The Ecology of Energy and Nutrient Fluxes in Hemlock Forests Invaded by Hemlock Woolly Adelgid

Abstract

The hemlock woolly adelgid (HWA, Adelges tsugae Annand) is currently causing a severe decline in vitality and survival of eastern hemlock in North American forests. We analyzed the effects of light HWA infestation on vertical energy and nutrient fluxes from the canopy to the forest floor. Canopy throughfall, litter lysimeters, and laboratory litter microcosms were used to examine the effects of HWA-affected and unaffected throughfall on litter type, leachate, and litter chemistry. Early in the season adelgid infestation caused higher dissolved organic carbon (DOC; +24.6%), dissolved organic nitrogen (DON; +28.5%), and K (+39.3%) fluxes and lower inorganic nitrogen fluxes (-39.8%) in throughfall and in adjacent litter solutions collected beneath infested compared to uninfested trees. Needle litter collected beneath uninfested hemlock had significantly lower N concentrations compared to needles collected beneath infested trees, while no difference in N concentrations was found in birch litter. Bacteria were significantly more abundant on hemlock and birch litter beneath infested trees, while yeasts and filamentous fungi showed no consistent response to HWA throughfall. Litter microcosms showed that less DOC was leaching from birch than from hemlock needles when exposed to HWA throughfall. Overall, NH4-N and DON leachate concentrations were higher from birch than from hemlock litter. Thus, HWA-affected throughfall leads to qualitative and quantitative differences in nitrogen export from the litter layer. The N concentration of hemlock litter did not change with time, but the N concentration in birch litter increased significantly during the course of the experiment, especially when HWA-affected throughfall was applied. We suggest a nonlinear conceptual model for the temporal and vertical transition of energy and nutrient fluxes relative to progressing HWA infestation from a pure hemlock to a birch/maple-dominated forest. Progressive needle loss and changes in needle chemistry are likely to produce a humped-shaped DOC curve, while N fluxes initially decrease as infestation continues but rise eventually with hemlock decline and immigration of hardwood species. These findings suggest that it is necessary to understand the biology and specific physiological/trophic effects of exotic pests on their hosts and associated ecosystem processes in order to decipher the temporal dynamics, direction of change, and functional consequences.

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

“…Although we have observed steady increases in light in the hemlock control plots as the adelgid increased in abundance [51], this is not yet strongly affecting understory vegetation in the hemlock controls. We expect that in coming years, additional nonlinear changes in soil N in the hemlock control plots as a function of the adelgid, as suggested by [42,43] and observed by [51] may affect understory composition. Soil N initially increases because carbon sloughing off from the waxy coating of the adelgid provides additional energy for microbial N immobilization of the relatively N-rich needles of infested trees [42].…”

“…We expect that in coming years, additional nonlinear changes in soil N in the hemlock control plots as a function of the adelgid, as suggested by [42,43] and observed by [51] may affect understory composition. Soil N initially increases because carbon sloughing off from the waxy coating of the adelgid provides additional energy for microbial N immobilization of the relatively N-rich needles of infested trees [42]. Consequently, Nitrogen fluxes initially decrease with infestation, but later rise as hemlock declines and is replaced by deciduous trees, whose litter has a higher percentage of N [42].…”

“…As we expected it to eventually colonize our hemlock control plots, HF-HeRE was designed explicitly to contrast the effects on these forests from physically disintegrating trees (resulting from girding them) with removal of them from the site (following logging). Since the adelgid colonized the hemlock controls-which occurred in 2009 and 2010-we have been able to contrast the effects of physical disintegration of eastern hemlock (in the girdled plots) with the effects of the adelgid (in the hemlock control plots), which includes not only physical disintegration but also changes caused by the adelgid directly, including, e.g., nitrogen inputs [42,43].…”

“…Soil N initially increases because carbon sloughing off from the waxy coating of the adelgid provides additional energy for microbial N immobilization of the relatively N-rich needles of infested trees [42]. Consequently, Nitrogen fluxes initially decrease with infestation, but later rise as hemlock declines and is replaced by deciduous trees, whose litter has a higher percentage of N [42]. The differences in interaction terms for PC-3 ( Figure 4) suggest increased rate of succession following logging, as the small hemlocks that were not removed during the logging operation have not yet succumbed to the adelgid.…”

Properties of dissolved and total organic matter in throughfall, stemflow and forest floor leachate of central European forests

“…Currie et al, 1996), only a few have investigated the dynamics (Carlisle et al, 1966;Sollins et al, 1980;le Mellec et al, 2009le Mellec et al, , 2010 and none the character of water-bound total OM (TOM) including the particulate organic matter fraction (POM; 0.45 µm < POM < 500 µm). Since water-bound element and nutrient concentrations are conventionally measured after filtration (frequently to < 0.45 µm pore size), the exclusion of the POM frac-tion potentially results in misleading inferences and budgeting gaps in studies of nutrient and energy fluxes in ecosystems (Michalzik and Stadler, 2005;Stadler et al, 2006). We are aware of only a handful of studies over the last 35 years which address POM or size equivalents in temperate broadleaved and coniferous ecosystems (e.g.…”