Use of boundary-line growth patterns as a basis for dendroecological release criteria

Identifying releases from suppression represents one of the most fundamental dendroecological procedures for quantifying forest disturbance histories. In tree-ring series, releases are typically defined as pulses in percent growth change that exceed a minimum threshold. Past methodologies have applied fixed growth-change thresholds that may overly generalize the ability of a tree to respond to a disturbance event. In this study, we develop a more precise method that scales each release by its maximum release potential as defined by prior growth rates, using an old growth hemlock (Tsuga canadensis L.) forest in central Pennsylvania as an example. On average, tree age, diameter, and crown class appear to influence the magnitude of release response. However, differences in release response between age, diameter, and crown classes are largely an artifact of the more fundamental relationship between release response and growth immediately prior to release. Specifically, maximum percent growth change declines at a steep, negative exponential rate as prior growth levels increase. Slow-growing hemlocks can exceed 800% growth increases while fast-growing hemlocks do not exceed growth increases of 20%. This negative exponential threshold, or boundary line,, represents the maximum percent growth change that is physiologically possible at a given level of prior growth. Thus, release potential is relative to prior growth rate. This relationship between prior growth and release potential is species specific and is demonstrated in 24 additional hemlock stands. We propose new criteria based on prior growth in which moderate and major releases are those falling within 20-49.9%, and 50-100% of the boundary line, respectively. Effects of short, moderate, and long-term climate events are mitigated by the percent-growth-change calculation and the high growth-change threshold applied to slow-growing trees. In addition, we demonstrate that similar boundary lines occur in Pinus strobus L and Quercus prinus L., and interspecific variations in boundary line configuration reflect differences in understory tolerance. Overall, this approach provides a more theoretically sound and flexible approach to developing disturbance histories, and we believe it will ultimately have broad applications in a variety of forest types.