Fire and canopy species composition in the Great Lakes-St. Lawrence forest of Témiscamingue, Québec

Large severe fires are typically rare in the northern hardwood forests of eastern North America, with estimated return intervals as high as 1400–4500 years. We investigated the history of large severe fires in western Québec, Canada, where sugar maple (Acer saccharum Marsh.), yellow birch (Betula alleghaniensis Britt.), and eastern hemlock ((Tsuga canadensis (L.) Carr.) dominate a landscape at northern limit of the Great Lakes-St. Lawrence forest region. Using a combination of provincial archives of fire data, interpretation of historic air photos and dendrochronological sampling, we estimated fire frequency in a 179,300 ha landscape and tested the hypothesis that time-since-fire is a principal determinant of canopy species composition. Our results indicated that despite its proximity to the boreal mixedwood forest, fires have been relatively infrequent in this landscape, with over 60% of the landscape remaining unburned during the 413 year period of study. The overall fire cycle, an estimate of the time required to burn an area of equivalent size to the study area, was 494 year (95% CI: 373–694). This estimate showed strong temporal partitioning, with an elevated frequency that coincided with the period of pioneering settlement beginning in 1880, and spatial partitioning according to surficial deposits, pine dominance and ecological site type. Multivariate analyses elucidated both (i) distinct assemblages of canopy species that differed in their association with time-since-fire and (ii) the relative importance of environmental variables affecting canopy species composition, where time-since-fire explained the largest amount of variation. This time-since-fire effect suggested an important role for secondary disturbances in influencing composition of the canopy. Given the current long fire cycles, it is likely this landscape will experience an increasing dominance of fire-avoiding species such as sugar maple and eastern hemlock, with consequences for fire-adapted tree species and fire-dependent communities.