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Effects of the Hemlock Woolly Adelgid

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Effects of the Hemlock Woolly Adelgid on Nitrogen Losses from Urban and Rural Northern Forest EcosystemsAbstractIntroductionSite DescriptionsMethodsStatistical AnalysesResultsNitrogen Inputs and LossesNatural Abundance Isotope ValuesDiscussionNitrogen InputsNitrogen LossesNitrate SourcesConclusionsAcknowledgmentsReferencesEffects of the Hemlock WoollyAdelgid on Nitrogen Losses fromUrban and Rural Northern ForestEcosystemsPamela H. Templer* and Tiffany M. McCannDepartment of Biology, Boston University, Boston, Massachusetts 02215, USAABSTRACTThe objectives of this study were to quantify ratesof nitrogen inputs to the forest floor, determinerates of nitrogen losses via leaching and to partitionthe sources of NO3-from healthy, declining, andsalvage or preemptively cut eastern hemlock (Tsugacanadensis) stands in both an urban forest at theArnold Arboretum in Boston, MA and a rural forestat Harvard Forest in Petersham, MA. Rates ofnitrogen inputs (NH4+and NO3-) to the forest floorwere 4–5 times greater, and rates of nitrogen lossesvia leachate were more than ten times greater, atthe Arnold Arboretum compared to Harvard Forest.Nitrate that was lost via leachate at Harvard Forestcame predominantly from atmospheric depositioninputs, whereas NO3-losses at the Arnold Arbo-retum came predominantly from nitrification. Al-though our study was limited to one urban and onerural site, our results suggest that current man-agement regimes used to control the hemlockwoolly adelgid (Adelges tsugae), such as salvagecutting, may be reducing nitrogen losses in urbanareas due to rapid regrowth of vegetation and up-take of nitrogen by those plants. In contrast, pre-emptive cutting of trees in rural areas may beleading to proportionately greater losses of nitrogenin those sites, though the total magnitude ofnitrogen lost is still smaller than in urban sites.Results of our study suggest that the combinationof the hemlock woolly adelgid, nitrogen inputs, andmanagement practices lead to changes in themovement and source of NO3-losses from easternhemlock forest ecosystems.Key words: forest insect outbreak; invasive spe-cies; natural abundance stable isotopes; nitratesources; nitrogen saturation; pest management;Tsuga canadensis.INTRODUCTIONHuman-induced environmental changes, such asthe introduction of non-native species, have led todramatic shifts in forest composition in the easternUnited States over the past 100 years. Introducedpest species, such as the Asian longhorned beetle(Anoplophora glabripennis), chestnut blight (Crypho-nectria parasitica), Dutch elm disease (Ophiostomaspp.), the emerald ash borer (Agrilus spp.), thegypsy moth (Lymantria dispar L), and the hemlockwoolly adelgid (Adelges tsugae Annand), have led tothe decline of native tree species throughout thisregion. The hemlock woolly adelgid, an introducedReceived 26 May 2010; accepted 2 September 2010;published online 30 September 2010Author Contributions: PHT took an active role in all parts of the study,including experimental design, sampling, data analysis, and writing of themanuscript. TMM was involved with sampling, data analysis, and editingof the text.*Corresponding author; e-mail: [email protected] (2010) 13: 1215–1226DOI: 10.1007/s10021-010-9382-x 2010 Springer Science+Business Media, LLC1215aphid-like insect from Japan, threatens hemlockstands throughout the eastern United States. Thehemlock woolly adelgid was first reported in forestsof the eastern United States in the early 1950s andis currently leading to mortality of eastern hemlocktrees from Georgia to Massachusetts (McClure andCheah 1999; Orwig and others 2002).The hemlock woolly adelgid suffers significantmortality at winter temperatures below -25 C(Skinner and others 2003). With the combinationof increasing temperatures and potentially greatercold tolerance of the hemlock woolly adelgid,stands of eastern hemlock trees could be affected bythis pest throughout the entire range of this treespecies within the next 30 years (Albani and others2010). Feeding, and possible injection of toxic sal-iva, by the hemlock woolly adelgid on youngbranches and twigs reduces the viability of newgrowth and typically leads to tree mortality within4–10 years (McClure 1987; Young and others1995). Eastern hemlock trees are recognized as afoundation species because of the large effect theyhave on forest ecosystems that they inhabit (Ellisonand others 2005). In healthy eastern hemlockstands, they produce a shady understory with athick organic soil layer due to the poor quality (forexample, low %N; Lovett and others 2004), acidiccomposition of their litter and because of slow ratesof decomposition (McClaugherty and others 1985).As a result, they tend to be associated with rela-tively slow rates of nitrogen cycling (Finzi andothers 1998; Jenkins and others 1999; Templer andothers 2003; Lovett and others 2004) and relativelyhigh amounts of ecosystem nitrogen retention(Templer and others 2005).Mortality of eastern hemlock trees impactsnutrient cycling and nitrogen retention of forestecosystems significantly. For example, canopythroughfall beneath infested hemlock trees hasgreater concentrations of NO3-(Stadler and others2005) and soil solution within stands of girdledhemlock trees has elevated concentrations of NH4+and NO3-(Yorks and others 2003). Girdling is atreatment used to mimic the decline of trees causedby the hemlock woolly adelgid. Trees typically re-main standing following both girdling and infesta-tion by the hemlock woolly adelgid, but trees canremaining standing longer when infested with thehemlock woolly adelgid. Changes in the microcli-mate caused by openings in the canopy fromdeclining hemlock stands lead to increased rates ofnet mineralization, net nitrification, and nitrogenturnover in the forest floor (Jenkins and others1999; Orwig and others 2008). Mixed hardwoodstands dominated by black birch (Betula lenta)typically replace hemlock stands following the de-cline of hemlock trees (Orwig and Foster 1998;Albani and others 2010). This shift in tree speciescomposition leads to increased rates of litterdecomposition, which will likely influence long-term changes in nitrogen cycling (Cobb 2010).In response to hemlock woolly adelgid out-breaks, many hemlock stands have been cut tosalvage usable wood before tree mortality (Orwigand others 2002). However, silvicultural manage-ment of infested hemlock stands may also play asignificant role in altering nitrogen cycling at


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