Gap succession and dynamics in eastern hemlock (Tsuga canadensis) dominated old-growth forest communities at Wachusett Mountain, Massachusetts

Torres, Diane E.
Peter Palmiotto, PhD
Department of Environmental Studies
Old growth forests in the eastern United States are rare and are therefore highly valued as legacies of the past. They are typically fragmented and surrounded by early to mid-successional forests. Historically these forests have been priorities for conservation and are valued for scientific study. More recently old-growth forests have become key components in broad-scale conservations efforts and there currently is a push to restore old-growth characteristics to existing forests as a means to protect species diversity. One of the more recently discovered old-growth forests in on Wachusett Mountain, Massachusetts. Several communities there are dominated by eastern hemlock, making it especially vulberable to an infestation of the hemlock wooly adelgid (HWA: Adelges tsugae Annand), an aphid-like insect that dwells exclusively on eastern hemlock in the Northeast and causes eventual mortality. Thus, this study of old-growth forest dynamics by characterizing primary gapmakers and determining potential post-gap vegetative succession. Thirty-six gaps were randomly selected from 166 existing gaps within four eastern hemlock dominated old-growth forest communities. Gap sizes ranged from 75 to 150 m² with a mean of 97.8 m². Single-tree gaps were the most common type of gap and they consisted of 70% softwood and 30% hardwood species. Basal area of tree species surrounding these gaps indicated that the dominant canopy species was eastern hemlock with a basal area of 26.4 m²ha-1. Eastern hemlock was likely to remain dominant unless a major disturbance occurred. If there were to be a major disturbance, black birch (Betula lenta L.), red maple (Acer rubrum L) or northern red oak (Quercus rubra L) would be the most likely species to replace the hemlock. Of these species, black birch has been known to profit the most from the demise and decline of eastern hemlock partly because it thrives in increased light created by thinning hemlock crowns. Striped maple (Acer pensylvanicum L.) dominated sapling densities and red maple (Acer rubrum L.) dominated seedling densities in gaps. Neither species would likely become dominant in the canopy due to growth habit and poor ability to compete for light. This information on old-growth forest gap dynamics provides an understanding of gapmaker characteristics was well as potential vegetative succession following a major disturbance event such as HWA infestation. This knowledge will support the conservation of old-growth forests by providing baseline information on gap characteristics that can be used in efforts to restore old-growth characteristics to our current forests for the protection of native species and species diversity.