The distribution of hemlock woolly adelgid in trees, forest stands, and regions

Eastern hemlock trees, Tsuga canadensis, provide wildlife habitat, supply timber, moderate climate, and form part of many old growth forests in eastern North America. The hemlock woolly adelgid (HWA), Adelges tsugae, is an invasive exotic insect that threatens to kill all eastern hemlock trees. As HWA and hemlock mortality have spread through eastern North America, it has generally been assumed that HWA population densities and rates of range expansion have been uniform. Those assumptions have led to potentially inaccurate crown sampling methods, obscured possibilities for mitigation, and prompted unnecessary pre-emptive salvage logging.; This dissertation examines variation in HWA population density and range expansion at multiple scales. It does so by establishing and applying appropriate sampling methods, measuring HWA populations in natural stands over multiple years, studying seedling infestation, and modeling the geographic variability of spread. The results of this research show that assumptions of uniformity are unfounded for both HWA density at the stand scale and for rates of range expansion at the regional scale. Some forest stands maintain significantly lower HWA populations than others and these differences remain consistent over multiple years. The probability of seedling infestation by MXA follows the same pattern across stands as overstory infestation. At the regional scale, HWA is spreading at a median rate of 12.5 km per year, but range expansion is significantly slower in areas with an average annual minimum temperature less than -26°C. Other significant results from this dissertation show that Chinese hemlock, T. chinensis, is a better candidate for reforesting hemlock stands than western hemlock, T. heterophylla . HWA densities are greater in the lower crowns of hemlock trees. However in trees with low HWA populations, the density trend appears to be reversed and lower crown populations are sparse, which suggests the need to adjust HWA sampling methods.; The multiple scales of analysis in this dissertation reveal ecologically significant patterns and highlight the spatial variability in an exotic invasion. The combination of consistent, low HWA densities in some forest stands and very slow spread in colder regions presents the opportunity for silviculture to encourage hemlock.