Early successional competition among boreal forest tree and shrub species and its effects on growth of commercial tree species have been a major source of uncertainty in establishing efficient precommercial thinning and brushing prescriptions. We examined the effect of prethinning competitor density, postthinning competitor regrowth density, prethinning stem diameter, and the timing of thinning operations on the growth response of black spruce (Picea mariana [Mill.] BSP) and jack pine (Pinus banksiana Lamb.). In addition, we examined the mortality rate of hardwoods after thinning and the number of new shoots produced per surviving thinned stem. For jack pine, growth response was greatest when thinning occurred between 4 and 9 years after establishment, whereas for black spruce we observed no significant relationship between growth response and the timing of treatment. For jack pine, growth response was significantly affected by pretreatment competitor density, posttreatment competitor regrowth density, and pretreatment stem diameter. For black spruce, no significant relationship was observed between growth response and any variables. Mortality rates and production of new shoots in hardwoods varied significantly between species. Considering the high regrowth potential of willow (Salix spp.) and alder (Alnus spp.), we recommend that stands exhibiting low densities of these species should be left unthinned. Our results help foresters identify stands that require precommercial thinning and call for modification of currently used thinning strategies.

In this paper, we model the post-fire recruitment dynamics of two aerial seedbank species, Picea mariana and Pinus banksiana, in response to salvage logging. The model incorporates: (1) initial seed availability as a function of source tree basal area and proportion of stand salvaged; (2) seed abscission as a function of time; (3) seedling survivorship as a function of seed mass, seedbed proportion, and granivory; and (4) seedling and seed mortality as a function of salvage operations. We also elaborate a simulation of the effect of direct seeding via cone-bearing branches fed into a moving chipper. The model performed adequately when tested against data sets from two fires in Quebec and one in Saskatchewan. In particular, it showed that P. mariana was more adversely affected by early salvage than P. banksiana because of its far slower seed abscission rate. The model predicted that a delay in salvage or a decrease in salvage proportion would enhance tree regeneration densities, especially for P. mariana. Finally, model projections indicate that the use of a chipper to disseminate seeds during the harvesting would permit either species to be adequately regenerated cheaply even with low pre-fire basal area per area or very early salvage.

For aerial seedbank species, the seed abscission schedule following fire is of practical interest as it affects the optimal timing of post-fire salvage operations designed to maximize natural regeneration. It is also of theoretical interest as we would expect that the rapid deterioration of the better (very thin duff or exposed mineral soil) post-fire seedbeds due to leaf-fall from regenerating plants ought to select for rapid dissemination of seeds following burning. Nonetheless, there are no published reports of the abscission schedule of an aerial seedbank species that include the full temporal range from the fire date to several years after. In northwestern Quebec, we used eight burnt, non-salvaged stands, four dominated by black spruce (Picea mariana) and four dominated by jack pine (Pinus banksiana), in three different fires to examine the seed abscission schedule of these aerial seedbank species for the first 3 years after fire. We found that (1) the abscission schedules of populations of each species differed between fires and (2) black spruce dispersed seeds from the cones at a significantly slower rate than jack pine at all fires. Extrapolating from the regressions (all fires lumped), we conclude that approximately 90% of jack pine and black spruce seeds will have been dispersed by 1 and 5 years, respectively, after a fire. Further, we argue that due to its protracted abscission schedule, early post-fire salvage will invariably require that black spruce be planted. The approach adopted here should be useful for optimizing post-fire salvage timing for all commercially valuable species with aerial seedbanks.

Les peuplements mélangés, caractérisés par une abondance de tiges résineuses prématures croissant sous un couvert dominant de peuplier faux-tremble mature, se prêtent bien à la coupe partielle qui favoriserait la croissan-ce des tiges supprimées et l'établissement de la régénération. Dans cette étude, nous décrivons la réponse initia-le de peuplements soumis à trois types de coupe où on a fait varié l'intensité de la récolte de la surface terrière du tremble par rapport à celle des peuplements témoins. Nous avons ainsi observé que la croissance des tiges inter-médiaires d'épinette blanche était favorisée par la coupe la plus sévère seulement. Après 5 ans, les coupes n'ont pas eu d'effet sur l'établissement et la croissance de la régénération résineuse. Un essai de scarifiage visant à favoriser l'établissement de semis a donc été réalisé. À la suite d'une année semencière, le taux d'établissement des semis d'épinette blanche sur le sol scarifié variait de 125 à 165 semis/m². En dépit de la mortalité élevée d'une grande proportion des semis, une régénération adéquate pourrait être assurée dans la zone scarifiée.

It is well known that post-fire duff layers that are thin or of lower porosity greatly enhance juvenile survivorship of sexually recruiting boreal plant species. Nonetheless, there has been no study on duff compaction by snow following charring. We examined post-fire duff depth for the first 3 years (two winters) after a 2006 wildfire in the boreal forest of north-western Quebec. We found that (1) significant compression was common, with (2) a positive relationship between the initial thickness of the burned organic layer depth and the subsequent amount of compression. The proportional compression rate was, however, roughly constant: ¡­14% after 2 years regardless of initial post-fire duff depth. We conclude that this amount of compression is too little to significantly impact post-fire seedling recruitment.

