David Orwig, Heidi Lux
Harvard Forest
Primary Interests:
Dendroecology, biogeochemistry, soils, old-growth forest dynamics, effects of disturbance and land use history on forest structure and composition
HWA Objectives:
Landscape-Level Analyses of Hemlock Woolly Adelgid Outbreaks in Massachusetts
Over the last 18 years, hemlock woolly adelgid (HWA) (Adelges tsugae), an introduced aphid-like insect from Asia, has expanded its range from southern Connecticut to northern Massachusetts, causing widespread mortality and decline of Tsuga canadensis (eastern hemlock). Recent USDA Focus Funding and an NSF Ecosystems grant has allowed us to build on similar analyses used in Connecticut (Orwig et al. 2002), and map the distribution of all T. canadensis stands (> 3 ha) prior to HWA infestation in a 4000 km2 transect through Massachusetts. We are interested in characterizing the temporal and spatial patterns of damage generated by HWA since the time of its arrival into the area in 1989. To date, over 5000 stands with > 10% hemlock have been mapped, representing over 86,000 ha, or 21% of the study area. We have sampled 80 stands during the summers of 2002- 2003 and collected information on forest structure and composition, crown vigor, site characteristics, potential replacement species, presence of HWA, and the extent and spatial patterns of canopy damage. Overstory T. canadensis importance ranged from 24 - 92% and total stand densities varied from 225 - 2025 ha-1. Many stands were found on northern and western aspects on slopes of 20 - 30%. HWA was found in close proximity (a few km) to VT, suggesting it is continuing to migrate and may already be in southern portions of VT and NH. Almost 50% of the stands sampled had HWA present, although overstory hemlock mortality levels are still very low. This information will be incorporated into a GIS analysis of landscape-level, biological, edaphic, and historical factors that control the damage patterns observed in hemlock.
Ecosystem Analyses of Hemlock Woolly Adelgid Outbreaks in Southern New England
In 1998 we began examining the magnitude and duration of Nitrogen (N) cycling dynamics in response to the stress and mortality caused by HWA at 8 study sites varying in HWA infestion level in southern New England. During the first five years of this study, thinning canopies from heavy HWA damage resulted in increased light, soil temperature, and mineral soil moisture, and decreased forest floor moisture content. Heavily infested sites continue to have larger extractable NH4-N and NO3 - N pools, and significantly higher net nitrification rates than healthy hemlock forests. In addition, resin bags captured more NH4-N and NO3 - N in infested versus uninfested stands. In 2001, we began examining 2 additional stands that contain high overstory hemlock mortality and a dense black birch understory. Under the direction of Sultana Jefts, we will continue to sample these stands as they deteriorate to determine the extent to which changes in overstory composition, microenvironment, and soil conditions produce fundamental changes in the cycling of nitrogen. Over the last 4 years we have been interested in how HWA-induced hemlock decline affects decomposition rates, because they are closely linked to the nitrogen cycling changes we have observed. Under the direction of Richard Cobb, we have studied 3 key drivers of decomposition in these forests: 1) changes in foliar quality due to HWA; 2) changes in microclimate associated with hemlock deterioration, and 3) the effect of hemlock replacement by black birch. Foliar % carbon and lignin did not appear to be affected by HWA infestation, but foliar %N (nitrogen) was higher in infested stands. Higher initial foliar N was found to increase the rate of N immobilization in decomposing foliage, likely leading to the increased N availability mentioned above. Microclimatic changes led to more rapid sub-surface decomposition and initially slower decomposition on the forest floor in infested vs. uninfested forests. We are continuing a 2 year comparative litter decomposition study of hemlock, black birch, and a mixture of the two, to examine longer term dynamics as these substrates break down over time.
HWA vs. Logging
Recent work by Kizlinksi et al. (2002) compared the magnitude and trajectory of vegetation and ecosystem function dynamics associated with the direct impact of HWA infestation versus the indirect consequences of HWA-induced damage in the form of salvage and pre-emptive logging of hemlock forests. Results suggest that both the decline associated with HWA infestation and the indirect effects of HWA in the form of logging are generating profound changes in structure, composition, and ecosystem function in these forests, although at different spatial and temporal scales. Hemlock harvesting imposed more abrupt microenvironmental changes, and rapidly reduced vegetative cover while chronic HWA infestation led to gradually thinning canopies. Both disturbances led to black birch dominated forests, although logging resulted in greater amounts of shade-intolerant regeneration, higher soil pH and nitrification rates, and reduced forest floor mass. Preemptive cutting of undamaged forests may lead to greater N losses than those associated with HWA infestation or logging of deteriorated hemlock forests, due to reduced vegetative uptake. Silvicultural methods that allow for vegetation establishment prior to harvesting will likely lessen the ecological impacts of hemlock removal. A newly funded NSF project was initiated to examine the vegetation and ecosystem consequences of hemlock removal by logging, the primary management response to the HWA. Dave and Laura Pustell are establishing permanent plots in logged sites in Massachusetts to compare the vegetation, microenvironment, and nitrogen cycling responses with those triggered by HWA infestations. Results from these studies will be incorporated into a series of outreach efforts targeted to a range of user groups that will ultimately lead to informed decision making on how best to manage hemlock forests.
