Variability in early height growth rate of forest trees: implications for retrospective studies of stand dynamics

Retrospective studies of forest stand dynamics may rely on estimates of tree ages. In some of these studies, trees are aged near the stem base, while in other studies trees may be aged at breast height. An age correction may be added to breast-height ages in an attempt to account for average time to reach breast height and thus provide better estimates of total ages. Aging at breast height can provide estimates of stem and stand ages that are sufficient and appropriate for many studies of stand dynamics, for example, those focusing on the dynamics of canopy recruitment. However, the various aging methodologies will provide similar interpretations of actual stand age structures only if early height growth rates are not variable among stems, an assumption not likely to be true. Thus, aging at breast height, with or without a correction factor, may be inappropriate in studies that rely on accurate determination of tree establishment times. In the present study, variability of early height growth rates for several tree species common to Populusgrandidentata Michx. forests is quantified by determining the number of years to reach breast height. Interpretations of stand age structures and dynamics are made based on total tree ages, breast-height ages, and corrected breast-height ages. The results are compared to explore the implications of ignoring variability in early height growth rates when interpreting development of the stands. For the study populations, early height growth rates were highly variable. Some variability was accounted for by differences in understory tolerance, establishment times, and regenerative modes. Intolerant species establishing early grew faster than more tolerant, later establishing species. Sprout-origin stems grew faster, and had much less variable growth rates, than did seed-origin stems. In the understory, hardwood regeneration grew faster than pine regeneration. Even after accounting for these factors, early height growth rates were still variable. Within replicate plots, cumulative establishment distributions based on breast-height ages always differed from those based on total ages, leading to different interpretations of stand age structures. Cumulative establishment distributions based on breast-height ages corrected for aging height differed 44% of the time from those based on total ages. The timing of understory reinitiation, an important dynamic in even-aged forests, was determined using the three aging methods and compared. The timing of understory reinitiation based on breast-height ages differed significantly from that derived using total ages, while that derived using corrected breast-height ages did not differ from the latter. These results suggest that interpretations of stand age structures and past dynamics based on breast-height ages should be viewed cautiously if the objectives of a study require accurate estimates of tree establishment times.