Fire has been part of the North American boreal and cordilleran ecosystems for thousands of years. Because fire and harvesting compete directly for the same wood supply, and provinces have, within the last few decades, tended to reach their annual allowable cut, salvage logging has emerged as a practice to minimize the impact of fire on long-term wood supply. In most parts of the boreal and cordilleran forests, fire-killed boles rapidly degrade after their death, as wood-boring insects, stain, wood-decay fungi and checking lead to significant loss of grade or volume in the months following the fire. Because of this impending degradation, salvage operations are often hurried and other considerations, including the potential ecological impacts of salvage logging, have seldom been taken into consideration when defining harvesting strategies. The ecological consequences of rapid salvage have been widely studied only in the last 5 years, and it is now clear that salvage logging can have negative impacts on natural regeneration by seed, water quality, and fire-associated animal species. In this paper, we review both industrial and ecological constraints to salvage logging and discuss how both can be integrated in salvage plans. In particular, we focus on the issues of salvage timing and retention. At this point, some type of retention of merchantable stands, even if only for a few years, appears to be the only way to alleviate the negative ecological impacts of post-fire logging. On-site operational constraints, e.g., stands that cannot be harvested due to lack of accessibility, represent an important starting point for any retention strategy.

In North America, Eurasia, and Australia, salvage logging is increasingly being used to mitigate economic losses due to fire, although the effects of this type of intervention are still essentially unknown. In a field experiment in a large recent boreal forest fire in central Quebec, we used 24 paired salvaged and non-salvaged stands to test the effect of salvage on the recruitment of two conifer species possessing an aerial seed bank (Pinus banksiana and Picea mariana). The seedbeds following salvage were, on average, more hospitable to germination, but, incongruously, engendered far lower regeneration densities. The poor recruitment on salvaged sites was due primarily to the loss of seeds following the immediate post-fire salvage, when cone-bearing branches were removed along with the trunks. By contrast, the density of the asexually-recruited Populus tremuloides was relatively unaffected. We suggest simple ways to modify current salvage procedures that would retard this transition from conifer to Populus forest, as well as leaving more wood in situ.

The SAFE (sylviculture et aménagement forestiers écosystémique) project was set up in 1998 in the Lake Duparquet Research and Teaching Forest to test stand-level silvicultural treatments designed to reflect different aspects of natural forest dynamics. In the winter of 1998–1999, four levels of forest harvesting, including a no-harvest and a clearcut treatment, were applied to even-aged trembling aspen (Populus tremuloides Michx.) stands according to a complete block design with three replications. Two partial cut treatments removed 33% and 61% of the stand basal area. During the first growing season, harvesting induced a large increase in indigenous understorey biomass that paralleled changes in the canopy opening. Aspen sucker density increased from 4916 stems/ha in the control to 28 751 and 63 333 stems/ha in the one-third and two-thirds harvesting treatments and 102 916 stems/ha in the clearcut. Most changes in nutrient cycling occurred in the second year and included an increase in forest floor organic C, total N, and base cation availability and a decrease in microbial C/N ratio. These changes may have occurred in response to reduced vegetation uptake and woody debris abundance.

Le projet SAFE (sylviculture et aménagement forestiers écosystémique) a débuté en 1998 dans la Forêt d'enseignement et de recherche du lac Duparquet afin de tester différents traitements sylvicoles inspirés de la dynamique naturelle des peuplements. Quatre intensités de récolte, incluant la coupe totale, la récolte de 33 et 61% de la surface terrière totale et un traitement témoin non coupé, ont été appliquées à des peuplements équiennes de tremble (Populus tremuloides Michx.) au cours de l'hiver 1998–1999, selon un plan en block complet avec trois répétitions. La végétation de sous bois a répondu par une augmentation de sa biomasse proportionnelle à l'ouverture du couvert. Au cours de la première année, la densité des drageons de tremble était de 4916 tiges/ha dans le traitement témoin, 28 751 et 63 333 tiges/ha dans les coupes partielles et 102 916 tiges/ha dans la coupe totale. La plupart des changements observés dans le cycle des nutriments sont survenus dans la couverture morte pendant la deuxième année suivant la coupe. Parmi ceux-ci figurent une augmentation du C organique, du N total et des cations basiques échangeables et une diminution du ratio C/N de la biomasse microbienne. La diminution du prélèvement par les plantes et les changements dans l'abondance des débris ligneux expliqueraient ces changements.©2004 NRC Canada

