Hemlock Infestation and Mortality: Impacts on Nutrient Pools and Cycling in Appalachian Forests

Abstract

E astern hemlock is a major component of riparian forests in the southern Appalachian Mountains (Narayanaraj et al., 2010). As the dominant conifer in riparian areas, this species plays an important role in regulating nutrient cycling processes and climatic conditions in both terrestrial and aquatic environments (Ellison et al., 2005). Th e hemlock woolly adelgid (Homoptera: Adelgidae), an exotic pest, was introduced to the mid-Atlantic region of North America from Asia in the 1950s and has spread throughout most of the range of eastern hemlock, moving north toward Canada and to the southern Appalachians. Pontius et al. (2006) examined the mortality of eastern hemlock following infestation with HWA and found that some trees can live for >10 yr following infestation. From county-level data on the spread of HWA, Evans and Gregoire (2007) found the rate of spread in Pennsylvania and north was 8.13 km yr −1 , while the rate of spread in the south was greater: 15.6 km yr −1 . Th e diff erence was attributed to colder temperatures in the north. Morin et al. (2009) modeled the spread of HWA north and south. While they concurred that spread to the north was limited by temperature, they concluded that the rate of spread to the south and west was related to host density. Indeed, in the southern part of hemlock's natural range, infestations were fi rst observed in the early 2000s and the rate of spread and mortality have been much faster than observed in northern areas Nuckolls et al., 2009 Eastern hemlock [Tsuga canadensis (L.) Carrière] trees serve an important ecological role in riparian ecosystems in the southern Appalachians. Signifi cant hemlock mortality is occurring due to infestation by the hemlock woolly adelgid (HWA) (Adelges tsugae Annand), a non-native invasive pest. Our objective was to quantify the impacts of HWA and hemlock mortality on nutrient cycling pools and processes. In 2004, we established eight research plots in riparian areas with >50% basal area in hemlock and four reference plots in riparian areas without hemlock (hardwood). All hemlock plots were infested with HWA. In four of the hemlock plots, all hemlock trees were girdled to induce defoliation and rapid mortality. By fall 2006, there was 90 and 10% mortality in the girdled and nongirdled hemlock plots, respectively. Measurements included soil temperature and moisture, nutrient pools, N transformations, litterfall and forest fl oor amount and chemistry, and throughfall and soil solution chemistry. From 2004 to 2008, litterfall composition changed, with an initial increase in the hemlock needle percentage followed by a decline. Hemlock plots had cooler spring soil temperatures than hardwood plots. Hemlock plots had greater surface soil and forest fl oor total C than hardwood plots; soil C content did not change during the 4 yr of measurement. Th ere were no diff erences in N mineralization rates or soil solution N concentrations among treatments. Diff erences between litterfall and forest fl oor nutrient contents in hemlock and hardwood plots ...

Experiments Are Revealing a Foundation Species: A Case Study of Eastern Hemlock (Tsuga canadensis)

“…This prediction was supported most clearly for changes in litterfall, which is one index of aboveground primary productivity [98]. In the girdled treatment plots at HF-HeRE, there was a sharp pulse in litterfall followed by a gradual decline [19]; similar patterns have been observed in other hemlock girdling experiments [22,39,99]. This shift reflects only the transient loss of standing biomass as the canopy died; litterfall recovered within four years to premanipulation (i.e., control-plot) levels as understory shrubs (primarily Rubus spp.)…”

“…Tsuga canadensis has little, if any, resistance to the hemlock woolly adelgid [34] and rarely recovers from chronic infestations [35,36]. In many infested stands, >90% of T. canadensis dies within 10 years of the arrival of the adelgid [37][38][39].…”

Hemlock loss due to the hemlock woolly adelgid does not affect ecosystem C storage but alters its distribution

“…This result is consistent with that of Jenkins et al (1999), who found no significant changes in C pools ;3 years following hemlock infestation at sites throughout southern New England. Knoepp et al (2011) also found no significant loss of soil C content 4-years following hemlock loss at the Coweeta Long Term Ecological Research. Hence, the initial impacts on the C cycle appear to be limited to a transfer of C into woody debris.…”

Foundation species loss affects vegetation structure more than ecosystem function in a northeastern USA forest

“…The hemlock woolly adelgid ( Adelges tsugae Annand), an invasive insect from Japan that in North America feeds exclusively on eastern hemlock and its southeastern (USA) endemic congener, Carolina hemlock ( T. caroliniana Engelmann), is moving rapidly both southward and northward (Fitzpatrick et al, 2012), killing >90% of hemlocks it encounters (Orwig, Foster & Mausel, 2002; Eschtruth et al, 2006; Knoepp et al, 2011). Hemlock has little resistance to the adelgid (Ingwell & Preisser, 2011) and as yet has shown no recovery from chronic infestations (McClure, 1995; Orwig et al, 2012).…”