Historical and current forest and range landscapes in the interior Columbia River basin and portions of the Klamath and Great Basins. Part 1: Linking vegetation patterns and landscape vulnerability to potential insect and pathogen disturbances

Management activities of the 20th century， especially fire exclusion， timber harvest， and domestic livestock grazing， have significantly modified vegetation spatial patterns of forests and ranges in the interior Columbia basin. Compositional patterns as well as patterns of living and dead structure have changed. Dramatic change in vital ecosystem processes such as fire， insect， and pathogen disturbances， succession， and plant and animal migration is linked to recent change in vegetation patterns. Recent change in vegetation patterns is also a primary reason for current low viability and threatened， endangered， or sensitive status of numerous native plant and animal species. Although well intentioned， 20th-century management practices have not accounted for the larger patterns of living and dead vegetation that enable forest ecosystems to function in perpetuity and maintain their structure and organization through time， or for the disturbances that create and maintain them. Knowledge of change in vegetation patterns enhances resource manager and public awareness of patterns that better correspond with current climate， site conditions， and native disturbance regimes， and improves understanding of conditions to which native terrestrial species have already adapted. In this study， we characterized recent historical and current vegetation composition and structure of 337 randomly sampled subwatersheds (9500 ha average size)， in 43 of 164 total subbasins (404 000 ha average size)， selected by stratified random draw on all ownerships within the interior Columbia River basin and portions of the Klamath and Great Basins (collectively referred to as the basin). We compared landscape patterns， vegetation structure and composition， and landscape vulnerability to 21 major insect and pathogen disturbances of historical and current vegetation coverages. For each selected subwatershed， we constructed historical and current vegetation maps from interpretations of 1932-66 and 1981-93 aerial photos， respectively. Areas with homogeneous vegetation composition and structure were delineated as patches to a minimum size of 4 ha. We then attributed cover types (composition)， structural classes (structure)， and series-level potential vegetation types (site potential) to individual patches within subwatersheds by modeling procedures. We characterized change in vegetation spatial patterns by using an array of class and landscape pattern metrics and a spatial pattern analysis program. Finally， we translated change in vegetation patterns to change in landscape vulnerability to major forest pathogen and insect disturbances. Change analyses results were reported for province-scale ecological reporting units. Forest and range ecosystems are significantly altered after their first century of active management， but there is reason for guarded optimism. Large areas remain relatively unchanged and intact， such as can be found on the east side of the Cascade Range in Washington and in the central Idaho mountains， and these areas may provide an essential “nucleus” for conservation strategies and ecosystem restoration. Strategies for improving the health of basin ecosystems can build on existing strengths. Improved understanding of change in vegetation patterns， causative factors， and links with disturbance processes will assist managers and policymakers in making informed decisions about how to address important ecosystem health issues.(388p.)