Since the 1980s, hemlock-dominated forests (Tsuga canadensis) of central New England have been increasingly infested by the invasive pest hemlock woolly adelgid (HWA, Adelges tsugae), predominantly resulting in its replacement by black birch-dominated forests (Betula lenta). To date there has been no long-term empirical analysis of HWA effects on forest carbon (C) cycling due to forest transition from hemlock to black birch. To address this question, I measured the C pools in five stand types at varying ages and stages of HWA infestation in Massachusetts and Connecticut. I also measured C fluxes in aboveground net primary production (ANPP) and soil respiration, and studied the drivers of these fluxes viz. litter production, rates of foliar decomposition, soil exoenzyme activity, temperature sensitivity of soil respiration and nitrogen (N) cycling. The mass of C stored in recovering forests was resilient to HWA infestation but the location of these stocks varied among stand types. There was a transition of C from live biomass in healthy, unaffected secondary hemlock forests to coarse woody debris (CWD) in recently girdled forests intended to simulate the effect of HWA on hemlock loss. Twenty years post-HWA infestation, however, ANPP was very high and there was a large increase in biomass-C pools in aggrading black stand types. C pools in mature, secondary black birch stand types ~135 years since pastureland abandonment were as large as those in primary hemlock stand types ~235 years of age, suggesting recovery of C storage within one century of HWA infestation. Soil respiration rates were positively correlated with inputs of hardwood leaf litter, fine root biomass and exoenzyme activity. Stand-type variations in ANPP were positively correlated with annual N requirements and N uptake from the soil. Nitrogen-use efficiency was highest in the girdled and post-HWA infestation stand types where ANPP was dominated by wood production which has a wide C:N ratio. Similar trends were found in soil respiration, but not to the same degree as that of ANPP. Collectively, my results indicate that southern New England forests C storage is highly resilient to the HWA-induced losses of hemlock, suggesting that these ecosystems will continue to be sinks for atmospheric carbon dioxide.

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Over the past several decades, naturally occurring populations of Fraser fir (Abies fraseri) in the Black Mountains of North Carolina have been heavily impacted by both direct and indirect anthropogenic disturbances, including logging and logging- associated fires, and high mortality rates due to the introduction of the exotic insect, balsam woolly adelgid (BWA) (Adelges piceae). The decline in Fraser fir is particularly concern because it serves as a foundation species within the spruce-fir forests of the Southern Appalachian Mountains. Our objectives for this research were to 1) use current stand structure to infer whether Fraser fir trees are experiencing a cycle of regeneration-mortality that will lead to eventual decline of the population, 2) determine what role, if any, the site-specific geographic variables of slope, elevation, aspect, and land use history have on stand structure, mortality, and BWA infestation level, and 3) analyze repeat aerial photography to examine broad trends of spruce-fir forest cover change caused by anthropogenic disturbance and the BWA. In order to understand stand structure, mortality, and infestation levels, we conducted detailed field surveys of Fraser fir trees throughout the Black Mountains using 44, fixed-radius circular sampling plots. These plots were placed throughout a series of aspects, elevations, and disturbance types in order to understand geographic variability among these variables. An analysis of 4 repeat aerial photographs and corroborating ground photographs revealed broad spatio-temporal trends of spruce-fir regeneration and mortality from 1954 to 2006. Our results indicate that Fraser fir stands at higher elevations are currently in a state of recovery; whereas stands at lower elevations appear to be more susceptible to BWA-induced mortality. Changes in forest cover area from 1954 to 2006 were influenced greatly by direct and indirect anthropogenic disturbance. Our results call attention to the significant impact that direct and indirect anthropogenic disturbance has had on Fraser fir stand structure, but also provide evidence for the ability of an imperiled ecosystem to recover from high rates of insect caused mortality. / Master of Science

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Thesis (M. En.)--Miami University, Institute of Environmental Sciences, 2004. / Title from first page of PDF document. Includes bibliographical references (p. 32-34).

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