Historical Condition and Change in Riparian Vegetation, Hood Canal and Eastern Strait of Juan de Fuca, Washington

Abstract

Pacific Northwest riparian forests occupy dynamic environments, host high biodiversity, and support important ecological services like nutrient storage/transformation, flood attenuation, and water quality protection. Lack of information on their historical condition and changes hinder effective ecosystem management. We combined archival records and contemporary field surveys to reconstruct historical riparian conditions and change in Hood Canal and the eastern Strait of Juan de Fuca, Washington. Historical riparian forest composition and structure varied by shoreline type and landform. Conifer species dominated all shoreline types, with hardwoods relatively abundant in estuaries and stream bottomlands. Older, more structurally diverse forests clustered in stream ravines, with the size of stream-riparian fir and redcedar dominants measuring 2.5 and 8 times larger than in upland stands, respectively. Over half (57.9%) of stream-riparian redcedar-spruce forest sites shifted to hardwood/mixed forest over the historical period, versus fir-hemlock, alder, and hardwood/mixed forest types which all showed ≤35.3% change. Classification and regression tree models highlighted historical forest type, elevation, and proximity to bottomlands as key predictors of vegetation change, with historical conifer sites in/near bottomlands two times more likely to undergo change than upland stands. Our findings provide spatially explicit historical context for ecosystem managers and modelers, and suggest new questions pertinent to riparian forest management.