Loss of Eastern Hemlock Will Affect Forest Water Use

by Sarah Farmer, SRS Science Delivery Group
Hemlock woolly adelgids are killing eastern hemlocks in the Southern Appalachian region. The loss of hemlock will have long-term implications for hydrological cycles as well as plant and animal communities. Photo by Chelcy Ford.

Hemlock woolly adelgids are killing eastern hemlocks in the Southern Appalachian region. The loss of hemlock will have long-term implications for hydrological cycles as well as plant and animal communities. Photo by Chelcy Ford.

Eastern hemlock grows in streamside areas throughout the southern Appalachian Mountains, where it is a keystone species. Because of its dense evergreen foliage, constant year-round transpiration (loss of water from needles) rate,  and dominance in riparian and cove habitats,  eastern hemlock plays an important role in the area’s water cycle, and regulates stream flow year round. 

“Eastern hemlocks are facing widespread decline and mortality because of an exotic invasive insect, the hemlock woolly adelgid. The loss of hemlock from southern Appalachian forests can be compared to the loss of American chestnut, which became functionally extinct in eastern forests after the introduction of an exotic fungus in the early 20th century. The loss of eastern hemlock is expected to have a major impact on forest processes, including transpiration, and could permanently change the areas hydrologic cycle,” says Steven Brantley, a post-doctoral researcher at Coweeta Hydrological Laboratory and lead author of a new paper about hemlock, future species composition, and the water cycle.

Coweeta researchers estimated changes in transpiration at the forest-level since hemlock woolly adelgid infestation by monitoring tree water use and changes in forest composition from 2004 to 2011. The four studied stands were once dominated by eastern hemlock trees, and are located in the Coweeta watersheds.

Changes in local forest hydrology from the loss of eastern hemlock will largely depend on which species replace it. Rhododendron, a woody evergreen shrub common in southern Appalachian forests, is one of the species replacing eastern hemlock trees. Although rhododendron is evergreen, it has lower leaf area than hemlock, and thus transpiration by rhododendron is lower than that of healthy hemlock trees.  Most of the other species replacing eastern hemlock trees are deciduous, such as sweet birch, which unlike the evergreen rhododendron and eastern hemlock, do not transpire during the winter when they lose their leaves. Sweet birch trees also have a much higher transpiration rate than eastern hemlock trees during the growing season.

“These species changes will probably mean permanent changes in seasonal transpiration patterns, which will affect streamflow,” says Brantley. “In the growing season, transpiration rates will likely rise, leading to lower streamflow in the summer. However, transpiration rates in the winter will be reduced, which could cause increased winter stream discharge.” Exploring how the loss of hemlock has affected annual and seasonal streamflow, as well as streamflow after individual storms is a current area of research at Coweeta.

Whichever species eventually replace eastern hemlock, there will be important long-term implications beyond stream flow. Without the shade provided by eastern hemlock, stream temperatures could rise, threatening aquatic animals such as eastern brook trout that require cold water for survival. The loss of eastern hemlock will not only affect the animal and plant communities in streamside habitats, but ecosystem function throughout these areas.

The study was conducted at the U.S. Forest Service Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina. Coweeta is one of the oldest continuous environmental studies in North America. Since 1934, precipitation, temperature, and stream flow have been continuously recorded at Coweeta, a U.S. Forest Service Southern Research Station facility.

For more information contact Steven Brantley at sbrantle@umn.edu or (828) 524-2128 ext. 116

Read the full text of the article (preprint version) .

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Posted in Experimental Forests, Forest Watersheds, Insects and Diseases, Upland Hardwoods