Fire history was reconstructed for an area of 15 000 km(2) located in the transition zone between the mixed and coniferous forests in Quebec's southern boreal forest. We used aerial photographs, archives, and dendroecological data (315 sites) to reconstruct a stand initiation map for the area. The cumulative distribution of burnt area in relation to time since fire suggests that the fire frequency has decreased drastically since the end of the Little Ice Age (about 1850) in the entire region. However, a large part of the area was burned between 1910 and 1920 during intensive colonization and when the climate was very conducive to fire. For the period 1920-1945, large fires have mainly been concentrated in the more populated southern area, while few fires have been observed in the virgin coniferous forest in the north. Despite slight differences between the south and the north, fire cycles or the average number of years since fire are not significantly different. Since 1945, there have been far more fires in the south, but the mean fire size was smaller than in the-north. These results suggest that the transition between the mixed and coniferous forests observed in the southern boreal forest cannot be explained by a difference in fire frequency, at least during the last 300 years. As climatic factors and species potential distribution did not vary significantly from south to north, we suggest that the transition from mixedwood to coniferous forests is mainly controlled by fire size and severity. Smaller and less severe fires would favor species associated with the mixedwood forests as many need survivors to reinvade burnt areas. The abundance of deciduous species in mixedwood forests, together with the presence of more lakes that can act as firebreaks, may contribute to decreases in fire size and severity. The transition between the two vegetation zones could be related to the initial setting following the vegetation invasion of the area during the Holocene. In this context, the limit of vegetation zones in systems controlled by disturbance regimes such as fires may not have reached a balance with current climatic conditions. Historical legacies and strong positive feedback between disturbance regimes and composition may filter and delay the responses to changes in climate.

Given that fire is the most important disturbance of the boreal forest, climatically induced changes in fire frequency (i.e., area burnt per year) can have important consequences on the resulting forest mosaic age-class distribution and composition. Using archives and dendroecological data we reconstructed the fire frequency in four large sectors along a transect from eastern Ontario to central Quebec. Results showed a dramatic decrease in fire frequency that began in the mid-19th century and has been accentuated during the 20th century. Although all areas showed a similar temporal decrease in area burned, we observed a gradual increase in fire frequency from the west to Abitibi east, followed by a slight decrease in central Quebec. The global warming that has been occurring since the end of the Little Ice Age (similar to 1850) may have created a climate less prone to large forest fires in the eastern boreal forest of North America. This interpretation is corroborated by predictions of a decrease in forest fires for that region of the boreal forest in the future. A longer fire cycle (i.e., the time needed to burn an area equivalent to the study area) has important consequences for sustainable forest management of the boreal forest of eastern Canada. When considering the important proportion of overmature and old-growth stands in the landscape resulting from the elongation of the fire cycles, it becomes difficult to justify clear-cutting practices over all the entire area as well as short rotations as a means to emulate natural disturbances. Alternative practices involving the uses of variable proportion of clear, partial, and selective cutting are discussed. ©2001 NRC Canada

Within the context of studying the ecological impacts of wildland fires in the boreal forest, a spatial analysis of a major wildfire was conducted. The fire covered nearly 500 km² in the north-western part of Quebec's boreal forest in the summer of 1995. The spatial distribution of different fire impacts on the forest canopy was obtained using timber damage assessment maps. Fire impacts varied throughout the burned area, ranging from areas where trees had completely burned crowns (43%) to remaining patches of trees with green foliage (3%). The effects of local stand and site factors on crown fire, as assessed by the fire impacts, were evaluated using geographic information systems. Despite the large extent and high intensity of the wildfire created by extreme fire weather conditions, stepwise logistic regression and analysis by log-linear models indicated that variations in surface material, stand composition, and estimated stand age played a role in the presence or absence of crowning at the stand level. However, it appears that height and density of stand, as well as topography, did not have a significant influence. Our study presents the variability of fire impacts and its implications, and it provides a better understanding of the relationships between landscape components and fire crowning.

On connaît peu la dynamique des feux de forêt dans le façonnement de la mosaïque forestière de la pessière noire à mousses. Dans le cadre de cette problématique, on a étudié le feu de forêt du lac Crochet, qui a couvert près de 50,000 hectares dans le nord-ouest de la forêt boréale au Québec entre le 16 août et le 20 octobre 1995. L'effet de certains facteurs biotiques et abiotiques locaux sur la sévérité du feu de forêt, la distribution des îlots de végétation résiduelle et le patron du feu ont été analysés. Les analyses ont été réalisées à l'aide des systèmes d'information géographique. Malgré l'ampleur et la forte intensité du feu, l'étude révèle que la sévérité du feu sur les peuplements forestiers semble définitivement reliée à certaines composantes locales du territoire. En effet, des modèles log-linéaires indiquent que certaines modalités du couvert forestier, du temps depuis le dernier feu et du type de dépôts de surface ont eu un effet déterminant sur la sévérité du feu. Contrairement à ce que l'on pourrait penser, la densité, l'âge et la hauteur du peuplement n'ont pas influencé la sévérité du feu du lac Crochet. Par ailleurs, les îlots de végétation résiduelle ont totalisé une superficie de 1292 hectares à l'intérieur du périmètre du feu, ce qui équivaut à 2.6% de l'étendue totale du feu. On retrouve seulement 0.06 îlot pour chaque 100 hectares que le feu a couvert mais l'étendue moyenne et médiane de ces îlots est grande, révélant des superficies de 43.1 et 19.8 hectares, respectivement. L'indice de configuration de l'incendie indique une valeur de 4.21 alors que l'indice de lisière s'établit à 5.33. On note que 84% de la région perturbée est à moins de 500 mètres d'une végétation résiduelle. Considérant uniquement les îlots de végétation résiduelle qui ont survécu au feu, plus que 30.8% de la zone brûlée se retrouve à moins de 500 mètres. Ce rapport met en évidence l'importance de ces découvertes pour l'étude de la mosaïque forestière boréale. © 1997 UQAT tous droits réservés.