Forest succession along a productivity gradient following fire exclusion

Numerous studies have documented significant change in conifer forests of the American West following the cessation of recurrent fire at the end of the 19th century. But the successional dynamics that characterize different forested settings in the absence of fire remain poorly understood. This study reconstructs structural and compositional change over 150 years across a productivity gradient that includes both mixed conifer and ponderosa pine forests within a 688,000 ha study area in the southern Blue Mountains of eastern Oregon. Moister and more productive forest stands dominated by grand fir did not support more tree basal area and were not appreciably denser in 1860 than drier and less productive stands dominated by ponderosa pine. The greatest magnitude of change over the last 150 years has occurred in the most productive mixed conifer stands. Drier ponderosa pine dominated stands with significant live old-growth structure have experienced relatively little change even after more than a century without fire. Reconstruction of historical forest density in productive mixed conifer forests suggests these sites were historically coupled to the broader dry forest landscape pattern via frequent fire disturbance and should be a priority for restoration if a return to historical conditions is a goal of management.