The impact that hemlock tree die-offs have had — and continue to have — on freshwater forest ecosystems is the focus of a research project at the University of Maine.
Hamish Greig, a UMaine assistant professor of stream ecology, and Jacquelyn Gill, an assistant professor of terrestrial paleoecology at the Climate Change Institute (CCI) and the School of Biology and Ecology, are leading a research team that is studying past and present declines of the conifers known for their dense shade. The resulting biomass the dying trees introduce into the watershed, as well as the other tree species that take their place on the forest floor, affect freshwater systems, including streams and lakes.
Understanding those implications is particularly important in Maine, where hemlocks are now being threatened by the same exotic pest that, in recent years, has decimated the tree species in the southeastern United States.
“People in Maine have a huge affinity to their rivers and lakes. It’s huge economically; it’s huge socially, and through recreational activities,” says Greig, who is joined on the research team by research assistant professor Krista Caps, postdoctoral scientist Robert Northington, as well as several graduate, undergraduate and high school students.
About 5,500 years ago, the hemlocks of eastern North America sustained a massive die-off that lasted about 1,000 years, brought on by severe drought and the hemlock looper, a native pest, Gill says. Today, the tree species has been nearly decimated in the southeastern United States by the hemlock woolly adelgid, an exotic insect from Asia.
Maine’s cold winters typically protect against exotic pests. However, warmer temperatures have allowed exotic pests to thrive and move north. Since 2004, the hemlock woolly adelgid has been in southwestern Maine. This year, it has made it as far north as Owls Head, according to the researchers.
“As the climate warms, there won’t be anything preventing the woolly adelgid from hitting our hemlocks in Maine as hard as they’ve been hit elsewhere,” Gill says.
As part of their study, the research team has set up 36 livestock water tanks as experimental freshwater mesocosms, or isolated experimental environments. Hemlock needles, along with rhododendron and maple leaves, have been added to the ecosystems to observe what happens when a hemlock dies.
The mesocosms allow the scientists to study these isolated environments as they develop over time — in this case, into the fall.
“You can’t really control something in a natural lake,” Greig says. “And if you do experiments in the lab, you’re really simplifying things down to two or three species of invertebrates. By having this happy medium, we can have natural complexity with the controlled replication of a true experiment.”
Next, Gill and Northington will study radiocarbon-dated records from the bottom of lakes and bogs in southeastern, coastal and central Maine regions to help understand how aquatic systems were affected by hemlock die-off in the past. By linking the paleo record with a modern experiment, the team hopes to will new light on hemlock’s role in changing ecosystems.