This paper assesses the change in coverage of prairie vegetation as estimated by current
occupancy of grasslands on mapped grassland soils. Southern Puget Sound prairies occur on
gravelly, well-drained soils or on other soils derived from materials with low water holding
capacity. These prairie soils are shallow, sandy to gravelly loam soils collectively referred to as
the Spanaway series with inclusions of Nisqually and Carstairs soils. The assessment area
contains approximately 1,497,600 acres. Within that, 149,360 acres are mapped as grassland
soils. The Washington Natural Heritage Program maps occurrences of native prairie on 12582
acres of the assessment area representing 8% of the historical grassland area. Currently native
prairie species dominate only 2993 acres or 3% of the grassland soils. The general reduction of
southern Puget Prairies to approximately 10% of its former abundance appears to be a
proportional loss across all size classes of prairie sites. Immediate conservation action of Puget
prairies is critical for all the remaining native grasslands since they will serve as the cores for
future conservation.
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