Restoring historical forest conditions in a diverse inland Pacific Northwest landscape

A major goal of managers in fire-prone forests is restoring historical structure and composition to promote resilience to future drought and disturbance. To accomplish this goal, managers require information about reference conditions in different forest types, as well as tools to determine which individual trees to retain or remove to approximate those reference conditions. We used dendroecological reconstructions and General Land Office records to quantify historical forest structure and composition within a 13,600 ha study area in eastern Oregon where the USDA Forest Service is planning restoration treatments. Our analysis demonstrates that all forest types present in the study area, ranging from dry ponderosa pine-dominated forests to moist mixed conifer forests, are considerably denser (273–316% increase) and have much higher basal area (60–176% increase) today than at the end of the 19th century. Historically, both dry pine and mixed conifer forest types were dominated by shade-intolerant species. Today, shade-tolerant tree cover has increased in dry pine stands, while mixed conifer stands are now dominated by shade-tolerant species. Federal managers in eastern Oregon are currently required to retain all live trees >53 cm diameter at breast height in the course of forest management activities because this size class is assumed to be under-represented on the landscape relative to historical conditions. However, we found the same or greater number of live trees >53 cm today than in the late 19th century. Restoring historical conditions usually involves removing shade-tolerant trees that established since Euro-American management significantly altered natural disturbance regimes. We evaluated a wide range of tree morphological and environmental variables that could potentially predict the age of grand fir and Douglas-fir, the most abundant shade-tolerant species found within the study area. We describe several morphological characteristics that are diagnostic of tree age and developed decision trees that predict the approximate age of trees using morphological characteristics that are easy to measure in the field such as height to live foliage or height to dead branches. Information about structural and compositional change over time combined with tree-age prediction tools provides a flexible framework for restoring historical conditions and meeting other resource management objectives.