Loss of eastern hemlock will affect forest water use

The loss of eastern hemlock from forests in the Southern Appalachian region of the United States could permanently change the area's hydrologic cycle, reports a new study by U.S. Forest Service scientists at the Coweeta Hydrologic Laboratory (Coweeta) located in Otto, North Carolina, published online in the journal Ecological Applications and available now in preprint format.

"The hemlock woolly adelgid, an exotic invasive insect, has caused widespread hemlock mortality," says Steven Brantley, a post-doctoral researcher at Coweeta and lead author of the paper. "Hemlock decline is expected to have a major impact on forest processes, including transpiration."

Transpiration describes the loss of water from plant leaves or needles. 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. Because of its dense evergreen foliage and dominance in riparian and cove habitats, eastern hemlock plays an important role in the area's water cycle, regulating stream flow year round.

The loss of hemlock from southern Appalachian forests can be compared to the loss of American chestnut from eastern forests, which became functionally extinct after the introduction of an exotic fungus in the early 20th century. 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 in rhododendron-dominated forest stands is lower than in previously-healthy hemlock forests. 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. Sweet birch trees also have a much higher transpiration rate than eastern hemlock trees during the growing season.

"The cumulative effect of these species changes will probably mean permanent changes in seasonal transpiration patterns," says Brantley. "In the growing season, transpiration rates will likely rise, leading to lower stream flow in the summer. However, transpiration rates in the winter will be reduced, which could cause increased winter stream discharge."

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