CANOPY DISTURBANCE INTERVALS, EARLY GROWTH RATES, AND CANOPY ACCESSION TRENDS OF OAK-DOMINATED OLD-GROWTH FORESTS

Using a radial growth averaging technique, changes in growth rates of overstory oaks were used to quantify canopy disturbance events at five old-growth sites. On average, at least one canopy disturbance occurred on these sites every 3 years; larger multiple-tree disturbances occurred every 17 years. Although there was some variation by site and by historical period, there has been little significant change in canopy disturbance rates for the 300-year period examined. A review of tree-ring chronologies yielded three growth strategies. For these oaks, a) the likelihood of originating in a large opening and achieving overstory status before canopy closure is about the same as the probability of requiring a major release, either from b) a smaller gap that closed, or c) from the understory. Half of the cored trees established individually, and 30 percent attained overstory status individually, members of no identifiable larger cohort. For trees that required a major canopy release, residence time in the understory averaged 89, 54, and 50 years for white oak, red oak, and black oak, respectively. These long-term understory residences averages suggest that the abundance of shade tolerant understory species was considerably less before 1900. These results are consistent with presettlement survey tree-tallies, the absence of shade tolerant species in the oldest cohorts, and fact that virtually no oak canopy recruitment after 1900 occurred after an extended period of below-average growth in the understory.