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One-year-old black spruce (Piceamariana (Mill.) B.S.P.) seedlings were preconditioned by exposing them to either one or two dehydration–rehydration cycles by using the osmoticum polyethylene glycol 3350. Preconditioned and unconditioned seedlings were then subjected to a more severe osmotic (water) stress by exposing them to a higher concentration of polyethylene glycol. Effects of repeated dehydration–rehydration cycles on cell-water relations were studied after 3, 7, and 13 days of stress relief using pressure–volume curve analysis. Repeated dehydration–rehydration cycles caused a cumulative increase in turgor potentials at full saturation. In these preconditioned plants there was also a progressive lowering of osmotic potentials and relative water contents at zero turgor, which increased over time with stress relief. The decline in osmotic potentials at zero turgor in osmotically stressed black spruce was associated with increased cell wall relaxation, followed by increased turgor potentials, in preconditioned but not in unconditioned seedlings. Saturated osmotic potentials were not altered by repeated, short-term conditioning stresses, suggesting that tissue elasticity was more important for turgor regulation than osmotic adjustment.

Periodic functions of Julian calendar dates were used to incorporate seasonal variation into logistic regression models designed to predict daily people-caused forest fire occurrence in the Northern Region of the province of Ontario. Three years of independent test data were used to evaluate predictions produced by the models.

At three previously thinned, even-aged northern hardwood stands affected by the 1998 ice storm, we evaluated the responses of trees in plots given a rehabilitation treatment, and compared findings to adjacent untreated areas. In both the rehabilitated and untreated areas mortality seemed consistent with regional norms for northern hardwood stands. Trees that died after the storm had essentially complete crown loss due to ice loading, and 12% of living trees lost at least half of their crown. In both sets of plots, epicormic branching within the crown led to crown rebuilding, resulting in high levels of tree survival and good post-storm diameter growth on all plots. Release of selected upper-canopy trees by the rehabilitation cutting significantly increased diameter growth rates compared to those in untreated areas. While some epicormics developed below the base of the original crown in both treated and untreated plots, they largely emanated at > 5.2 m from the ground and did not degrade the butt log of most trees.Keywords: ice storm, even-aged northern hardwoods, rehabilitation treatment, stand growth, stand mortality, epicormic branching. RéSuméAu moyen de mesures effectuées dans des parcelles, nous avons évalué la réaction des arbres à la suite du verglas de 1998 dans trois peuplements équiennes de feuillus nordiques éclaircis qui avaient bénéficié d'un traitement de réhabilitation, et comparé les résultats à ceux des parcelles voisines n'ayant reçu aucun traitement. Tant dans les parcelles réhabilitées que dans celles non traitées, la mortalité semblait conforme aux standards régionaux pour les peuplements de feuillus nordiques. Les arbres qui sont morts après la tempête de verglas avaient essentiellement perdu l' ensemble de leur cime sous le poids de la glace et 12 % des arbres vivants avaient perdu au moins la moitié de leur cime. Dans les deux groupes de parcelles, la croissance des pousses adventives dans la cime a permis à celle-ci de se rétablir ce qui a donné un fort niveau de survie chez les arbres et une bonne croissance en diamètre post-tempête dans toutes les parcelles. Le dégage-ment des arbres choisis parmi les dominants au cours de la coupe de réhabilitation a considérablement augmenté les taux de croissance en diamètre, comparativement aux arbres des parcelles non traitées. Même si certaines pousses adventives se sont développées sous le niveau de la cime originale, tant dans les parcelles traitées que dans celles non traitées, elles se trouvaient en grande partie à plus de 5,2 m au-dessus du sol et n' ont pas déclassé la bille de souche de la plupart des arbres.

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