Windthrow disturbance, forest composition, and structure in the bull run basin, oregon

This study examined relationships among forest landscape dynamics, environmental factors (climate and landforms), and disturbance history in forests dominated by Douglas‐fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and Pacific silver fir (Abies amabilis) in the Bull Run basin in northwestern Oregon and evaluated the findings in a broader geographic context. Three sets of analyses were conducted: mapping of historical windthrow disturbance patches in the 265‐km² Bull Run basin over the past century and analysis of their relationships with meteorological conditions, landforms, and vegetation; comparison of forest structure and species composition as a function of mapped windthrow and wildfire disturbance history in 34 1‐ha vegetation survey plots in Bull Run; and canonical correspondence analysis of environmental factors and forest overstory species composition in 1637 vegetation plots in the Mount Hood and Willamette National Forests. Nearly 10% of the Bull Run basin has been affected by windthrow since 1890, but only 2% was affected prior to the onset of forest harvest in 1958. Most of the mapped windthrow occurred in areas with 500‐ to 700‐yr‐old canopy dominants and no mapped disturbance by fire in the past 500 yr. Most mapped windthrow occurred during three events in 1931, 1973, and 1983 that were characterized by extreme high speed east winds from the Columbia River Gorge. Forest harvest modified the effects of climate, landforms, and vegetation on windthrow disturbance, reducing the importance of topographic exposure to east and northeast winds, and creating a strong influence of recent clearcut edges, which accounted for 80% of windthrow in the 1983 event. Shade‐tolerant overstory species (western hemlock and Pacific silver fir) are abundant in present‐day forest stands affected by windthrow as well as by fire in the past century. In the western Cascade Range, Douglas‐fir and western hemlock decline and Pacific silver fir increases with elevation (summer moisture stress declines but temperature variability increases), but this transition occurs at lower elevations in the Bull Run, perhaps because of the interaction between regional climate processes and disturbance along the Columbia Gorge. Complex landscape dynamics result from these contingent interactions among climate, landform and stand conditions, and disturbance.