Effects of Tsuga canadensis mortality on soil water chemistry and understory vegetation: possible consequences of an invasive insect herbivore

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Abstract
Soil water chemistry and vegetation were monitored for almost 1 year before and 4 years after treatment in two healthy Tsuga canadensis (L.) Carrière (eastern hemlock) stands and two T. canadensis stands subjected to a girdling treatment. The girdling treatment was applied to simulate infestation and mortality by the introduced pest Adelges tsugae Annand. The girdling of T. canadensis trees resulted in elevated concentrations of NO3– and most cations in soil water within 2–3 months, and concentrations of several ions remained high relative to control stands for the duration of the study (e.g., NO3–, Ca2+, K+, Mg2+). Maximum mean monthly concentrations were generally observed 12–18 months after girdling (e.g., 473–2272 µequiv. NO3–/L, 22–126 µequiv. NH4+/L). Maximum mean annual weighted ion concentrations and fluxes (e.g., 236 µequiv. NO3–/L and 18 kg NO3–-N/ha, respectively) were observed 2–3 years after girdling and were comparable to values reported in the literature regarding clear-cutting effects on stream water ion concentrations and losses. Betula alleghaniensis Britt. seedling densities and percent cover of several herbaceous species (Dennstaedtia punctilobula (Michx.) Moore, Dryopteris intermedia (Muhl.) A. Gray, Oxalis acetosella L., Thelypteris noveboracensis (L.) Nieuwl.) increased in response to T. canadensis mortality. Total percent cover of understory vegetation more than doubled in the first 3 years after girdling.
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