Canopy mortality and stand-scale change in a northern hemlock-hardwood forest

Ring-width patterns from 348 trees on a 5.25-ha mapped plot in the Sylvania Wilderness, Upper Michigan, were used to infer the pattern of canopy mortality for the past 140 years. Estimates of average canopy turnover rate (0.54%/year) and average canopy residence time (186 years) are similar to other estimates for the region. The temporal pattern of release events detected in the cores is closely related to the occurrence of extreme drought with sugar maple (Acer saccharum Marsh.) trees showing increased growth in the years directly following drought and eastern hemlock (Tsuga canadensis (L.) Carr.) trees showing increased growth 3-4 years following drought. Drought seems to be synchronizing canopy recruitment by influencing the temporal pattern of canopy mortality. In particular, 22% of all sampled stems show a response to canopy turnover during the 1930s and a large portion of the recruitment consisted of sugar maple stems originating in gaps. Individuals of this sugar maple cohort are clustered up to 30-40 m. Along the border between the hemlock and hardwood stands, remnant hemlock logs are present beneath these sugar maple clusters suggesting that canopy replacement of hemlock by sugar maple has occurred. Diameter distributions of stems indicate that sugar maple is regenerating in hardwood stands and in areas of mixed species composition. In contrast, smaller size classes of hemlock are conspicuously absent from areas of mixed species composition and are restricted to the hemlock stand. In this hemlock-hardwood forest mosaic, sugar maple has been slowly increasing in abundance at the expense of hemlock, gradually increasing the extent of the hardwood patch.