Direct and indirect ecosystem consequences of an invasive pest on forests dominated by eastern hemlock

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Abstract
Aim  This study compares the magnitude and trajectory of vegetation and ecosystem function dynamics associated with the direct impact of hemlock woolly adelgid (Adelges tsugae Annand; HWA) infestation vs. the indirect consequences of HWA‐induced damage in the form of salvage and pre‐emptive logging of hemlock [Tsuga canadensis (L.) Carriere] forests.Location  The study was conducted within an area extending from southern Connecticut up to and including the Connecticut River lowlands west to the Berkshire Plateau in central Massaschusetts, USA.Methods  Overstorey and understorey vegetation and ecosystem function parameters such as decomposition and nitrogen cycling were examined in logged and unlogged portions of ten hemlock stands varying in HWA damage intensity.Results  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 vs. HWA‐damaged and undamaged sites. High black birch sapling densities (>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 favourable 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.Main conclusions  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. Pre‐emptive cutting of undamaged forests may lead to greater N losses than those associated with HWA infestation or logging of deteriorated hemlock forests, because of reduced vegetative uptake. Silvicultural methods that allow for vegetation establishment prior to harvesting will probably lessen the ecological impacts of hemlock removal.