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Extensive [.sup.14]C dating and botanical identification of charcoal fragments located in the organic surface soil layer and buried in the mineral podzolic solum were used to reconstruct the successional pathways of a balsam fir forest site. The studied forest site developed in a context of continuous fire disturbance over the last 9000 years with at least 26 fires occurring at a mean interval of 330 years. Tree vegetation of the site followed a four-step trajectory consisting of an early-Holocene spruce forest and a late-Holocene mixedwood balsam fir forest. Boreal-like spruce-birch and temperate-like white pine-birch forests dominated the site between 7900 and 5900 cal. B.P. and 5600 and 1275 cal. B.P., respectively. Because all forest types developed repeatedly after fire since early deglaciation, changes in forest composition, in particular the shift of white pine forest to balsam fir forest, and concurrent decline of birch (yellow birch and/or paper birch) and pine populations were most likely related to progressive cooler and wetter conditions from mid- to late Holocene. Fire disturbance on this part of the southern boreal biome has been a continuous, positive regenerative process over the Holocene, allowing the successional turnover of boreal and temperate trees under the influence of climatic change. Key words: Abies balsamea (L.) Mill., Betula, boreal forest, climate change, fire disturbance, Holocene, Finns strobus La datation au [.sup.14]C et l'identification botanique de fragments de charbon de bois situés dans l'horizon organique de surface et enfouis dans le solum minéral podzolique ont servi à la reconstitution des trajectoires de succession d'un milieu occupé actuellement par une sapinière boréale. La station forestière étudiée a été marquée par au moins 26 feux différents survenus au cours des derniers 9000 ans, soit à un intervalle moyen de 330 ans. La végétation de ce milieu a été marquée par la succession de quatre types forestiers représentés par une forêt d'épinette au début de l'Holocène et une sapinière mixte à l'Holocène supérieur. Une forêt boréale dominée par l'épinette et le bouleau et une forêt tempérée de pin blanc et de bouleau ont occupé respectivement la station entre 7900-5900 cal. B.P. and 5600-1275 cal. B.P. Les changements de composition floristique observés au cours de la succession, plus particulièrement le passage d'une forêt de pin blanc à une forêt de sapin baumier, ainsi que le déclin concomitant des populations de bouleau (bouleau jaune et/ou bouleau à papier) et de pin blanc, semblent tous reliés à des conditions climatiques plus froides et plus humides depuis le début de l'Holocène supérieur. Les perturbations par le feu dans cette partie de la forêt boréale méridionale de l'Est nord-américain ont agi comme un processus positif de régénération forestière pendant tout l'Holocène, assurant ainsi le remplacement des arbres boréaux et tempérés sous l'influence directe du climat. Mots-clés : Abies balsamea (L.) Mill., Betula, forêt boréale, changement climatique, feu, Holocène, Pinus strobus, Introduction Despite the small number of species, conifers cover large areas of land in the northern hemisphere. Among the conifers, boreal and temperate species of the Pinaceae and Cupressaceae families [...]

The Earth's climate has been warming rapidly since the beginning of the industrial era, forcing terrestrial organisms to adapt. Migration constitutes one of the most effective processes for surviving and thriving, although the speed at which tree species migrate as a function of climate change is unknown. One way to predict latitudinal movement of trees under the climate of the twenty-first century is to examine past migration since the Last Glacial Maximum. In this study, radiocarbon-dated macrofossils were used to calculate the velocity of past migration of jack pine (Pinus banksiana) and black spruce (Picea mariana), two important fire-adapted conifers of the North American boreal forest. Jack pine migrated at a mean rate of 19 km per century (km-cent) from unglaciated sites in the central and southeastern United States to the northern limit of the species in subarctic Canada. However, the velocity increased between unglaciated and early deglaciated sites in southern Quebec and slowed from early to mid-Holocene in central and eastern Quebec. Migration was at its lowest speed in late-Holocene times, when it stopped about 3,000 y ago. Compared with jack pine, black spruce migrated at a faster mean rate of 25 km-cent from the ice border at the last interstadial (Bølling/Allerød) to the species tree limit. The modern range of both species was nearly occupied about 6,000 y ago. The factors modulating the changing velocity of jack pine migration were closely associated with the warm-dry climate of the late Pleistocene-Holocene transition and the more humid climate of the mid- and late-Holocene. [ABSTRACT FROM AUTHOR]