Canopy disturbance history in old-growth white oak sites throughout the eastern United States: Implications for oak management

Background/Question/Methods

In the Eastern Deciduous Forest Formation, disturbance regimes are dominated by localized events that remove relatively small portions of the forest canopy, increasing growing space and resource availability for residual trees and allowing for the establishment of new individuals. Such disturbances are therefore important for the maintenance of moderately shade-tolerant taxa such as oak, a genus that is failing to regenerate and declining in dominance throughout the eastern United States. Numerous local-scale canopy disturbance reconstructions have been developed and have shaped our understanding of natural disturbance frequency and changes in forest systems through time. To examine canopy disturbance trends across a broader scale, we analyzed tree-ring series from 44 old-growth white oak (Quercus alba) sites located throughout the species’ distributional range. We used a radial growth averaging technique to document the occurrence of release events in the tree-ring record. We included a five-year release duration criterion to serve as a proxy for gap size and ensure only large gap-scale disturbance events were identified. To account for unequal sample sizes and tree ages, we calculated the mean releases tree^-1 century^-1 and a ratio of releases to sample sizes for all sites and forest regions. 

Results/Conclusions

The disturbance chronology contained 311 release events and several pulses in release and establishment frequency that were likely related to stand developmental processes, drought, and anthropogenic impacts. A decline in large gap-scale disturbances beginning in the mid-1600s was apparent in the mean releases tree^-1 century^-1 and ratio of releases to sample sizes. This gradual decrease in canopy disturbance is neither the result of forest developmental processes occurring in synchrony nor the result of tree age. Possible broad-scale phenomena that may explain this decline include changes in drought regimes, changes in land use, and the extinction of Passenger Pigeon (Ectopistes migratorious). Prior to the decline of Passenger Pigeon in the late 19^th Century, the species represented a profound disturbance agent in eastern forests and the decline of the species is concomitant with the decline in large gap-scale disturbance. The oak-maple transition documented throughout the eastern U.S. may be partly explained by the decrease in large gap-scale disturbance. Land managers aiming to mimic historical canopy disturbances should employ a combination of harvest prescriptions to result in both more and larger gaps (e.g. group-selection and shelterwood harvests). These larger, more frequent gaps may facilitate the regeneration of mid-successional taxa such as oak.