Filling in the Gaps: Understanding the Effects of Hemlock Decline at MacLeish

With climate change altering species’ distributions as suitable habitats move, and with our increasing rates of global travel, biological invasions are becoming more widespread.  The hemlock woolly adelgid (Adelges tsugae), for example, an exotic insect accidentally introduced to eastern North America that feeds on eastern hemlock trees (Tsuga canadensis), has been steadily moving northward as a result of a warming climate and relatively easy transport via the wind, deer and human activity.  The woolly adelgid has now just reached the Ada and Archibald MacLeish Field Station, where the hemlocks that dominate the forest there are starting to show signs of infestation.  Hemlocks can’t defend themselves against these insects and can die within as few as four years of becoming infested, leaving skeletal forests behind.

Woolly adelgid on an eastern hemlock tree at MacLeish.

The decline of hemlocks is of particular concern because eastern hemlock is a foundation species.  This means that the species (e.g., eastern hemlock) influences the conditions of the ecosystem they inhabit in such a way that their presence helps determine which other organisms can live there.  Because hemlocks create a unique “microclimate” in the forests they dominate – one that is cool, dark, and damp – and tend to acidify the soils beneath them, the removal of this species and replacement by deciduous tree species is bound to produce large changes in the forest.  These changes will be both in environmental conditions and ecosystem processes as well as in the animal and plant communities that inhabit these forests.

I started working on the question, “What are long-term effects of hemlock removal on New England forests?” early in my Smith career and, after completing a senior thesis on this topic, I am now spending the summer before grad school as a SURF intern continuing to collect long-term data.

The Field Station offers a unique opportunity for us to look at what forests devastated by adelgid infestations might look like in twenty years, a result of logging that occurred before MacLeish became a research site.  In the late 1980s, patches of hemlock trees were logged as part of a larger cut to help promote the growth of maple and other economically valuable trees.  Young black birch trees (Betula lenta) have since grown in these logged patches.  This logging activity produced an “accidental experiment” for us to use, in that we can compare environmental conditions in the young black birch patches to those in the hemlock-dominated forest next to them.  This data will help us predict what the forests in New England may look like decades after the wooly adelgid infestation causes hemlock populations to decline and disappear. 

Several labs at Smith have been using these plots and exploring this question.  I’ve been working since my junior year with both Jesse Bellemare in Biological Sciences and Amy Rhodes in Geosciences, and with several other students in the professors’ labs, to predict how nutrient cycling and other ecosystem processes change with the loss of eastern hemlock.  I examined several facets of this question for my thesis, exploring precipitation chemistry, leaf litter decomposition, and soil nutrient cycling.  This summer, I am primarily continuing our work on soil nutrient cycling, comparing nitrogen cycling between a hemlock forest, a young black birch forest, and a mature deciduous forest.

To do this we measure the production of nitrate (NO₃^–) and ammonium (NH₄⁺), nitrogen-containing compounds that plants can use and that are produced when microbes digest leaf litter on the forest floor, by performing regular soil incubations.  For each incubation, we take soil cores from each plot and measure “initial” concentrations of nitrate and ammonium in our lab at Smith.  For each soil core we take back to lab, we leave a second soil core, obtained next to the first, in the ground in a PVC sleeve.  We come back to the forest after at least three weeks, bring those incubated cores back to lab, and measure the “final” concentrations of nitrate and ammonium in those samples.  We then use the difference between the initial and final nitrogen concentrations to calculate the rate at which these compounds are being produced in the soil, and use these rates as a quantitative way to compare nutrient cycling between the three forest types.

Soil cores ready to go back to the lab for processing (left), and a soil core incubating in a PVC sleeve in the field (right).

Our research hasn’t always gone as planned.  In 2010, professor Rhodes’s lab established their hemlock plot as one representing a “healthy, mature hemlock forest.”  Last year, however, we started to notice the adelgid near this plot, and began to see an increase in the amount of nitrogen produced in the soil as well, a typical sign of adelgid invasion.  This summer we want to see if this trend is still evident, and though we can’t really think of this hemlock plot as our “healthy hemlock forest control” any more, now we can track how the soil changes at the beginning of an infestation, using the data collected before the infestation as our baseline.

Our work and the work of other Smith students and professors will contribute to the growing body of literature on the effects of the hemlock woolly adelgid infestation on our New England forests.  This will help scientists, foresters, and policymakers confronted with the daunting task of dealing with the adelgid infestation to make more informed management decisions.  And though I’m not actively working to eradicate the adelgid, I’m proud to be doing my own small part, gathering information that might be useful for managing this invasion and other similar biological invasions as well.

–       Jenna Zukswert ‘13

Jenna graduated in May with a double major in Biological Sciences and Environmental Science and Policy.  She will be attending the University of British Columbia in Vancouver this fall to pursue a Master of Science degree in Forestry. She hopes to pursue a career in science education.