Habitat Descriptions
The descriptions below were developed by The Nature Conservancy
Herbaceous Balds, and Rock Outcrops
Herbaceous balds and rock outcrops occur in the driest environmental settings that support continuous vegetation: generally south- to west-facing slopes on shallow or sandy/gravelly soils. They typically occur as isolated sites within a forest matrix or on coastal bluffs. Burning by Native Americans was probably an important “management” factor on most of these sites, and some of them are threatened by tree invasion due to modern-day fire suppression.
Vegetation is dominated by perennial bunchgrasses, forbs (herbaceous flowering plants that are neither grasses, rushes or sedges), and mosses. Scattered trees, especially Quercus garryana (Oregon white oak), Arbutus menziesii (madrone), and Pseudotsuga menziesii (Douglas-fir) are often present on the fringes.
Oregon White Oak Woodlands
This system is associated with dry sites and/or frequent pre-settlement fires. In the Willamette Valley, soils are mesic (sites characterized by intermediate moisture conditions neither decidedly wet nor decidedly dry), yet well-drained. In the absence of fire, succession tends to favor increased shrub dominance in the understory and greater tree density. The vegetation is a woodland dominated by deciduous broadleaf trees, mostly Quercus garryana. Co-dominance by the evergreen conifer Pseudotsuga menziesii is common, the climax result being conifer forest.
Upland Prairie and Savannas
This ecosystem formed a complex mosaic of varying patch sizes of open grasslands often associated with low-density tree cover (5-30%) over much of the Willamette Valley. It occurs on well-drained soils and was maintained by the Native American practice ofsetting frequent low-intensity fires. With fire suppression or in abandoned pastures, many such areas have reverted to forest.
The dominant vegetation of these native grasslands were perennial bunchgrasses such as Festuca roemeri (Roemer’s fescue) and Danthonia californica (California oatgrass), with abundant and diverse herbaceous plants. Scattered, open-growth trees such as Quercus garryana, Pseudotsuga menziesii, or Pinus ponderosa (ponderosa pine) within the grassland characterize a savanna. Uncommon now, such savannas once covered about 1/3 of the Willamette Valley.
Vernal Pools
Vernal pools are a component of wet prairies (see below) characterized by freshwater inundation for much of the winter and spring, followed by summer drought. They are found in isolated small depressions with no inflow or outflow, and a restrictive subsurface soil layer (clay or bedrock). Vegetation is, typically, dominated by annual forbs.
Wet Prairie
Wet prairies covered large areas of the Willamette Valley where they were maintained by a combination of wet soil and regular burning. These are high nutrient wetlands that are temporarily or seasonally flooded. They have been reduced to tiny fragments of their former range. They are dominated primarily by grasses, sedges, and rushes, especially Deschampsia cespitosa (tufted hairgrass), Danthonia californica, and Carex spp.
Mixed Woodlands
The Willamette Valley, particularly its margin forests, features a mixed woodland type characterized by Douglas-fir, western redcedar, and grand fir, with mixed hardwood species such as alder and bigleaf maple found in the understory as well as shrubs such as salal and vine-maple. This forest can be wet or dry with wetland forests often bordering creeks. Bottomland forests are found along river systems. The Willamette Valley bottomland forest is a mix of riparian and wetland habitats, dominated by hardwood trees like red alder, bigleaf maple, and black cottonwood.