Puget Sound Prairies

Edited from University of Washington Prairie Ecology web page:     EHUF 473 Restoration Ecology.   Kern Ewing, Instructor

The prairie ecosystem is unusual in the otherwise forested and urbanized Puget Sound :
  1. Puget Sound prairies have undergone large-scale destruction (as much as 97%). From 150,000 acres down to 3,000 acres (Dorner 1999).
  2. Are a disjointed mosaic of prairies separated by Douglas-fir forest.
  3. Many have had human influence since long before European settlers arrived (Gee 1998).
  4. Mainly due to development, the basic ecological processes that maintain prairies have disappeared or been altered.

The origin and persistence of prairies are attributed to two main factors (Franklin and Dyrness 1988) :
  1. "the occurrence of droughty, gravelly soils derived from glacial outwash materials coupled with low summer precipitation"
  2. "frequent burning of the prairies by natural causes, Indians, and possibly early white settlers" .

Two major criteria describe Puget Sound prairies (Gee 1998):
  1. Dominance of native grasses and forbs.
  2. Well-drained glacial outwash or bedrock soil.

Major sections of prairie characterized by location, rainfall, and disturbance (urban or rural) are :
  1. Northern prairies (south of Victoria to approximately Whidbey Island) < 20 inches of rain.
  2. Southwestern prairies (east side of Olympic Mountains, Skokomish area) < 90 inches of rain.
  3. Southern prairies (south Puget Sound area: Fort Lewis, etc) < 40 inches of rain.

South Puget Sound Prairie soils have low nutrient availability (low productivity), and prone to drought (due to both soil-type and rain shadow effects) (Dunn and Ewing 1997).

Three major plant associations occur in the South Puget Sound prairies (Dorner 1999):
  1. Idaho Fescue dominated: high cover of native prairie species
  2. Bentgrass - Idaho Fescue dominated: reduced diversity / abundance of native species.
  3. Bentgrass - Scotch broom dominated: dominated by invasives

Reasons for the decline of these systems include :
  1. the exclusion of fire.
  2. conversion to agricultural, industrial, and residential use by humans.
  3. invasion by weedy non-native plant species.
  4. encroachment by opportunistic native species.

Preservation and restoration of these habitats means learning how to :
  1. control invasive species.
  2. establish native plants.
  3. combine the species into a stable community.