STRUCTURE OF NORTHERN SPOTTED OWL NEST STANDS AND THEIR HISTORICAL CONDITIONS ON THE EASTERN SLOPE OF THE PACIFIC-NORTHWEST CASCADES, USA

The northern spotted (Strix occidentalis caurina) uses a wide array of nesting habitat throughout its current range and successfully reprodu ces in a variety of stand types on the eastern slope of the Pacific No rthwest Cascades. The species has the ability to utilize dynamic fores t stands that continue to undergo significant changes in tree density, proportion of tree size classes, and tree species composition. Curren t stand structure and composition reflect the results of timber harves t, reduced fire effects and ongoing successional and stand development processes. In nest stands, multi-layered canopy was more strongly exp ressed in numbers of both small (< 13 cm DBH) and large (> 41 cm DBH) trees than in unoccupied stands of the same type within the owl neighb orhoods. Tree density and the proportion of shade-tolerant tree specie s have increased significantly in spotted owl nest sites in both dry a nd wet forests since Eurosettlement. Barring disturbance, further incr eases in the dominance of shade-tolerant species should occur over tim e with continual change in nest stand structure and composition. The d evelopment of dense forest stands and 'old-forest structural attribute s' as a result of reduced fire effects could be potential mitigating f actors to the loss of old-forest habitat from harvest and should be co nsidered in determining the available owl habitat in the eastern Casca des. However, old-forest structural attributes in dense, overstocked s tands are at high fire hazard and should be viewed as transitional unt il old-forest habitat with improved sustainability becomes available. (C) 1997 Elsevier Science B.V.