Associations Between Eastern Hemlock (Tsuga canadensis) and Avian Occurrence and Nest Success in the Southern Appalachians

Keller, David

ABSTRACT: The recent decline of eastern hemlock (Tsuga canadensis) is a dramatic example of the effects exotic insect pests can have on forest composition and structure. Hemlock decline is attributed to defoliation caused by hemlock woolly adelgid (HWA, Adelges tsugae), an exotic Hemipteran introduced to the northwest U.S. from Asia during the 1920s and the eastern U.S. during the early 1950s. Within the past several years, HWA infestations have reached the southern portions of the Appalachian mountains and threaten to eliminate both eastern and Carolina hemlock (Tsuga carolinensis) from theregion. Recognized as nationally important bird areas by the American Bird Conservancy, lands administered by the U.S. Forest Service and National Park Service in the southern Appalachians form one of the largest networks of contiguous forested habitat in the eastern United States and provide great quantities of high quality habitat for numerous breeding bird species. Changes in southern Appalachian forest ecosystems resulting from loss of hemlock may have impacts on the distribution and demography of several avian species in the region. This study had two primary objectives designed to investigate the potential effects of eastern hemlock decline on southern Appalachian breeding birds: 1) develop models of avian associations with eastern hemlock in southern Appalachian national forests comparable to existing models developed for Great Smoky Mountains National Park (GSMNP) (Shriner 2001) and 2) compare nest success and the mechanisms influencing productivity of three hemlock-associated species [black-throated blue warbler (Dendroica caerulescens), dark-eyed junco (Junco hyemalis), and blue-headed vireo (Vireo solitarius)] in hemlock and hardwood-dominated stands. Using the U.S. Forest Service R8BIRD database containing point-count data for Jefferson and George Washington National Forests (JGWNF), Virginia, and Cherokee National Forest (CNF), Tennessee, I developed logistic regression models predicting occurrence probabilities as a function of hemlock presence for 29 avian species. Two sets of models were developed based on single-year and multiple-year occupancy criteria for classifying a point-count location as a presence point for a given species. On JGWNF, the presence of Acadian flycatcher (Empidonax virescens), blue-headed vireo, black-throated blue warbler, black-throated green warbler (Dendroica virens), Canada warbler (Wilsonia canadensis), dark-eyed junco, northern parula (Parula americana), and veery (Catharus fuscescens) had significant positive associations with hemlock presence at a Bonferroni-corrected α-level of 0.0017. Eastern towhee (Pipilo erythrophthalmus) and indigo bunting (Passerina cyanea) displayed negative associations. On CNF, four species—Acadian flycatcher, black-throated blue warbler, black-throated green warbler, and northern parula—were positively associated with hemlock presence and one species—indigo bunting—was negatively associated at an α-level of 0.0017. Three species showed a consistent positive association with eastern hemlock in JGWNF, CNF, and GSMNP: Acadian flycatcher, black-throated blue warbler, and blackthroated green warbler. Although these species may experience negative impacts from hemlock decline, analysis of distributional data ignores mechanisms driving the observed species associations with hemlock, and, consequently, potential plasticity in habitat selection and other life history traits that may mitigate the impacts of hemlock loss are unknown. A possible proximate mechanism for the hemlock associations identified by the models for JGWNF, CNF, and GSMNP may be greater reproductive fitness afforded by hemlock sites for hemlock-associated species. My second objective was to test the prediction that nest success of three hemlock-associated species in the southern Appalachians—black-throated blue warbler (BTBW), dark-eyed junco (DEJU), and blue-headed vireo (BHVI)—was greater in breeding...