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Keywords: beta diversity; biotic homogenization; pre-colonial forests; pre-industrial forests; pre-settlement forests Abstract 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 km.sup.2) 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 km.sup.2; 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. CAPTION(S): Supplementary Material Byline: Victor Danneyrolles, Mark Vellend, Sebastien Dupuis, Yan Boucher, Jason Laflamme, Yves Bergeron, Gabriel Fortin, Marie Leroyer, Andre Romer, Raphaele Terrail, Dominique Arseneault, Glenn Matlack

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]