Decline of the Hemlock, Part I


Most readers have probably noticed that hemlock trees in the southern Appalachians are losing their needles and dying due to the hemlock woolly adelgid (HWA). The hemlock woolly adelgid is a tiny but devastating insect that has spread throughout the area, decimating the hemlock population. These losses threaten not just the views, but the functioning of the forest ecosystem and water balance of the region. Coweeta researchers Chelcy Ford and Jim Vose have been studying how hemlocks affect the larger ecosystem and the potential impacts of their loss, the spread of the adelgid, and the most effective ways to protect them. This column will review the important role that hemlocks play in stream ecology, while the next will describe how Coweeta researchers are discovering more effective ways to protect these trees.

Here is a little background for anyone not familiar with the plight of the hemlocks: HWA, an invasive pest that kills by sucking the stored carbon out of trees, was first described in the western U.S. in 1924 and was likely brought in from nursery stock from Asia. It first appeared in the eastern U.S. east in 1951 in Richmond, VA and has been rapidly spreading north-, eastand southward ever since. We now know that the HWA in the eastern U.S. originated from southern Japan. Unfortunately, HWA thrive in the southern Appalachians due to the milder winters that allow it to remain active year round. Sick trees, identifiable by a grayish cast to their normally dark green hue, dropping needles, and thinning branches, are usually dead within five to 10 years.

Hemlocks are one of the most important riparian species (meaning they grow along streams) in the southern Appalachians. Unlike most streamside trees in the area, hemlocks are evergreen. Keeping most of their leaves all year round has important implications for the forest ecosystem; most notably, they maintain deep shade year round. Even during summer months, hemlocks filter out more light than deciduous trees, meaning streams are cooler and less light is available to fuel photosynthesis and growth of aquatic plants and algae. It has been estimated that shade from riparian hemlocks can cool a stream by as much as 9° F. Cooler streams benefit brook trout, the only native trout in the region.

Hemlocks also have a significant impact on transpiration rates, and thus soil moisture and stream flow. Transpiration is the process by which plants lose the water that they take in through their roots. Transpiration happens through the leaves, so in winter when deciduous trees lose their leaves, transpiration comes largely to a halt. Hemlocks, on the other hand, can transpire all year round. Coweeta researchers working on understanding the impacts of hemlock decline on water balance have found that a large hemlock tree can transpire up to about 50 gallons of water a day – that results in an extra 50 gallons of water available for streamflow after the hemlocks die.

This is important because it means that the loss of these trees will have significant effects on stream flow and hydrology. The loss of hemlock will likely cause overall stream flow to increase and become much more seasonally variable as the deciduous trees that replace hemlocks drop and regain their leaves each year. Researchers in hemlock forests in the Northeast have found that streams draining hemlock forests were less prone to drought disturbance than those draining hardwood forests. They explain that soils in hemlock forests can hold moisture better than mixed hardwood forests, allowing more consistent recharge of groundwater aquifers during summer droughts and more stable runoff. The steady temperatures and flow of hemlock-shaded streams support unique salamanders, fish, and invertebrates that cannot tolerate seasonal drying of streams or large variations in temperature. Hemlocks also provide habitat for a number of terrestrial animals such as whitetailed deer, which often take cover under hemlocks in winter snows. Additionally, several species of birds, such as the Wood Thrush and Winter Wren, prefer to build their nests in the dense canopy of hemlock. Because of all these unique functions, researchers are rightfully concerned that the loss of the hemlock could result in cascading changes throughout the forest ecosystem.

 

This column is produced by members of the Coweeta Listening Project (CLP), a branch of the Coweeta Long Term Ecological Research Program. Views expressed here are not representative of the USDA Forest Service or the Coweeta Hydrologic Lab. Please share questions, comments, or suggestions for future topics at cwtlistn@uga.edu or Coweeta Listening Project, UGA, 210 Field St., Room 204, Athens, Georgia 30602.

 

Original Citation: The Coweeta Listening Project. Franklin Press. Column on "Science, Public Policy, Community." Page B6. July 29, 2011.