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Understanding the Impact of Hemlock Woody Adelgid on Forest Ecology    

Coring trees gives researchers precise dates on the age of a forest. Jessamine Finch ’12  and graduate student Kaila Matatt ’10 work to get an increment borer into the hard wood of a red oak.

/ Published September 9, 2010

There's nothing as inviting as the cool shade of a stand of hemlock trees stretching out along a forested trail on a sunny day. Unfortunately, this might not be an option for much longer: in the northeastern United States, the invasive pest known as hemlock woolly adelgid is killing off these inviting shady evergreens, the eastern hemlocks. The bug arrived in western Massachusetts some 10 years ago and is steadily making its way into southern Vermont and New Hampshire, destroying hemlocks along the way; infested trees often die within a few years.

Now a team of Smith professors and students is examining the changes taking place as the hemlock trees die and studying the long-term implications if the trees are succeeded by another species such as black birch. The fieldwork is under way at Smith’s Ada and Archibald MacLeish Field Station, 200 acres of mixed farmland and woodland and located just 12 miles north of campus, in Whately, Massachusetts.

The Changing Forest: As an invasive pest spreads among hemlocks and the trees begin to die out, Smith researchers are looking at what that will mean to the soil composition of the forest floor and the water cycles in hemlock and deciduous forests of New England.

Scientific field research can be messy and dirty, but Smith undergraduates working this summer at MacLeish welcomed the opportunity. Indeed, collecting data under the dense canopy of 100-year-old trees can be a welcome assignment, especially when it is July and it is hot and humid back in the city of Northampton and on the Smith campus.

For Stacie Mansen, an Ada Comstock scholar, and Ellen Maley ’11, it's just another day in the field. In their case, the field is a patchwork of forest, meadows, vernal pools, the occasional pile of bear scat and remnants of old lead and zinc mines. To get to their research plots that muggy morning, they hiked along an old dirt roadway bordered by century-old stonewalls until they cut into the forest along a flagged trail that brings them to the marked stands of hemlock and black birch trees. The undergraduates carry their field notebooks, first-aid kits, a cooler, water, snacks and various digging implements that usually included shovels and pick axes.

“A lot of this soil is pretty rocky,” says Maley, applying a bandage to her shin where she had banged it with a shovel. “This is land sitting on top of glacial till. We're looking at climate change and forest succession due to the invasion of the woolly adelgid.”

For 10 weeks this summer, Mansen and Maley were at MacLeish digging 3-foot-by-3-foot pits in plots of black birch and hemlock trees and collecting soil samples from the layers. They then took the samples back to the lab where they were analyzed for ammonium and nitrate contents.

Maley and Mansen, both geoscience majors working through Smith's Summer Research Fellows program and under the direction of Associate Professor of Geosciences Amy Rhodes, were part of Team Soil. They were collaborating with Andrew Guswa, associate professor of engineering, and Jesse Bellemare, assistant professor of biological sciences, and their teams of students to investigate the differences in nutrient cycling in soils underlying different species of trees and to predict how the nitrogen, phosphorous and other nutrients in the soil may change as a result of forest succession from hemlock trees to black birch.

The broad goal is to assess the hydrologic cycle and how it will impact soil moisture and water uptake and measure the differences in hydrologic fluxes between hemlock and deciduous forest stands. Ultimately, the ongoing project will help researchers determine how nitrogen cycling in soils, the hydrology and microclimate of the forest might be altered should the hemlock species die out.

Top: Engineering majors Jake Pecht ’12 and Meghan Mussehl ’12 collect and measure rainwater as it falls through the thick canopies of hemlock trees. Bottom: Back in the Smith lab, Stacie Mansen, an Ada Comstock scholar, sieves through the samples she has collected from soil pits at the MacLeish Field Station.

Only a 10-minute hike away, under the dense outstretched branches of hemlocks and along another flagged trail blanketed by dry leaves, Team Hydro did their own measurements and sample gathering. Engineering majors Meghan Mussehl ’12 and Jake Pecht ’12 worked in a spot where the forest floor is mapped out in plots, marked by grids of string and graduated cylinders that collect and measure rainwater as it falls through the thick canopies of trees.

“As hemlock forests die out, new growth is intruding. This is going to affect forest ecology and hydrology,” Mussehl says. Their hydrologic research project measures both throughfall, the amount of rain that falls to the ground, and sap flux, the fluid transported through plant tissues.

The impact of the hemlock woolly adelgid on the hemlock forests and the water cycle in New England could be significant, according to Associate Professor of Engineering Andrew Guswa and director of the Smith Center for the Environment, Ecological Design and Sustainability (CEEDS). The destruction of eastern hemlock forests by the invasive pest has the potential to transform the water cycle throughout New England.

Analyzing and collecting sap flux involves the methodology of tree coring, in which researchers have to drill into the trunk of a tree to allow for insertion of a probe. This comes with its own technological challenges, says Pecht with a smile. “We've broken six drill bits so far trying to core.”