Impacts of the loss of hemlock canopy on southern Appalachian herbaceous communities

Ty B Boyd, Purdue University

Abstract

Eastern hemlock (Tsuga canadensis) was a dominant canopy tree in the southern Appalachian Mountains that has been nearly extirpated by the exotic hemlock woolly adelgid (Adelges tsugae). The Great Smoky Mountains National Park harbored 35,414 hectares of forests that had a significant hemlock component, with 7,287 hectares of hemlock-dominated forests. Forty-six permanent vegetation plots were established in 1998 in the Cataloochee watershed in forest containing hemlock before the adelgid invasion, and I resampled 40 of these plots in 2010. I collected data, following the North Carolina Vegetation Survey protocol, to quantify changes in the herbaceous plant communities and overstory strata of the forest. By 2010, nearly all mature hemlock trees were dead in the study plots, substantially altering the canopy. Diverse herbaceous communities adapted to shady, moist conditions changed in density, diversity and species composition as a result of changes in microclimate following death of hemlock. Calculations of similarity in 2010 species composition compared to 1998 show that most plots have changed by more than 40%. These changes were particularly strong in low-diversity plots that had dense hemlock canopy. Species diversity has increased in most plots, consistent with previous studies showing hemlock suppression of understory diversity. Herbaceous species’ responses to hemlock loss varied widely. Approximately one third of common species (those found in many plots and at substantial densities) have at least doubled in density between 1998-2010 following hemlock loss, and the densities of another third of species were reduced by half or more. Population changes corresponded with habitat affinities: declining species are shade-tolerant and increasing populations belong to open-canopy species. Multivariate community analysis showed that loss of hemlock from the canopy, abundance of co-dominant birch species, and shrub density were the environmental variables strongly correlated with changes in herbaceous composition. Although distinct herbaceous associations exist, reorganization of species composition is consistently high across the study area. Appalachian forests have been altered by other exotic insects and pathogens, and this study helps establish the demise of hemlock as one of the best-documented and most dramatic anthropogenic ecosystem modifications.

Degree

M.S.

Advisors

Rabenold, Purdue University.

Subject Area

Ecology|Forestry

COinS