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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 long-standing hypothesis that sugar maple (Acer saccharum Marshall) communities are maintained at equilibrium by present climate and small-scale disturbances is questioned because empirical evidence is accumulating about the ability of the species to withstand several stand-scale disturbances. The fire history of a sugar maple site at the northeastern range limit of the species (Gaspé Peninsula, eastern Canada) was documented to test the hypothesis that this forest type is resilient to fire disturbance. The fire history was reconstructed using radiocarbon-dated soil macrocharcoals. Two main fire periods were recorded during the Holocene. The oldest period occurred between 9055 and 8265 cal. years BP, and was characterized by the presence of conifers, including spruce. After 6900 years of fire-free activities, the second period covered the last 1335 years, and was characterized by the presence of sugar maple in the charcoal assemblage. The dominance of sugar maple after more than 1000 years of recurrent fires underlines the species resilience to frequent site disturbances. The soil of the forest stand was heavily disturbed by earthworms. However, the dense seedling and sapling bank of sugar maple suggests that earthworms do not affect negatively the regeneration and survival of the species. Key words: Acer saccharum, earthworm, forest fire, Holocene, resilience, advance regeneration. Résumé : L'hypothèse voulant que les érablières (Acer saccharum Marshall) soient à l'équilibre avec le climat actuel et les perturbations à fine échelle est de plus en plus remise en question par les données empiriques montrant la capacité de l'espèce de se régénérer après des perturbations à l'échelle des peuplements. L'histoire des feux d'une station dominée par l'érable à sucre à la limite nord-est de son aire naturelle (Péninsule gaspésienne, Est canadien) a été reconstituée afin de tester l'hypothèse de sa résilience face aux feux. La radiodatation de macrocharbons (identifiés botaniquement) du sol de la forêt a servi à la reconstitution. Les feux sont survenus à l'Holocène inférieur, entre 9055 et 8265 ans avant aujourd'hui, et ont brûlé des conifères. Après une longue période sans feu, ces derniers ont repris à l'Holocène supérieur. Cette deuxième période, de 1335 ans à aujourd'hui, est caractérisée par la forte présence de l'érable à sucre dans l'assemblage macrofossile. La dominance de l'espèce après plusieurs feux récurrents souligne sa résilience face à ce type de perturbation. Actuellement, le sol de la forêt est fortement perturbé par les vers de terre. Cependant, l'importante régénération préétablie suggère que l'activité des vers de terre n'affecte pas la régénération et la survie de l'érable à sucre. Mots-clés : Acer saccharum, feux de forêt, Holocène, régénération préétablie, résilience, ver de terre., Introduction The main distributional range of sugar maple (Acer saccharum Marshall) covers the temperate areas of eastern North America, from the Great Lake states to Alabama and northern Georgia and [...]

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]

A better understanding of how disturbance impacts tree diversity at different scales is essential for our ability to conserve and manage forest ecosystems in the context of global changes. Here we test the impacts of land use‐related disturbances on tree diversity since the 19th century across a broad region (>150,000 km2) of northern temperate forests in eastern Canada.We used a large and unique dataset of early land surveys conducted during the 19th century (>130,000 species lists), along with modern forest inventories (>80,000 plots), to analyse long‐term changes in taxonomic and functional tree diversity at several scales (grid cell resolutions ranging from 12.5 to 1,600 km2; we refer to one grid cell as a 'landscape').Our results show that land use‐related disturbances have led simultaneously to (a) increased diversity within landscapes and a (b) homogenization at the regional scale (i.e. decreased composition dissimilarity among landscapes). These trends were found for both taxonomic diversity and functional diversity, with temporal changes more pronounced for taxonomic than functional diversity. We also found an increase over time in the strength of correlations between environmental variables and diversity both within and among landscapes.Synthesis. Our results support the idea that human‐induced impacts on biodiversity are strongly scale‐dependent and not necessarily associated with biodiversity loss. This highlights possible ways that human‐driven changes in tree diversity might impact forest resistance and resilience to future global changes. [ABSTRACT FROM AUTHOR]