Ecosystem and Vegetation Response to Hemlock Logging in Southern New England 1999-2005

This study compares the magnitude and trajectory of vegetation and ecosystem function dynamics associated with the direct impact of hemlock woolly adelgid (Adelges tsugae; HWA) infestation versus the indirect consequences of HWA-induced damage in the form of salvage and pre-emptive logging of hemlock (Tsuga canadensis (L.) Carriere) forests. The study was conducted within an area extending from southern Connecticut up to and including the Connecticut River lowlands west to the Berkshire Plateauin central Massachusetts, USA. Overstory and understory vegetation and ecosystem function parameters such as decomposition and nitrogen cycling were examined in logged and unlogged portions of 10 hemlock stands varying in HWA damage intensity. Intensive hemlock logging generated more rapid and pronounced microenvironment and vegetation changes than chronic HWA damage. Black birch (Betula lenta L.) seedling densities and percent cover of brambles (Rubus L. spp.), sedges (Carex L. spp.) and hay-scented fern (Dennstaedtia punctilobula Michx.) were significantly higher in recent harvests versus HWA-damaged and undamaged sites. High black birch sapling densities (greater than 7000 ha –1) were common in the older harvests but not in adjacent, HWA-damaged portions of these sites. Undamaged sites had 20% more forest floor mass than HWA-damaged sites and double the mass of older cuts. Mass loss rates of cellulose paper suggest that conditions were more favorable for decomposition in the damaged and older logged sites. Recently cut sites had significantly larger inorganic N pools than undamaged forests, although total net nitrogen (N) mineralization rates were not significantly different among treatments. Nitrification rates of 0.2 kg ha-1 day-1 measured in the oldest cuts were three times greater than in HWA-damaged sites and over 200 times greater than in undamaged hemlock sites. However, resin bag capture in the older cuts was similar to amounts captured in undamaged and damaged forests, suggesting that excess nitrogen was being utilized in vegetative uptake. In contrast, relatively large amounts of ammonium and nitrate captured in recent harvests indicate higher N availability, less vegetative uptake, and a greater potential for N leaching. Results suggest that both the decline associated with HWA infestation and the indirect effects of HWA in the form of logging are generating profound changes in structure, composition, and ecosystem function in these forests, although at different spatial and temporal scales. Hemlock harvesting imposed more abrupt microenvironmental changes, and rapidly reduced vegetative cover while chronic HWA infestation led to gradually thinning canopies. Both disturbances led to black birch dominated forests, although logging resulted in greater amounts of shade-intolerant regeneration, higher soil pH and nitrification rates, and reduced forest floor mass. Preemptive cutting of undamaged forests may lead to greater N losses than those associated with HWA infestation or logging of deteriorated hemlock forests, due to reduced vegetative uptake. Silvicultural methods that allow for vegetation establishment prior to harvesting will likely lessen the ecological impacts of hemlock removal.