A methodology for estimating canopy disturbance frequency and intensity in dense temperate forests

Analysis of the frequency of past moderate and high-intensity disturbances has been hindered in forests of complex age structure by methodological problems. A methodology is proposed for developing a disturbance chronology in such stands by identifying the probable date of canopy accession for each sample tree. Canopy accession dates are based on an evaluation of radial growth pattern and early growth rates of existing canopy trees. Canopy disturbance intensity is defined as the percentage of sample trees with canopy accession events in each decade. Rotation periods for disturbances of various intensities are calculated from the chronology. The method was evaluated using 893 increment cores from 70 plots in northern hardwood stands of western Upper Michigan. The estimated average disturbance rate for all plots and decades was 5.7–6.9% of land area per decade, with an implied average canopy tree residence time of 145–175 years. These estimates are similar to those obtained by on-site estimates of canopy tree residence time and studies in the literature on the rate of gap formation. Problems in radial increment analysis and possible solutions are discussed.