DOI: 10.6073/pasta/4ae5b2c9147cf1b580e28c5bc75a3c8d
title: Ecosystem Response to Hemlock Woolly Adelgid in Southern New England since 1998

description: In 1998 we began examining the response of ecosystem processes to the stress and mortality caused by the introduced hemlock woolly adelgid (HWA) in southern New England. Healthy hemlock forests typically have slow decomposition and N cycling rates due to their low foliar N content and cool microclimate. However, thinning canopies associated with HWA infestations are starting to reverse this trend, due to dramatic increases in light levels and soil temperature. Within 8 study sites varying in HWA infestion level, we continue to investigate the magnitude and duration of N dynamics associated with HWA infestations by measuring nitrogen (N) mineralization rates using close-topped soil cores during the last five years. In addition, ion-exchange resin bags are used to estimate the spatial availability of N within sites and the extent to which NO3 is being lost. Measurements of gravimetric moisture content and soil temperature were used with hemispherical photographs to assess microenvironmental changes. During the first five years of this study, thinning canopies from heavy HWA damage resulted in increased light, soil temperature, and mineral soil moisture, and decreased forest floor moisture content. Heavily infested sites continue to have larger extractable NH4 and NO3 – N pools, and significantly higher net nitrification rates than healthy hemlock forests. In addition, resin bags captured more ammonium and nitrate in infested versus uninfested stands. Results indicate that introduced pests and selective tree decline can rapidly and dramatically alter ecosystem processes, even prior to the onset of extensive tree mortality. In 2001, we began examining 2 additional stands that contain high overstory hemlock mortality and a dense black birch understory. We will continue to sample these stands as they deteriorate to determine the extent to which changes in overstory composition, microenvironment, and soil conditions produce fundamental changes in the cycling of nitrogen. Several decomposition studies have also been undertaken to examine how decomposition may be driving N-cycling changes in infested stands. Our study has focused on three key drivers of decomposition: (i) changes in foliar quality due to HWA herbivory, (ii) changes in forest floor microclimate that occur as the canopy thins, and (iii) the effect of species composition change. All three have had an impact on decomposition. Furthermore, our data suggest that these changes are coupled with N-cycling dynamics and may helped to elucidate the mechanisms driving increased N availability in these forests. We sampled hemlock foliar N, C, and lignin along an extensive gradient of hemlock forests. These stands range from uninfested to forests with complete hemlock mortality and currently dominated by black birch (Betula lenta). Foliar % C and lignin were not affected by HWA infestation however, foliar N was higher in infested stands. Higher initial foliar N was found to increase the rate of N immobilization in decomposing foliage. Additionally, our data suggest that in many forests infested foliage may switch from a net sink to a net source of N more rapidly compared to uninfested foliage. This may be an important contribution to increased N availability in infested forests. Altered microclimate has had an important effect on foliar decomposition. Surface litter decomposition was slowed in many infested forests due to poor conditions for microbial establishment on litter. However, our work suggests this effect may be limited only to litter at the forest floor surface. Cellulose paper buried at the forest floor mineral soil interface had significantly greater mass loss in infested stands. We also observed that as surface litter became buried, its rate of mass loss increased in infested stands. In 2001 we began a comparative study of hemlock, black birch, and mixed litter decomposition. During 2002 we collected litter bags after six and twelve months of decomposition. Results after six months show that black birch litter decomposed more rapidly than hemlock and mixed litter. The higher rates of litter decomposition that occur after the switch from hemlock to black birch litter-fall are likely a major mechanism of forest floor mass loss after hemlock mortality and logging. Taken as a whole, our studies illustrate the dynamic effects of hemlock woolly adelgid infestation on decomposition in eastern hemlock forests. All stages of infestation studied had significant effects on decomposition by altering the chemical changes in litter, altering litter inputs (hemlock to black birch), and soil microclimate conditions.