Hemlock Hydrology

A dead and dying hemlock stand in Connecticut, with black birch coming into the understory below. Photo by David Alan Orwig.

Hemlock wooly adelgid, the invasive insect from Japan that is wiping out most of the hemlock forests in the eastern United States, is also having a significant effect on the availability of surface water. The long-term implications could be serious.

That’s the conclusion drawn by University of Indiana hydrologist Taehee Hwang, who assessed the changes to water yield – the amount of water that reaches streams and rivers – over a 10-year period in hemlock-dominated stands of the Harvard Forest in Petersham, Massachusetts. Hwang said that trees damaged by the adelgid use less rainfall, allowing more water to reach the ground and run off into waterways. By measuring the rates of evaporation and transpiration in his forest plots, Hwang and postdoctoral researcher Jihyun Kim calculated that the watershed yielded 16 percent more water because of the hemlock decline.

Although an increase in water to a watershed sounds like a positive result, Hwang said that numerous other considerations come into play that make his results much less comforting.

“Water quality may suffer as rainfall runs off more quickly from forested areas and carries higher concentrations of nutrients,” he said, noting that rising water temperatures from penetrating sunlight could also affect water quality by promoting more frequent algal blooms. “And the long-term picture may change as hemlocks are replaced with broad leaved trees that have a different impact on water resources.”

Hemlocks use much less water than deciduous trees like birches and maples, and deciduous trees use most of their water during the growing season – also a critical period for human water use.

“If we have a transition from hemlock to deciduous trees, like many ecologists predict, then we’ll have a decrease in water yield, affecting the seasonal availability of fresh water,” Hwang said.

In Virginia, where the adelgid was first discovered in 1951, hemlock forests that were killed by the insect have been replaced by birches. The same thing happened recently at Devil’s Hopyard State Park in East Haddam, Connecticut, where the process took just eight years after the hemlocks died, according to Hwang.

“Surface water is extremely important across the eastern US because it provides the main drinking-water supply in many places,” Hwang said. “Our research was conducted in the headwater area of the Quabbin Reservoir, one of the primary water-supply reservoirs for the Boston area. So a decline in water availability could be serious.”

Northern New England has not yet felt the effect of the hemlock wooly adelgid infestation because the insect is less likely to thrive in the region’s cold winters. But Hwang believes that the changing climate may accelerate its move northward.

In the next step of his research, he plans to use remote sensing data to estimate how extensive the hemlock decline is in Massachusetts and calculate its net effect on the entire water basin.

“With warming temperatures and changing patterns of precipitation, where we’ll see more extreme rainfall events and more frequent droughts, we’re in for big changes in seasonal water availability,” Hwang concluded. “And the hemlock decline is just making things worse.”


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