Potential Effects of Eastern Hemlock Decline on Avian Distributions and Reproduction in Great Smoky Mountains National Park and Surrounding National Forests.

Buehler, David

"Researchers measured territory density, provisioning rates at nests, and nest survival rates for three species: blue-headed vireo (Vireo solitarius), dark-eyed junco (Junco hyemalis), black-throated green warbler (Dendroica virens), and black-throated blue warbler (D. caerulescens). They compared three plots with hemlocks treated for hemlock wooly adelgid and three untreated plots, in 2008/09. The plots on Jim's Branch had almost complete hemlock mortality, regardless of treatment, while the plots at Salt Branch had over 80% healthy hemlocks where treated and 50% healthy where untreated. There was no difference in avian species richness observed between treated and untreated plots. Black-throated Blue Warblers, Black-throated Green Warblers, Hooded Warblers, and Blue-headed Vireos were the most abundant species associated with the hemlock forests. They found significantly lower densities of black-throated green warblers in untreated plots and a non-significant trend for lower densities amongst the other three species. Raw numbers also indicated a decline in territory densities from 2008 to 2009. Over all, only 5 of 22 nests monitored across the three species successfully fledged young in 2008. There is some supposition that decline of hemlocks may provide a temporary increase in understory that may benefit species like the black-throated blue warbler in the short run. Most foraging observed for this species was in the mid-story or understory and most was in either hemlock or rhododendron. This study started in 2002/03 comparing nesting in hemlock plots vs. hardwood plots. They found much higher nesting survival for the warblers in hemlock plots, presumably because of greater cover, but found no difference for the junco or the vireo. They made an effort to determine if there was a difference in mammalian or avian predator densities between the two habitats, but essentially found no difference."