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

Abstract

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·g -1 vs. 1.06 ± 0.05 g·g -1 ) 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.Résumé : Cette étude examine les impacts du puceron lanigère de la pruche (Adelges tsugae Ann.), un petit insecte invasif, sur les propriétés chimiques du feuillage, le microclimat dans la couverture morte et la décomposition subsé-quente du feuillage vert dans les forêts de pruche du Canada (Tsuga canadensis (L.) Carrière). Nous avons étudié les effets directs du puceron lanigère de la pruche et les changements indirects du microclimat sur la décomposition du feuillage en incubant du feuillage sain et du feuillage infesté par le puceron lanigère de la pruche dans huit peuplements dominés par la pruche du Canada dans le sud de la Nouvelle-Angleterre qui avaient différents historiques d'infestation par le puceron lanigère de la pruche. Les peuplements infestés étaient caractérisés par un état de santé des cimes beaucoup plus détérioré (3,4 vs. 1,4 d'indice de perte de cime), un pourcentage plus élevé d'ouverture du couvert (10,9 ± 2,4 vs. 5,3 ± 0,5 de coefficient de trouées lumineuses) et une humidité du sol organique plus faible (0,83 ± 0,02 vs. 1,06 ± 0,05 g·g -1 ) que les peuplements sains. Il n'y avait pas de différence significative dans le pourcentage de C, N ou de lignine du feuillage excisé au début de l'étude mais, après 120 jours, le feuillage...

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

“…Th ey suggested that decreases in the forest fl oor mass could have resulted from greater decomposition rates in HWA-infested stands due to changes in environmental variables or litter quality. Cobb et al (2006) found that aft er 120 d of decomposition, foliage from infested trees had a greater N concentration and lower C/N ratio than foliage from uninfested trees, suggesting N immobilization. We expect the decline in hemlock foliage inputs with hemlock mortality (Fig.…”

Foundation Species Loss and Biodiversity of the Herbaceous Layer in New England Forests

“…In forest stands of the cove forests in the southern part of its range, in mixed forests of New England and southern Canada, and along riparian corridors throughout its >10,000 km 2 range, eastern hemlock can account for >50% of the total basal area [17,23]; see [2,[24][25][26] for detailed discussions of the foundational role of eastern hemlock in stands where it is the dominant species. The forest floor beneath the eastern hemlock canopy is cool and dark [16,27], and the slowly decomposing hemlock needles give rise to a deep organic layer, which is very acidic and low in nutrients [28]. Unique faunal assemblages, including groups of birds [29], arthropods [30][31][32], and salamanders [33] live in eastern hemlock stands.…”

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

“…air temperatures are above freezing, but photosynthetic rates are slow, water-use efficiency is high, and peak carbon fixation by T. canadensis occurs in spring and fall, not in the summer when cooccurring broadleaved, deciduous trees are in full leaf [16]. Because the needles of T. canadensis are slower to decompose than all of the other regionally cooccurring trees [17], organic matter accumulates unusually rapidly beneath T. canadensis canopies [18], but the soils are acidic and nutrientpoor [19]. Overall, both carbon flux [16,[20][21][22][23] and nutrient cycling [19,[24][25][26] are much slower in T. canadensis stands than in stands dominated either by hardwoods or other conifers.…”