Restoration of presettlement age structure of an Arizona ponderosa pine forest

The age structure in 1876, the last year of the natural frequent-fire regime, of an unharvested ponderosa pine forest in northern Arizona was reconstructed from living and dead dendrochronological samples. Approximately 20% of the trees were >200 yr old in 1876 with ages ranging to 540 yr. If dead trees had not been included in the reconstruction, the distribution would have been biased toward younger trees and a 40% shorter age range. The presettlement age distribution was multimodal with broad peaks of establishment, consistent with the model of regeneration in 'safe sites' where herbaceous competition and fire thinning are reduced. Although fire disturbance regimes and climatic conditions varied over the centuries before 1876, a clear relationship between these variations and tree establishment was not observed. Due to fire exclusion, reduced grass competition, and favorable climatic events, high levels of regeneration in the 20th century raised forest density from 60 trees/ha in 1876 to >3000 trees/ha in 1992. An ecological restoration experiment initiated in 1993 conserved all living presettlement trees and reduced the density of young trees to near-presettlement levels. Two important components for evaluating the restoration treatment effects are monitoring of old-tree persistence and patterns of future regeneration in the context of the presettlement reference age structure.