Examining the Impact of Hemlock Woolly Adelgid Herbivory on Southern Appalachian Forest Landscapes

Yu Zeng1, Robert Coulson1, Szu-Hung Chen1, Charles Lafon2, David Cairns2, John Waldron3, Andrew Birt1, Maria Tchakerian1, Weimin Xi4, Douglas Streett5

1Knowledge Engineering Laboratory, Texas A&M University, College Station, TX 77843;
2Department of Geography, Texas A&M University, College Station, TX 77843;
3Department of Environmental Studies, University of West Florida, Pensacola, FL 32514;
 4Department of Forest and Wildlife Ecology, University of Wisconsin-Madison, Madison, WI 53706;
USDA Forest Service Southern Research Station, 2500 Shreveport Hwy, Pineville, LA 71360


The Southern Appalachian Mountains are one of the most diverse forest landscapes in the United States. Invasive insect and plant species are one of the most immediate environmental threats to the structure, composition and utility of these forests

An immediate and pressing example of an invasive pest is provided by the Hemlock Woolly Adelgid (Adelges tsugae) which is gradually killing all Hemlocks in the eastern United States. In the absence of effective Adelgid control, research must focus on how the change in Hemlock abundance may affect vegetation dynamics, and forest utility.

Hemlock mortality may have direct and indirect impacts on the forest. First, Hemlock mortality will significantly affect the aesthetics and biotic and abiotic characteristics of the forest. Second, Hemlock mortality may affect long-term vegetation dynamics by changing the interactions amongst competing vegetation including both native and non-native fauna.

The goal of this research is to simulate the dynamics of vegetation in a specific Southern Appalachian Landscape following Hemlock mortality. In addition to native fauna, we also simulate Mimosa (Albizia julibrissin) , an established alien species, to illustrate the synergistic effects of invasive species on forest dynamics.

We focused on the effects of Hemlock Wooly Adelgid in the Grandfather Ranger District, Pisgah National Forest, NC. Forests in this area are internationally important, have high biodiversity and are therefore well studied. They are also at risk from currently established and future invasive species. By choosing a specific, well studied area (Grandfather) we aim to develop simulations that have immediately practical results, and that serve as model studies for other invasive problems.

  • We used a landscape simulation model LANDIS-II to simulate 36 native and non-native tree species in the Grandfather Ranger District.
  • To parameterize and calibrate LANDIS we examined the historical development of Grandfather Ranger District based on vegetation research starting from 1940s (Braun, 1942; Whittaker, 1956), current stand maps for the Grandfather Region, and field data from Carolina Vegetation Survey (Peet, et al 1998). Our research shows :
    • Forest landscape in Southern Appalachian Mountains has gradually reached a relatively stable state since the establishment of National Forest
    • With the expectation of consistent forest management of this forest, this forest landscape without large scale major disturbance can be considered as a dynamically equilibrium ecosystem; the trajectory of this forest landscape should maintain a statistically stable state

  • Once parameterized, the model can be used to simulate vegetation dynamics under almost any ecological threat or restoration scenario and/or for any specific landscape. Examples of threats include climate change, Southern Pine Beetle, Hurricane Damage, or any other invasive plants. The model can also be used to simulate MULTIPLE synergistic effects.

  • In this example, we have used the model to compare forest dynamics predictions with and without Hemlock Woolly Adelgid induced Hemlock mortality.

Mimosa   Hemlock Woolly Adelgid (HWA)
County-level distribution of Mimosa   County-level progress map of HWA since 1971

  • Following widespread Hemlock mortality, the area occupied by Mimosa increases by 28% in the Shortleaf Pine-Oak Heath ecoregion (Eco3). However, Mimosa abundance does not significantly increase in the Acidic Cove ecoregion (Eco2) (Figure 1).

Figure 1 Mimosa with vs. Mimosa without
Hemlock in two ecoregions
  • Pine and Oak abundance also increase in Pine-Oak Heath and Acidic Cove ecoregions. Figures 2 and 3 illustrate the temporal change in Shortleaf pine in Shortleaf Pine-Oak Heath ecoregion and Scarlet oak in Acidic Cove ecoregion, respectively.
Figure 2 Shortleaf pine with vs. Shortleaf pine
without Hemlock in Shortleaf
Pine-Oak Heath ecoregion
Figure 3 Scarlet oak with vs. Scarlet oak
without Hemlock in Acidic Cove ecoregion
  • Simulating a specific, well studied ecosystem allows practical interpretation of simulation results, enabling local experts to critique and discuss the effects of HWA in the context of a well studied landscape.
  • Restoration strategies need to account for multiple forest threats. In this case, we highlight the importance of simultaneously investigating two established species - Hemlock Wooly Adelgid and Mimosa
  • Given current trends in invasive species, climate change and other threats, there is no shortage of potential forest stressors. The research we describe here goes some way to developing a tool that can be quickly modified to investigate future threats.
  • Although LANDIS model is a useful tool for understanding long range vegetation dynamics, it is also important to understand how changes in vegetation affect the utility of the forest system - for example, water quality, soil erosion, aesthetics, ecosystem function. To address these questions, future research will link LANDIS simulation results to more detailed models.
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Peet, R.K., T.R. Wentworth & P.S. White. 1998. The North Carolina Vegetation Survey protocol: a flexible, multipurpose method for recording vegetation composition and structure. Castanea, 63:262-274.

Perry, D.A. 1994. Forest Ecosystems, The Johns Hopkins University Press, Baltimore, MD.

Shugart, H.H. 1998. Terrestrial Ecosystems in Changing Environments. Cambridge University Press, Cambridge, U.K.

Whittaker, R.H. 1956. Vegetation of the Great Smoky Mountains. Ecol. Monog., 26: pp. 1-80.