Willamette Valley Prairie and Savanna

This grassland system is endemic to the Puget Trough and Willamette Valley. It formed a complex mosaic of varying patch sizes with wet prairies and riparian forests over much of the Willamette Valley during the pre-European settlement era. In parts of the Puget Trough, it occurred as large patches in more forested landscapes, usually associated with deep, coarse outwash deposits. Historically, it also occurred as large patches on glacially associated soils of variable texture in localized portions of the Georgia Basin in both Washington and British Columbia. It occurs on well-drained deep soils and was maintained historically by frequent anthropogenic burning. Landforms are usually flat, rolling, or gently sloping, and often part of extensive plains. Dominant vegetation is perennial bunchgrasses, especially Festuca idahoensis ssp. roemeri and, to a lesser degree, Danthonia californica, with abundant and diverse forbs. Scattered deciduous (Quercus garryana) and/or coniferous (Pseudotsuga menziesii, Pinus ponderosa) trees are rarely found now, but such savannas historically covered about one-third of the total acreage. In the absence of disturbance, many of them have succeeded to forest and others continue to do so.

This ecosystem occurs on well-drained deep soils and was maintained historically by frequent anthropogenic burning. Landforms are usually flat, rolling, or gently sloping, and often part of extensive plains.

Fires are thought to have occurred every few years (Chappell and Kagan 2001, as cited in WNHP 2011). Annual soil drought during the summer made it difficult for woody species (especially trees) to establish in these grasslands. However, occasionally Quercus garryana and Pseudotsuga menziesii would establish and survive long enough to be resistant to frequent fires thereby creating savanna conditions (Chappell and Kagan 2001, as cited in WNHP 2011). Following European settlement of the region, anthropogenic fire became less frequent resulting in widespread encroachment of the prairies and savannas by woody vegetation, especially conifers (WNHP 2011).

Conversion of this type has commonly come from residential and agricultural development (WNHP 2011). Common stressors and threats include exclusion of fire, ground-disturbing activities like grazing or off-road vehicle use, continuous heavy grazing, military activity, and conifer encroachment (WNHP 2011).

In the Pacific Northwest, regionally downscaled climate models project increases in annual temperature of, on average, 3.2°F by the 2040s. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), but some models project wetter autumns and winters and drier summers. Increases in extreme high precipitation (falling as rain) in the western Cascades and reductions in snowpack are key projections from high-resolution regional climate models (Littell et al. 2009). Warmer temperatures will result in more winter precipitation falling as rain rather than snow throughout much of the Pacific Northwest, particularly in mid-elevation basins where average winter temperatures are near freezing. This change will result in: Less winter snow accumulation, Higher winter streamflows, Earlier spring snowmelt, Earlier peak spring streamflow and lower summer streamflows in rivers that depend on snowmelt (most rivers in the Pacific Northwest) (Littell et al. 2009). Potential climate change effects could include: increased vigor of some non-native species; increased fire frequency which may reduce conifer cover; increase drought length and intensity which may benefit native prairie species; and predicted shifts in vegetation favoring mixed evergreen from moist conifer forests, which may indicate less conifer encroachment (PRBO Conservation Science 2011).

This system is endemic to the Puget Trough and Willamette Valley.