Megan Buchanan* - University of Minnesota
Justin L Hart - University of Alabama
In the eastern US, forest disturbance regimes are dominated by localized events that remove relatively small portions of the canopy and increase growing space and resource availability for adjacent trees. We analyzed tree-ring series from 44 old-growth white oak sites located within the species' range with the goal of identifying changes in disturbance characteristics throughout the tree-ring record. We documented large gap-scale disturbances via the identification of release events with a minimum five-year duration. The range-wide disturbance chronology contained 311 releases and displayed several pulses in release and establishment frequency related to stand developmental processes, drought, and anthropogenic impacts. Throughout the 44 sites, we documented a steady decline in large gap-scale disturbances that began in the mid-1600s. 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, land use, and extinction of Passenger Pigeon. Historically, Passenger Pigeon exerted a profound influence on forest composition and structure through canopy disturbance. The observed decline of large gap-scale disturbance coincides with the loss of the species. The widespread successional shift reported in oak stands throughout the East may be partly explained by the decrease in large gap-scale disturbance. If land managers aim to mimic historical disturbance regimes, a combination of harvest prescriptions should be used that result in both more and larger gaps. These larger, more frequent gaps may facilitate the regeneration of mid-successional taxa such as oak.