Canopy disturbance history of old-growth Quercus alba sites in the eastern United States: Examination of long-term trends and broad-scale patterns

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. Numerous local-scale canopy disturbance reconstructions have been developed and have shaped our understanding of natural disturbance frequency, community dynamics, and changes in forest systems through time. We analyzed tree-ring series from 44 Quercus alba old-growth sites located throughout the species' distributional range with the ultimate goal of identifying long-term, broad-scale changes in canopy disturbance regime characteristics throughout the tree-ring record. We documented large gap-scale disturbances via the identification of release events with a five-year duration. 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. The disturbance chronology contained 311 release events and displayed several pulses in release and establishment frequency that were likely related to stand developmental processes, drought, and anthropogenic impacts. Throughout the 44 old-growth Q. alba sites, a steady decline in large gap-scale disturbances beginning in the mid-1600s was apparent in the both 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 throughout the 44 sites nor a result of tree age. Possible broad-scale phenomena that may explain this decline in canopy disturbance include changes in drought frequency and intensity, changes in anthropogenic land use (i.e. timber harvesting and localized fire), and the extinction of Ectopistes migratorious (Passenger Pigeon). Prior to the decline of E. migratorious in the late 19th Century, the species exerted a profound influence on eastern hardwood forest composition and structure through canopy disturbance. The decline of large gap-scale disturbance is concomitant with the loss of the species. The widespread successional shift reported in Quercus stands throughout the Eastern Deciduous Forest may be partly explained by the decrease in large gap-scale disturbance. If land managers aim to mimic the hypothesized canopy disturbance from centuries prior, a combination of harvest prescriptions should be employed to result in both more and larger gaps. Single-tree, group-selection, and shelterwood harvests may mimic the hypothesized impact of E. migratorious on eastern hardwood forests (i.e. medium to large gap-scale and incomplete stand-scale disturbances). These larger, more frequent gaps may facilitate the regeneration of mid-successional taxa such as Quercus. (C) 2011 Elsevier B.V. All rights reserved.