Can forest management based on natural disturbances maintain ecological resilience?

Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance

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 Populus grandidentata 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