The effects of different harvest intensities, including uncut, 1/3 and 2/3 partial cuts, clearcuts with and without slash, were investigated on the germination and cumulative survivorship of white spruce and balsam fir over 2 consecutive years. We also investigated the regenerative capacity of both species on three different seedbeds across all harvest intensities. The seedbeds included were mineral, humus, and organic soil. At the germination stage, both species were strongly affected by seedbed type (p < 0.032). The germination rates of fir seeds in partial cuts were significantly greater than clearcut treatments, but spruce remained unaffected at this stage by harvest intensity. The addition of slash improved the germination rates of fir relative to the clear-cut plots without slash. The germination rates the following year were reduced on mineral soil for spruce. The cumulative survivorship at the end of the third summer still showed a significant seedbed response for both species (p < 0.007) and a significant harvest response for fir (p < 0.005). The cumulative survivorship of the second fir cohort was no longer affected by either harvest or seedbed type. Spruce, however, was still affected by seedbed type (p=0.006). The data from this study provide us with a more detailed description of the fate of cohorts recruited following a harvest operation. Still, what remains to be studied is the fate of these cohorts over the next 5–10 years.

Les effets de différentes intensités de récolte sur la germination et la survie cumulative de l'épinette blanche et du sapin baumier, incluant l'absence de coupe, la coupe partielle d'un tiers ou des deux tiers du peuplement et la coupe totale avec ou sans rémanents, ont été étudiés pendant 2 années consécutives. Nous avons aussi étudié la capacité de régénération des deux espèces sur trois lits de germination à toutes les intensités de récolte. Les trois lits de germination retenus incluaient le sol minéral, l'humus et le sol organique. Au stade de la germination, les deux espèces étaient fortement affectées (p < 0,032) par la nature du lit de germination. Le taux de germination des graines de sapin était significativement plus élevé dans les coupes partielles que dans la coupe totale mais à ce stade l'épinette n'était pas affectée par l'intensité de la récolte. L'addition de rémanents a amélioré le taux de germination du sapin comparativement aux parcelles de coupe totale sans rémanents. La deuxième année, le taux de germination de l'épinette chutait sur le sol minéral. Le taux cumulatif de survie à la fin du troisième été était toujours significativement influencé par le lit de germination chez les deux espèces (p < 0,007) et par l'intensité de la récolte dans le cas du sapin (p < 0,005). Le taux cumulatif de survie de la seconde cohorte de sapin n'était plus affecté ni par le type de récolte ni par la nature du lit de germination (p=0,006). Les résultats de cette étude nous fournissent une description plus détaillée du sort des cohortes recrutées après une opération de récolte. Il reste tout de même à étudier le sort de ces cohortes au cours des 5–10 prochaines années.©2004 NRC Canada

Most studies of postfire tree recruitment have occurred in severely burned portions, despite the fact that partial burning is common. In this study we examined regeneration following a 1997 fire in the boreal forest of Quebec. A model of postfire recruitment was elaborated using parameters such as the proportion of trees killed (severity), the proportions of postfire seedbed types and their associated juvenile survivorship, the available seed supply, the available bud supply (for Populus tremuloides Michx.), and the granivory rate. All three species had peak recruitment in the first or second summer, and the recruitment episode was essentially finished after the third year. Mineral soil and surviving Sphagnum were the best seedbeds for both conifer species. Seedbed frequency was essentially independent of crown fire severity except for surviving Sphagnum, which was concentrated primarily where severity was light. Conifer fecundity was much lower in the lightly burned stands, a result we attribute to a higher granivory rate. The fecundity (seedlings/basal area for the conifers or suckers/basal area for Populus) in the severe sites was typical of the few other North American studies of postfire recruitment, where the published data permit us to make the comparison.

La plupart des études sur le recrutement après feu ont été réalisées dans des zones sévèrement brûlées, malgré le fait qu'il y ait fréquemment des zones légèrement brûlées. Dans cette étude, nous avons examiné la régénération qui s'est établie après un feu survenu en 1997 dans la forêt boréale québécoise. Nous avons élaboré un modèle de recrutement après feu à l'aide de paramètres tels que la proportion d'arbres tués (sévérité), la proportion de types de lits de germination après feu et la survie des jeunes tiges associées à ces lits de germination, le stock de graines disponibles, le stock de bourgeons disponibles (pour le Populus tremuloides Michx.) et le taux de prédation des graines. Les trois espèces ont connu un pic de recrutement durant le premier et le second été, et l'épisode de recrutement était à toute fin pratique terminé après la troisième année. Le sol minéral et la sphaigne qui avait survécu étaient les meilleurs lits de germination pour les deux espèces de conifères. La fréquence des lits de germination était essentiellement indépendante de la sévérité du feu de cime, à l'exception de la sphaigne qui avait survécu, laquelle était principalement concentrée aux endroits où la sévérité du feu était légère. La fécondité des conifères était beaucoup plus faible dans les peuplements légèrement brûlés; un résultat que nous attribuons à un plus haut taux de prédation des graines. La fécondité (nombre de semis/surface terrière pour les conifères ou nombre de drageons/surface terrière pour les peupliers) dans les sites sévèrement brûlés était typique de celle observée dans les quelques autres études nord-américaines sur le recrutement après feu dont la publication des résultats nous permet d'établir une comparaison.©2004 NRC Canada