Tree mortality following partial harvests is determined by skidding proximity.

Faculty of Forestry, University of Toronto, 33 Willcocks St., Toronto, Ontario M5S3B3, Canada.
Ecological Applications (Impact Factor: 3.82). 11/2008; 18(7):1652-63. DOI: 10.1890/07-1697.1
Source: PubMed

ABSTRACT Recently developed structural retention harvesting strategies aim to improve habitat and ecological services provided by managed forest stands by better emulating natural disturbances. The potential for elevated mortality of residual trees following such harvests remains a critical concern for forest managers, and may present a barrier to more widespread implementation of the approach. We used a harvest chronosequence combined with dendrochronological techniques and an individual-based neighborhood analysis to examine the rate and time course of residual-tree mortality in the first decade following operational partial "structural retention" harvests in the boreal forest of Ontario, Canada. In the first year after harvest, residual-tree mortality peaked at 12.6 times the preharvest rate. Subsequently, mortality declined rapidly and approached preharvest levels within 10 years. Proximity to skid trails was the most important predictor both of windthrow and standing death, which contributed roughly equally to total postharvest mortality. Local exposure further increased windthrow risk, while crowding enhanced the risk of standing mortality. Ten years after harvest, an average of 10.5% of residual trees had died as a result of elevated postharvest mortality. Predicted cumulative elevated mortality in the first decade after harvest ranged from 2.4% to 37% of residual trees across the observed gradient of skid trail proximity, indicating that postharvest mortality will remain at or below acceptable rates only if skidding impacts are minimized. These results represent an important step toward understanding how elevated mortality may influence stand dynamics and habitat supply following moderate-severity disturbances such as partial harvests, insect outbreaks, and windstorms.

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    ABSTRACT: Across forest ecosystems worldwide, alternative silvicultural practices such as partial or “structural retention” harvesting are gaining increasing use as means to balance economic and ecological management goals. Recent studies have documented highly elevated rates of residual-tree mortality and growth, both of which are likely to influence post-harvest stand development. Explicit consideration of residual-tree responses to partial harvest is therefore required to make accurate growth and yield predictions. In this paper we incorporate empirically based, spatially explicit tree growth and mortality predictors into a forest simulation model in order to quantify stand dynamics following a range of simulated partial harvests. We consider lowland black spruce boreal forests and use a common strip-cutting technique, Harvest with Advance Regeneration Protection, as a reference against which to compare alternative scenarios. Predicted stand development proceeded most rapidly in scenarios with reduced skidding impacts and relatively high levels of retention, since these treatments were associated with lower residual-tree mortality rates and larger growing stocks respectively. Stand development was also influenced on a proportional basis by residual-tree growth rates, which increased with declining neighbourhood competition. Across a range of harvest scenarios with equivalent retention levels (20%), differences in residual-tree responses among treatments led to a 35-year range in projected basal area return intervals (70–105 years). Our results demonstrate that short-term responses of residual trees affect stand development rates over long (>100-year) time scales, and show that post-harvest stand recovery rates are influenced not only by the retention level but also by the specific parameters of partial harvest prescriptions, including the layout of skid trails and the spatial arrangement of residual trees. Results from this study provide essential information for evaluating partial harvests in the black spruce boreal forest and supply modelling tools for assessing alternative silvicultural treatments in other forest ecosystems.
    Ecological Modelling. 01/2010;
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    ABSTRACT: Summary1. Tree mortality has important influences on forest structure and composition, but the mechanisms that cause tree mortality are not well understood. Asymmetric competition is known to be a dominant cause of plant mortality, but this idea has not received much attention in studies of long-lived trees.2. We hypothesised that while tree mortality is dependent on size relative to neighbours as a result of asymmetric competition, tree mortality of shade-tolerant species varies little with size because of their physiological and morphological adaptations to shaded environments. Furthermore, we hypothesised that tree mortality is higher in more crowded stands because of higher average resource competition, in conspecific stands because of potential negative intra-specific interactions, and in older stands because of the physiological limitations and susceptibility to minor disturbances of large trees.3. Using data from repeatedly measured permanent sampling plots that covered a wide range of tree sizes, stand developmental stages and stand compositions in boreal forests, we simultaneously tested, by boosted regression tree models, the effects of an individual’s relative size, stand crowding, species interaction and ageing on mortality of Pinus banksiana, Populus tremuloides, Betula papyrifera and Picea mariana.4. Mortality increased strongly with decreasing relative size for all study species, and the size-dependent mortality was stronger for shade-intolerant than for shade-tolerant species. With increasing stand basal area, mortality increased for Pinus banksiana, Populus tremuloides and Picea mariana but decreased for Betula papyrifera. Mortality was higher in stands with more conspecific neighbours for Populus tremuloides, Betula papyrifera and Picea mariana, but was slightly lower for Pinus banksiana. Mortality also increased with stand age for all species. Furthermore, the size-dependent mortality was generally stronger in more crowded stands.5. Synthesis. Our findings show that tree mortality over a wide range of tree sizes, stand developmental stages and stand compositions in non-equilibrium boreal forests was strongly controlled by competition, but species interactions and ageing were also important mechanisms. Furthermore, the relative importance of these mechanisms to tree mortality differed with the shade tolerance of species.
    Journal of Ecology 10/2011; 99(6):1470 - 1480. · 5.43 Impact Factor
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    Forest Ecology and Management 11/2013; 308:116-127. · 2.77 Impact Factor

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