Abstract
Eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are ecologically important tree species in eastern North America forests that are currently threatened by the hemlock woolly adelgid (HWA, Adelges tsugae Annand, Hemiptera: Adelgidae). HWA has spread rapidly from its original introduction site into new areas. Once present, HWA kills its hosts over a period of 4 to 10 years leading to a phenomenon that is known scientifically and colloquially as hemlock decline. To date, quarantine, chemical management, and biocontrol efforts have failed to curb the spread of the HWA. As such, forest management efforts are now being redirected towards developing an understanding of the effects of hemlock removal on vegetation dynamics, changes in forest composition, and changes in ecosystem function. In this study, we parameterize a spatially explicit landscape simulation model LANDIS II for a specific forested region of the southern Appalachians. Parameterization involves defining the life-history attributes of 37 tree species occupying 11 ecological zones and is based on knowledge of: current vegetation composition data, recent historic management and fire regimes, and life-history traits of each species. The parameterized model is used to explore a simple scenario of catastrophic hemlock mortality likely to occur as a result of HWA herbivory. Our results emphasize that hemlock is an important foundation species. When hemlock is removed from the system, forest composition changes considerably with a greater presence of shade intolerant pine and oak species. Additionally, hemlock removal leads to a period of transient, relatively unstable vegetation dynamics as the forest communities restructure.
Highlights
The eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are foundation species in eastern US forests (Ellison et al, 2005)
Species abundance is defined as the proportion of cells in each ecological zone occupied by each species (LANDIS II does not provide a means for calculating the abundance of each species in each cell)
Removal of hemlock leads to an increase in the abundance of the remaining 36 species in all ecological zones
Summary
The eastern hemlock (Tsuga canadensis Carriére) and the Carolina hemlock (Tsuga caroliniana Engelmann) are foundation species in eastern US forests (Ellison et al, 2005). Eastern hemlock forests are under threat from the hemlock woolly adelgid (Adelges tsugae Annand; Hemiptera: Adelgidae) (HWA), an exotic invasive sap sucking insect that feeds on both eastern and Carolina hemlock. Since its introduction in 1953 in Richmond, Virginia, HWA populations have expanded and currently occupy approximately half of the range of native eastern hemlock forests (USDA, 2010). Professional foresters, and increasingly, the general public, this pattern of expanding HWA populations and gradual tree mortality is often referred to as hemlock decline. The potential impact of HWA on eastern North American forests has been compared to that of chestnut blight (Cryphonectria parasitica) on the American chestnut (Castanea dentata) and Dutch elm disease (Ophiostoma ulmi) on the American elm (Ulmus americana) (Small et al, 2005)
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