Windthrow disturbance, forest composition, and structure in the Bull Run basin, Oregon

This study examined relationships among forest landscape dynamics, environm ental factors (climate and landforms), and disturbance history in forests d ominated by Douglas fir (Pseudotsuga menziesii), western hemlock (Tsuga het erophylla), and Pacific silver fir (Abies amabilis) in the Bull Run basin i n northwestern Oregon and evaluated the findings in a broader geographic co ntext. Three sets of analyses were conducted: mapping of historical windthr ow disturbance patches in the 265-km(2) Bull Run basin over the past centur y and analysis of their relationships with meteorological conditions, landf orms, and vegetation; comparison of forest structure and species compositio n as a function of mapped windthrow and wildfire disturbance history in 34 1-ha vegetation survey plots in Bull Run; and canonical correspondence anal ysis of environmental factors and forest overstory species composition in 1 637 vegetation plots in the Mount Hood and Willamette National Forests. Nea rly 10% of the Bull Run basin has been affected by windthrow since 1890, bu t only 2% was affected prior to the onset of forest harvest in 1958. Most o f the mapped windthrow occurred in areas with 500- to 700-yr-old canopy dom inants and no mapped disturbance by fire in the past 500 yr. Most mapped wi ndthrow occurred during three events in 1931, 1973, and 1983 that were char acterized by extreme high speed east winds from the Columbia River Gorge. F orest 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 clearcu t edges, which accounted for 80% of windthrow in the 1983 event. Shade-tole rant overstory species (western hemlock and Pacific silver fir) are abundan t 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 hem lock decline and Pacific silver fir increases with elevation (summer moistu re stress declines but temperature variability increases), but this transit ion occurs at lower elevations in the Bull Run, perhaps because of the inte raction between regional climate processes and disturbance along the Columb ia Gor re. Complex landscape dynamics result from these contingent interact ions among climate, landform and stand conditions, and disturbance.