Says Ford, “With the loss of this species, we predict changes to streamflow, streamside forest structure, and soil moisture that will have to be addressed by land managers.”

In the June 2007 issue of Ecological Applications, researchers Chelcy Ford and Jim Vose from the FS Southern Research Station (SRS) Coweeta Hydrologic Laboratory present findings on eastern hemlock rates of transpiration, the amount of soil water taken up by trees.

Hemlock woolly adelgids attach themselves to the base of the needles of the eastern hemlock, feeding on carbon fixed by the trees, slowing growth and causing the needles to drop. Needle loss causes the crown of the tree to thin and dieback in branches; in a surprisingly short time – usually 5 to 10 years – the tree fades away and dies.

To estimate the impact the loss of hemlock will have on the water balance, the researchers measured transpiration rates over a range of tree sizes for 2 years. “We found quite substantial transpiration rates for individual hemlocks, with large trees transpiring as much as 49 gallons of water a day.” says Ford. “As hemlock woolly adelgid infestations increase, we expect to see at least short term reductions in forest transpiration rates. For southern Appalachian forests specifically, we estimate that eastern hemlock mortality could reduce annual forest transpiration by 10 percent, and winter and spring transpiration by 30 percent. We expect this will increase soil moisture and alter both the amount and timing of stream flow.?EUR??,,????'??

Source: Southern Research Station, USDA Forest Service.