Forest Response to the Introduced Hemlock Woolly Adelgid in Southern New England, USA

The hemlock woolly adelgid (Adelges tsugae Annand) is an invasive hemipteran that poses a major threat to eastern hemlock (Tsuga canadensis (L.) Carrière) forests in the United States. We conducted three surveys over a five-year period that assessed the density of hemlock woolly adelgid (HWA) and a second invasive pest, the elongate hemlock scale (EHS; Fiorinia externa Ferris), overstory hemlock mortality, and hemlock regeneration in ~140 hemlock stands (mean size, 44 ha; range, 7–305 ha) within a 7500 km2 north–south transect of southern New England (USA). In each stand, we rated HWA and EHS density on 50 hemlock trees using a 0–3 scale (0, none; 1, 1–10 organisms/m branch; 2, 11–100 organisms/m branch; 3, >100 organisms/m branch). Data on the presence or absence of regeneration were taken in 2005; in 2007 and 2009, we quantitatively assessed regeneration by counting the number of hemlock seedlings in three 16 m2 plots per stand. In 2005, 81% of sampled stands had HWA, 72% had EHS, and 66% had hemlock regeneration. In 2007, 86% of sampled stands had HWA, 79% had EHS, and 46% had hemlock regeneration. In 2009, 91% of stands had HWA, 87% had EHS, and 37% had hemlock regeneration. The proportion of stands with hemlock regeneration declined 46% between 2005 and 2009, and hemlock seedling density declined 71% between 2007 and 2009. A best-fit model selection algorithm found that this decrease was inversely correlated with stand-level adelgid density. There was no correlation between the change in seedling density and stand-level density of the elongate hemlock scale. The apparent decline in regeneration suggests that the ecosystem-level changes currently occurring in southern New England may be difficult to reverse.

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Decomposition of green foliage in eastern hemlock forests of southern New England impacted by hemlock woolly adelgid infestations

Canadian Journal of Forest Research ◽

10.1139/x06-012 ◽

2006 ◽

Vol 36 (5) ◽

pp. 1331-1341 ◽

Cited By ~ 35

Author(s):

Richard C Cobb ◽

David A Orwig ◽

Steve Currie

Keyword(s):

Soil Moisture ◽

New England ◽

Mass Loss ◽

Organic Soil ◽

Hemlock Woolly Adelgid ◽

Adelges Tsugae ◽

Eastern Hemlock ◽

Functional Changes ◽

Southern New England ◽

Crown Health

This study examined the impacts of hemlock woolly adelgid (Adelges tsugae Annand) (HWA), a small, invasive insect, on foliar chemistry, forest floor microclimate, and subsequent green foliage decomposition in eastern hemlock (Tsuga canadensis (L.) Carrière) forests. We investigated the direct effects of HWA feeding and indirect changes in microclimate on foliar decomposition by incubating HWA-infested and uninfested foliage across eight eastern hemlock dominated stands in southern New England that had different histories of HWA infestation. Infested stands had much poorer average crown health (3.4 versus 1.4 crown loss ratings), higher percent open sky (10.9 ± 2.4 vs. 5.3 ± 0.5 gap light index), and lower organic soil moisture (0.83 ± 0.02 g·g1 vs. 1.06 ± 0.05 g·g1) than uninfested stands. There were no significant differences in percent C, percent N, or percent lignin for the excised foliage at the start of the study. However, after 120 days, decomposing foliage from infested trees had significantly higher N concentrations (1.83% ± 0.05% vs. 1.69% ± 0.02 %) and lower C/N ratios (29.9 ± 0.8 vs. 31.6 ± 0.2) than uninfested foliage, suggesting that HWA herbivory resulted in alterations of litter chemistry as decomposition progressed. Mass loss of common uninfested foliage was lower in uninfested hemlock stands than in infested ones (30.9% ± 0.7% vs. 34.2% ± 0.1%). These rates of mass loss were significantly correlated with microclimate factors and indicate that organic soil moisture levels are controlling decomposition in HWA-infested forests. Infestation by HWA causes virtually no direct feeding damage to foliage, but it does lead to several indirect and significant ecological and functional changes over the 10- to 20-year course of infestation and stand decline.

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Multi-year ecosystem response to hemlock woolly adelgid infestation in southern New England forests

Canadian Journal of Forest Research ◽

10.1139/x07-196 ◽

2008 ◽

Vol 38 (4) ◽

pp. 834-843 ◽

Cited By ~ 100

Author(s):

David A. Orwig ◽

Richard C. Cobb ◽

Anthony W. D’Amato ◽

Matthew L. Kizlinski ◽

David R. Foster

Keyword(s):

New England ◽

Forest Floor ◽

Hemlock Woolly Adelgid ◽

Adelges Tsugae ◽

Net N Mineralization ◽

Eastern United States ◽

Ecosystem Response ◽

Total N ◽

Invasive Pests ◽

Southern New England

The introduced hemlock woolly adelgid (HWA) ( Adelges tsugae Annand) has generated widespread tree decline and substantial mortality of eastern hemlock ( Tsuga canadensis (L.) Carrière) throughout the eastern United States. To assess the magnitude of ecosystem response to this disturbance, we conducted a multi-year study of forests with and without damage from HWA. Infested forests had significantly higher HWA-induced foliar loss and significantly lower forest floor C:N ratios and soil organic matter than uninfested forests. There were no significant soil temperature differences among stand types, although infested stands did have lower forest floor soil moisture than uninfested stands. Net nitrification and net N mineralization rates were significantly higher in infested versus uninfested forests by the second and third year of this study, respectively. In addition, total N pools and resin bag capture of NH4 and NO3 were significantly higher in infested versus uninfested forests throughout this study. Increases in N were likely due to a combination of factors including enhanced decomposition, reduced uptake of water and N by declining trees, sparse understory vegetation, and N-enriched throughfall from infested canopies. These results confirm that invasive pests can initiate substantial changes in ecosystem function soon after infestation occurs, prior to substantial overstory mortality or understory reorganization.

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