Vegetative patterns, disturbances, and forest health in Eastern Oregon and Washington
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Vegetation patterns in eastern Oregon and Washington, USA are largely a result of environmental conditions, species distributions, plant ecology, and disturbances operating at multiple scales and in different environments. In turn, vegetative patterns strongly influence the amount, severity, and distribution of disturbances generated by various agents. This paper focuses on the latter - the relations between vegetation pattern, disturbance, and forest health and productivity. At all scales, vulnerability to disturbance appears to increase when vegetation condition and pattern differs from the historical or expected range for a given environment. Generally, forests that are older, composed of larger trees, denser, more homogeneous, or more contiguous than would be expected under natural or historical disturbance regimes are more vulnerable to mortality from insects and disease. Factors related to vulnerability include site potential, host abundance, canopy structure, host size, patch vigour, patch density, patch connectivity, topography, and logging disturbance. Mortality from insects and disease contributes to diverse habitat, but current levels of tree mortality from insects and disease are often outside the historical or expected range given site environment. High levels of mortality may continue because many forests have become more homogeneous, contiguous, and dominated by shade-tolerant species owing to fire suppression and management. Uncharacteristically severe fires will likely increase in the next 100 years even with restoration management because of changed vegetation patterns and other factors. Information at stand or site scales is relatively abundant in the scientific and management literature. Much broad-scale information is based on models and expert opinion. Research at broad scales is scanty and difficult.