We compared prefire and postfire organic-layer depths in boreal forest types (14 fires) across Canada, and examined tree recruitment as a function of depth. There was extensive within-stand variation in depth, much of it due to clustering of thinner organic layers around boles. There were no significant differences in postfire organic-layer depth among sites with different prefire forest species composition, but sites in the eastern boreal region had thicker postfire organic layers than those in the western boreal region. Mean organic-layer depth was much greater in intact stands than after fires; overall, fire reduced organic-layer depth by 60%, largely because of increases in the area of thin (<3 cm) organic layers (1% in intact stands vs. 40% in postfire stands). There was more variation in organic-layer depth within postfire than within prefire stands; notably, some areas in postfire stands were deeply combusted, while adjacent parts were only lightly combusted. We speculate that the diminished role of energy loss to latent heat around tree boles increased organic-layer consumption around tree boles. Seedlings were clustered around burned tree bases, where organic layers were thinner, and the dependence of a species on thin organic layers was an inverse function of seed size.Résumé : Nous avons comparé l'épaisseur de la matière organique avant et après un feu dans différents types forestiers boréaux (14 feux) à travers le Canada et nous avons étudié le recrutement d'arbres en fonction de l'épaisseur de la matière organique. L'épaisseur de la matière organique variait beaucoup à l'intérieur des peuplements surtout parce que les couches plus minces étaient concentrées autour des arbres. Après un feu, il n'y avait pas de différence significative dans l'épaisseur de la matière organique entre les stations dont la composition avant le feu était différente, mais la matière organique était plus épaisse dans les stations de la forêt boréale de l'est que dans celles de la forêt boréale de l'ouest. Avant le feu, la matière organique des peuplements était plus épaisse qu'après le feu. De façon générale, le feu a diminué l'épaisseur de la couche de matière organique de 60 %, en grande partie à cause de l'augmentation de la surface couverte par une mince (<3 cm) couche de matière organique (1 % dans les peuplements non brûlés vs 40 % dans les peuplements brûlés). L'épaisseur de la matière organique était plus variable à l'intérieur des peuplements après le feu qu'avant le feu, notamment en raison des surfaces profondément brûlées qui côtoyaient des surfaces légèrement brûlées à la suite du passage du feu. Nous avançons l'hypothèse que la diminution de la perte d'énergie en chaleur latente autour des arbres cause une augmentation de la consommation de la couche organique autour des troncs. Les semis étaient regroupés autour de la base des arbres brûlés, là où la matière organique était plus mince, et la dépendance d'une espèce à la présence d'une mince couche de matière organique était une fonction inverse de la taille des...
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.
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