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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Date of Award

Spring 2010

Document Type


Degree Name

Master of Science (MS)


Department of Biology


Heather Griscom


The exotic invasive insect, hemlock woolly adelgid (Adelges tsugae Annand), is causing mortality in eastern hemlocks (Tsuga canadensis [L.] Carr.) throughout the eastern U. S. Hemlocks are being replaced by hardwood species that cannot fill the structural and functional role of hemlock in forest ecosystems. Because hemlocks produce dense shade, their loss may increase understory light levels. In the southern Appalachians, increases in understory light could cause changes in stream ecosystems because riparian hemlocks may help maintain cool stream temperatures for cold water species such as brook trout (Salvelinus fontinalis). I studied changes in light levels with eastern hemlock decline at a southern Appalachian brook trout stream using hemispherical photography and multi-temporal satellite images (ASTER). My results indicate that stream light levels have increased significantly with adelgid infestation. Leaf-on light levels are currently significantly higher (P < 0.02) in plots containing high basal areas of hemlock (mean global site factor (GSF)(SE) = 0.267(0.01)) compared with plots containing no hemlock (mean GSF(SE) = 0.261(0.01)), suggesting that increases in light have occurred with hemlock decline. The Normalized Difference Vegetation Index (NDVI) decreased with hemlock decline from 2001 to 2008. In 2001, NDVI showed no relationship (R2 = 0.003; F = 0.14; P = 0.71) with hemlock basal area, but by 2008, there was a significant negative relationship (R2 = 0.352; F = 19.55; P < 0.001) between NDVI and hemlock basal area. I also conducted a gap experiment that showed that light levels may increase by up to 64.7% more (mean increase in GSF = 27.5%) as hemlocks fall, creating gaps in the canopy. However, by comparing light levels between plots containing hemlock and those containing only hardwood species, I found that if hemlocks are replaced by hardwood species, light levels under an all-hardwood canopy (mean GSF(SE) = 0.240(0.005)) are unlikely to be higher than they are under the current forest (mean GSF(SE) = 0.254(0.007)). These results suggest that loss of hemlock along southern Appalachian streams could have short-term impacts on light levels and stream temperatures, potentially threatening brook trout populations, but that long-term changes in light levels may be unlikely.

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