine species. The extent of wildfire is projected to increase by the late century; however, there is high uncertainty due to varied precipitation projections.

Conservation

The Northwest Forest Plan (of the U.S. Department of the Interior) continues to provide the foundation for ecosystem management and biodiversity conservation on federal lands in this and related Pacific Northwest ecoregions. The plan's reserve network, combined with inventoried roadless areas on federal lands, provide protection for the bulk of the region's remaining late-successional forests.

There are gaps in the reserve network, however. Climate change requires building onto the existing reserves so they are robust, expanding the network along elevation and latitudinal-longitudinal gradients to accommodate climate-forced shifts in plant and wildlife communities, and restoring aquatic floodplains to prepare for potential flooding if precipitation increases. Such measures can help to recover wild salmon runs and reduce land-use stressors (e.g., livestock grazing) that contribute to a shift from native communities toward more invasive ones.

Restoration of native grasslands, shrublands, and riparian areas is essential for climate adaptation. More research, monitoring, and adaptive management are needed on fire to determine what truly constitutes “uncharacteristic” events and whether contemporary fire management is likely to be restorative or create novel ecosystems that worsen climate-related impacts. The development of renewable energy sources (e.g., biofuels) is a growing concern in this and surrounding regions. Partnerships with conservation researchers, land managers, and energy developers are essential for reducing habitat fragmentation problems.

Bibliography

Agee, James K. "Historical Range of Variability in Eastern Cascades Forests, Washington, USA." Landscape Ecology, 26 May 2003, www.fs.usda.gov/rm/pubs/rmrs‗gtr292/2003‗agee.pdf. Accessed 9 Nov. 2024.

Baker, W. L. "Implications of Spatially Extensive Historical Data From Surveys for Restoring Dry Forests of Oregon's Eastern Cascades." Ecosphere, vol. 3, no. 1, 2012, pp. 1-39.

Crawford, R. "East Cascades Oak-Ponderosa Pine Forest and Woodland." NatureServe Explorer, 2 June 2022, explorer.natureserve.org/Taxon/ELEMENT‗GLOBAL.2.740345/East‗Cascades‗Oak-Ponderosa‗Pine‗Forest‗and‗Woodland. Accessed 9 Nov. 2024.

Ricketts, T. H., et al. Terrestrial Ecoregions of North America: A Conservation Assessment. Island Press, 1999.