No. 11. Westside Grasslands

Christopher B. Chappell and Jimmy Kagan

Geographic Distribution. This habitat is restricted primarily to the Puget Lowland and Willamette Valley ecoregions, with most now occurring in Pierce, Thurston, and San Juan counties, Washington. It also occurs in scattered small outliers in the Coast Range of Oregon and the western Columbia Gorge of Washington (Skamania County) and in adjacent southwestern British Columbia.

Physical Setting. The climate is mild and moderately dry (17-55 inches [43-140 cm] mean annual precipitation), with moist winters and dry summers. Elevation is mostly low and ranges up to a maximum of about 3,500 ft (1,067 m). Topography varies from flat, to mounded or rolling, to steep slopes. Most sites are topoedaphically dry and experience extreme soil drought in the summer. Much of what currently remains of this habitat is found on the South Puget prairies, which are underlain by very deep gravelly/sandy glacial outwash that is excessively well drained. Many other small sites, often called "balds", have shallow soils overlying bedrock and typically are on south- or west-facing slopes. The habitat also occurs rarely in Oregon on deeper soils that are not excessively drained.

Landscape Setting. This habitat occurs adjacent to or in a mosaic with Westside Riparian-Wetlands, Westside Oak and Dry Douglas-fir Forests and Woodlands, Agriculture, or Urban habitats. Westside grassland habitat occurs less commonly in a matrix of Westside Lowland Conifer-Hardwood Forest. In the San Juan Islands, the habitat sometimes occurs on bluffs or slopes adjacent to marine habitats. Currently this habitat is used for grazing, recreation, and, in the southern Puget Sound area, for military training.

Structure. This habitat is grassland or, less commonly, savanna, with <30% tree or shrub canopy cover. Bunchgrasses predominate in native-dominated sites, with space between the vascular plants typically covered by mosses, fruticose lichens, or native forbs. Montane balds are sometimes dominated in part by short forbs (<1.6 ft [0.5 m]) or dwarf-shrubs.  Degraded sites are dominated by rhizomatous exotic grasses with some native herbaceous component still present. Scattered trees are either evergreen conifers or deciduous broadleaves. Shrubs may be absent, scattered, or very prominent, and include evergreen and deciduous broadleaf physiognomy.

Composition. The major native dominant bunchgrass is Roemer’s fescue (Festuca idahoensis var. roemeri). Red fescue (F. rubra) and California oatgrass (Danthonia californica) are frequently dominant or co-dominant on a local basis. Long-stolon sedge (Carex inops) is occasionally co-dominant, especially in savannas and in the Columbia Gorge. Slender wheatgrass (Elymus trachycaulus), blue wildrye (Elymus glaucus), prairie junegrass (Koeleria macrantha), and Lemmon’s needlegrass (Stipa lemmonii) can be important locally. Major exotic dominant species are colonial bentgrass (Agrostis capillaris), sweet vernalgrass (Anthoxanthum odoratum), Kentucky bluegrass (Poa pratensis), tall oatgrass (Arrhenatherum elatius), medusahead (Taeniatherum caput-medusae), tall fescue (F. arundinacea), and soft brome (Bromus mollis). Common camas (Camassia quamash) is probably the most important forb in terms of cover, but it rarely dominates. The bracken fern (Pteridium aquilinum) is sometimes co-dominant. A rich diversity of native forbs is typical of sites in good condition.

The most common savanna tree is Douglas-fir (Pseudotsuga menziesii). Oregon white oak (Quercus garryana) formerly was part of extensive savannas, but is now rare in that structural condition. Ponderosa pine (Pinus ponderosa) is very local. The most common shrub is the exotic species Scot’s broom (Cytisus scoparius), which frequently forms open stands over the grass. Common snowberry (Symphoricarpos albus), Nootka rose (Rosa nutkana), poison- oak (Toxicodendron diversilobum), and serviceberry (Amelanchier alnifolia) are other common shrubs. The dwarf shrubs kinnikinnick (Arctostaphylos uva-ursi) and common juniper (Juniperus communi) sometimes dominate small areas in montane balds, and the former sometimes on the Puget Sound prairies. Racomitrium canescens is the most common ground moss.

Other Classifications and Key References. Portions of this habitat have been referred to as prairies by many authors. Franklin and Dyrness ⁸⁸ described this habitat as prairie in the Puget Sound area, grassland in the San Juan Islands and Interior Valley zone of Oregon, and grass balds in the Oregon Coast Range. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Types ¹²⁷ effort did not map this type; it was inadvertently aggregated with the agriculture classification. The Washington Gap project mapped this habitat as part of nonforested in the Woodland/Prairie Mosaic Zone. Other references describe elements of this habitat^{7, 40, 41, 54, 99, 142, 211}.

Natural Disturbance Regime. Historically, fire was a major component of this habitat. In addition to occasional lightning strikes, fires were intentionally set by indigenous inhabitants to maintain food staples such as camas and bracken fern ¹⁶⁵. Although there is no definitive fire history information, evidence suggests that many, if not most, of these grasslands burned every few years. Annual soil drought naturally eliminated or thinned invading trees and promoted higher frequency fire regimes in the past.

Succession and Stand Dynamics. Historically, regular fires or extreme environmental conditions on the most xeric sites prevented the establishment and continued growth of most woody vegetation, thereby maintaining the grasslands and oak savannas. In some patches, scattered oaks or even Douglas-fir survived long enough to obtain some fire resistance and the frequent light fires then helped to maintain savannas ^{1, 41}. Oaks were also able to resprout if the above-ground stem was killed. High fire frequencies combined with digging of roots by Native Americans could have favored the abundance of forbs over that of grasses in many areas of the pre-European landscape.

Effects of Management and Anthropogenic Impacts. The exclusion of fire from most of this habitat over the last 100+ years has resulted in profound changes. Oak savanna has, for all practical purposes, disappeared from the landscape ⁴¹. Douglas-fir encroachment, in the absence of fire, is a "natural" process that occurs eventually on the vast majority of westside grasslands, except perhaps on the very driest sites. This encroachment leads to the conversion of grasslands to forests. Fire exclusion has also resulted in increases in shrub cover and the conversion of some grasslands to shrublands. Exotic species are prominent in this habitat and generally increase after ground-disturbing activities like grazing or off-road vehicle use. Scot’s broom, tall oatgrass, colonial bentgrass, sweet vernalgrass, tall fescue, common velvetgrass (Holcus lanatus), Kentucky bluegrass, soft brome, common St. Johnswort (Hypericum perforatum), and hairy catsear (Hypochaeris radicata) are among the most troublesome species. The dominant native grass, Roemer's fescue, can be eliminated with heavy grazing. Prescribed fire and other management tools have been used recently to control Scot’s broom, Douglas-fir encroachment, and to attempt to mimic historical conditions in some areas ⁷⁸.

Status and Trends. This habitat is very rare and limited in areal extent. In the southern Puget Sound area, only about 10% of the original area of the habitat is extant, and only 3% is dominated by native species ⁵⁴. Overall decline is significantly greater than these figures suggest because the habitat is even more decimated and degraded elsewhere. Causes of the decline are fire suppression, conversion to agriculture and urban, and invasion of exotic species. Most of what remains is dominated or co-dominated by exotic species. Current trends are continued decline both in area and condition. Ongoing threats include urban conversion, increase of exotic species, ground disturbance via tracked vehicle use for military training, and effects of fire suppression. Eleven out of 12 native plant associations representing this habitat listed for the National Vegetation Classification are considered imperiled or critically imperiled ¹⁰.