Modeling change in potential landscape vulnerability to forest insect and pathogen disturbances: methods for forested subwatersheds sampled in the midscale interior Columbia River basin assessment

In the interior Columbia River basin midscale ecological assessment， including portions of the Klamath and Great Basins， we mapped and characterized historical and current vegetation composition and structure of 337 randomly sampled subwatersheds (9500 ha average size) in 43 subbasins (404 000 ha average size). We compared landscape patterns， vegetation structure and composition， and landscape vulnerability to 21 major forest insect and pathogen disturbances of historical and current forest vegetation coverages. Forest vegetation composition， structure， and patterns were derived from attributes interpreted and mapped from aerial photographs taken from 1932 to 1966 (historical)， and from 1981 to 1993 (current). Areas with homogeneous vegetation composition and structure were delineated as patches to a minimum size of 4 ha. Results of change analyses were reported for province-scale ecological reporting units (ERUís). In this paper， we report on methods used to characterize historical and current patch and subwatershed vulnerability to each of 21 insect and pathogen disturbance agents.　 We assessed landscape vulnerability to defoliator， bark beetle， dwarf mistletoe， root disease， blister rust， and stem decay disturbances. We used patch composition， structure， logging disturbance， and physical environment attributes to compare vegetation vulnerability of historical subwatersheds with that of their current condition. Patch vulnerability factors included items such as site quality， host abundance， canopy layers， host age or host size， patch vigor， patch (stand) density， connectivity of host patches， topographic setting， and presence of visible logging disturbance. Methods reported here can be used in landscape or watershed analysis to evaluate or monitor change in the magnitude and spatial pattern of vegetation vulnerability to insect and pathogen disturbances， and in planning to compare potential disturbance futures associated with alternative vegetation management scenarios.(Interior Columbia Basin Ecosystem Management Project: Scientific Assessment. Thomas M. Quigley， Editor)