Arii, K., B.R. Hamel, and M.J. Lechowicz (Department of Biology, McGill University, 1205 Av. Docteur Penfield, Montréal, QC H3A 1B1). Environmental correlates of canopy composition at the Gault Nature Reserve in southwestern Quebec, Canada. J. Torrey Bot. Soc. 132: 90–102. 2005.—The environmental basis for variation in canopy composition was investigated in an extensive old-growth forest at Mont St. Hilaire, Quebec, Canada. Based on 144 permanent plots, spatial variation in canopy tree species and the effect of environmental variables on canopy composition were examined using canonical correspondence analysis (CCA). Slope and the amount of direct solar radiation received during the growing season, both of which are good indicators of soil moisture, were the main factors explaining plot-to-plot variation in canopy composition. Quercus rubra, Betula papyrifera, Ostrya virginiana and Pinus strobus predominated on plots with high insolation and steeper slope, while species such as Acer saccharum, Fagus grandifolia, Betula alleghaniensis, and Tilia americana occurred on sites with gentler slope and lower insolation during the growing season. Additionally, plots with greater dominance of Acer saccharum in the canopy had higher soil nitrogen availability, and plots with greater dominance of Fagus grandifolia had lower Ca availability.