Willamette Valley Prairie

Benton County Prairie Species Habitat Conservation Plan

What is a Prairie?

A prairie is an open grassland that has little cover by trees or shrubs. Prairies in the Willamette Valley typically occur at fairly low elevations in the valley bottom or surrounding foothills. The open structure of prairies will persist only if a regular disturbance prevents the natural encroachment of trees and shrubs.  Natural disturbances such as wildfire can maintain prairies, and anthropogenic (human caused) disturbances such as prescribed fire or mowing can have similar effects.  If natural disturbance is suppressed, and anthropogenic disturbance is not actively used, prairies will become dominated by woody vegetation.

Upland Prairie

Upland prairies occur on drier sites with well-drained soils. These are frequently located along the rim or margins of valleys. Fires are the key disturbance that maintains these habitats. Upland prairies are typically dominated by perennial grasses and annual or perennial forbs. Key native and perennial grasses in upland prairie include Roemer’s fescue (Festuca roemeri), California oatgrass (Danthonia californica), junegrass (Koeleria macrantha), blue wildrye (Elymus glaucus), and Lemmon’s needlegrass (Achnatherum lemmonii). Native forbs that are commonly intermixed with the grasses include Oregon sunshine (Eriophyllum lanatum), slender cinquefoil (Potentilla gracilis), dwarf checkermallow (Sidalcea virgata), and Tolmie startulip (Calochortus tolmiei).

Upland prairie at Jackson Place, OSU.  Photo: C Menke

Wet Prairie

Wet prairies usually occur on poorly drained soils or well drained but shallow soils above bedrock. These prairies and are usually flooded during part of the year, and are dominated by herbaceous plants, often including facultative or obligate wetland plant species. Wet prairies are maintained by seasonal flooding and summer fires. Common native grass species found in wet prairies include tufted hairgrass (Deschampsia caespitosa), and meadow barley (Hordeum brachyantherum). Also common is one-sided sedge (Carex unilateralis). Native forbs found in wet prairie include common camas (Camassia quamash), Oregon sunshine (Eriophyllum lanatum), elegant downingia (Downingia elegans), and coyote-thistle (Eryngium petiolatum).

Wet prairie at Fisher Butte.  Photo: TN Kaye

What are Threats to Prairies?

Native prairies are an extremely rare and endangered ecosystem in the Pacific Northwest. Much of Willamette Valley prairie has been converted to agricultural and urban uses, leaving small isolated islands of remaining prairie. Almost all remaining fragments of upland prairies have been invaded in some degree by introduced plant species. Introduced species are often from Europe or Asia, and can be very aggressive or weedy, out-competing more desirable native species from our plant communities.

Native species are critical to support native wildlife and insects, and are crucial components of healthy, functioning ecosystems. Common and problematic introduced species in upland prairie include scotch broom (Cystisus scoparius), tall oatgrass (Arrhenatherum elatius), soft chess (Bromus hordeaceus), false-brome (Brachypodium sylvaticum), dogtail (Cynosurus echinatus), and medusahead (Taeniatherum caput-medusae).  Wet prairies are most at risk from invasion by reed canarygrass (Phalaris arundinacea), sweetbriar rose (Rosa eglanteria), and sweet vernalgrass (Anthoxanthum odoratum).