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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.

In the past 10 to 15 years, alternative silvicultural treatments involving partial harvesting have been developed for boreal forests, with the goal of achieving a balance between biodiversity maintenance and continued timber production. Most prior research has focussed on the impacts of partial harvesting on biological diversity, while stand dynamic responses remain little studied. In this paper we explore partial stand harvesting in the Canadian boreal-its rationale, current extent, and impact on stand dynamic patterns. Empirical studies from the boreal and elsewhere indicate that residual trees of many species respond to partial harvesting with enhanced growth, commonly showing a lagged response after which peak growth occurs five to 25 years following harvest. Post-harvest mortality is also prevalent but much more variable, with losses of residual trees ranging from nearly zero to more than 50% above background mortality rates in the initial years following harvest. With the exception of strip cutting in parts of northern Ontario and Quebec (HARP/CPPTM), operational partial harvesting in the Canadian boreal currently involves very low levels of retention. Available data suggest that such low retention levels, particularly when spatially dispersed, generally result in unacceptably high rates of post-harvest mortality, which are unlikely to be offset by increases in residual tree growth. There is an urgent need for development of spatially explicit stand simulation models that will allow accurate yield predictions for partial harvest systems to assess their feasibility in boreal forest management.Key words: ecosystem management, natural disturbance emulation, boreal forest, partial cut, structural retention, growth response, windthrow, post-harvest mortality RÉSUMÉ Au cours des dix à quinze dernières années, des traitements sylvicoles alternatifs comprenant une coupe partielle ont été élaborés en forêt boréale, dans le but d'obtenir un équilibre entre le maintien de la biodiversité et la production continue de matière ligneuse. La majeure partie de la recherche réalisée à ce jour a porté sur les impacts de la coupe partielle sur la diversité biologique, tandis que les réactions au niveau de la dynamique du peuplement recevaient peu d'attention. Dans cet article, nous explorons la coupe partielle des peuplements dans les forêts boréales du Canada : le rationnel, le niveau actuel d'utilisation et les conséquences sur les patrons de dynamique des peuplements. Les études empiriques en provenance des forêts boréales et des autres forêts indiquent que les arbres résiduels de plusieurs espèces réagissent à la coupe partielle par un accroissement de la croissance, démontrant généralement une réaction déphasée suivie d'un pic de croissance survenant entre 5 et 25 ans après la récolte. La mortalité après coupe est également présente quoique plus variable, avec des pertes parmi les arbres résiduels avec des taux de zéro à plus de 50 % de mortalité par rapport au milieu naturel au cours des premières années suivant...

Tree mortality is a critical driver of stand dynamics, influencing forest structure, composition, and capacity for ecosystem service provision. In recent years, tree mortality has been gaining attention as dramatic occurrences of forest dieoff have been linked to climate change. Using permanent sample plot data, we examined tree mortality rates in mature forests in west-central Alberta from 1956 to 2007. We quantified mortality risk at an individual-tree level as a function of size, local competition, and calendar year, a proxy for increasing temperature, and used maximum likelihood methods to estimate species-specific model parameters. Tree size and local competition were both important predictors of mortality risk. However, once these factors were included in our model, no additional variation could be attributed to calendar year, indicating that the trend of increasing tree mortality over time found in our raw data is primarily a result of stand development processes. This finding is supported by the changes in forest structure and composition that we documented over the study period. Stands generally increased in basal area and stem density, and lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson) declined in abundance relative to the more shade-tolerant black spruce (Picea mariana (Mill.) B.S.P.) and white spruce (Picea glauca (Moench) Voss). Our results indicate that warming-related changes did not affect background tree mortality rates in mature forests in the Alberta foothills over the study period. These results also provide critical information for future studies of forest dynamics in the region.Résumé : La mortalité des arbres est un facteur important de la dynamique des peuplements qui a un impact sur la structure, la composition et la capacité des peuplements à préserver l'écosystème forestier. Au cours des dernières années, la mortalité a fait l'objet d'une attention croissante à cause d'épisodes de mortalité massive liés aux changements climatiques. En utilisant des données de placettes échantillons permanentes, nous avons étudié le taux de mortalité des arbres dans les forêts matures du centre-ouest de l'Alberta de 1956 à 2007. Nous avons quantifié le risque de mortalité à l'échelle de l'arbre en fonction de la taille, de la compétition locale et de l'année civile, qui reflète l'augmentation des températures, et nous avons utilisé des méthodes de maximum de vraisemblance pour estimer les paramètres de modèles propres à chaque espèce. La taille des arbres et la compétition locale étaient deux prédicteurs importants du risque de mortalité. Toutefois, lorsque ces facteurs étaient inclus dans le modèle, aucune variation additionnelle ne pouvait être expliquée par l'année civile, ce qui indique que la tendance à l'augmentation de la mortalité avec le temps observée dans nos données brutes est surtout produite par les processus de développement du peuplement. Ce résultat est appuyé par les changements de structure et de composition forestière observés au cours de la période d'étude. La su...

Variants of partial harvesting are gaining favour as means to balance ecosystem management and timber production objectives on managed boreal forest landscapes. Understanding how residual trees respond to these alternative silvicultural treatments is a critical step towards evaluating their potential from either a conservation or a wood supply perspective. We used dendroecological techniques combined with a chronosequence approach to quantify the temporal radial growth response pattern of residual black spruce ( Picea mariana (Mill.) BSP) trees to partial harvest in northeastern Ontario. At its peak, 8–9 years after harvest, radial growth of residual trees had doubled. The growth pattern was characterized by a 2-year phase of no response, a subsequent period of increase 3–9 years after harvest, and a stage of declining rates 10–12 years after harvest. The magnitude of tree growth response depended strongly on tree age: peak postharvest growth was substantially higher for young trees, while old trees displayed only modest growth increases. Both the large magnitude and the time delay in postharvest growth responses have important implications for the development of more accurate quantitative tools to project future yields and, more generally, for determining whether partial harvesting is a viable management option for the boreal forest.