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East Cascades Oak-Ponderosa Pine Forest and Woodland

This narrowly restricted ecological system appears at or near lower treeline in foothills of the eastern Cascades in Washington and Oregon within 65 km (40 miles) of the Columbia River Gorge. It also appears in the adjacent Columbia Plateau ecoregion. Elevations range from 460 to 1920 m. Most occurrences of this system are dominated by a mix of Quercus garryana and Pinus ponderosa or Pseudotsuga menziesii. Isolated, taller Pinus ponderosa or Pseudotsuga menziesii over Quercus garryana trees characterize parts of this system. Clonal Quercus garryana can create dense patches across a grassy landscape or can dominate open woodlands or savannas. The understory may include dense stands of shrubs or, more often, be dominated by grasses, sedges or forbs. Shrub-steppe shrubs may be prominent in some stands and create a distinct tree / shrub / sparse grassland habitat, including Purshia tridentata, Artemisia tridentata, Artemisia nova, and Chrysothamnus viscidiflorus. Understories are generally dominated by herbaceous species, especially graminoids. Mesic sites have an open to closed sodgrass understory dominated by Calamagrostis rubescens, Carex geyeri, Carex rossii, Carex inops, or Elymus glaucus. Drier savanna and woodland understories typically contain bunchgrass steppe species such as Festuca idahoensis or Pseudoroegneria spicata. Common exotic grasses that often appear in high abundance are Bromus tectorum and Poa bulbosa. These woodlands occur at the lower treeline/ecotone between Artemisia spp. or Purshia tridentata steppe or shrubland and Pinus ponderosa and/or Pseudotsuga menziesii forests or woodlands. In the Columbia River Gorge, this system appears as small to large patches in transitional areas in the Little White Salmon and White Salmon river drainages in Washington and Hood River, Rock Creek, Moiser Creek, Mill Creek, Threemile Creek, Fifteen Mile Creek, and White River drainages in Oregon. Quercus garryana can create dense patches often associated with grassland or shrubland balds within a closed Pseudotsuga menziesii forest landscape. Commonly the understory is shrubby and composed of Ceanothus integerrimus, Holodiscus discolor, Symphoricarpos albus, and Toxicodendron diversilobum. Fire plays an important role in creating vegetation structure and composition in this habitat. Decades of fire suppression have led to invasion by Pinus ponderosa along lower treeline and by Pseudotsuga menziesii in the gorge and other oak patches on xeric sites in the east Cascade foothills. In the past, most of the habitat experienced frequent low-severity fires that maintained woodland or savanna conditions. The mean fire-return interval is 20 years, although variable. Soil drought plays a role, maintaining an open tree canopy in part of this dry woodland habitat.

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