New Insights on Changing Hemlock Forests

Three recent studies co-authored by Harvard Forest ecologist David Orwig detail the impacts of the invasive hemlock woolly adelgid (HWA) on forests, carbon, and the New England economy.

In a paper published in Southeastern Naturalist, researchers estimated the financial loss caused by the decline of hemlock trees in a 2,800-square-mile area across 9 counties in central Connecticut and Massachusetts. Aside from the value of harvestable timber, mature hemlock trees contribute scenic beauty, dense shade, stream-cooling, and other values to communities. During the 5-year study period (2007-2011), the number of households that were financially affected by hemlock decline tripled, and property values declined by $24.6 million.

A study published in the journal Biological Invasions investigated the same CT-MA study region for 14 years (1997 to 2011), assessing damage caused by 4 invasive insects that feed on hemlock trees, including the hemlock woolly adelgid. The study revealed the complex relationships between these insects - finding, for example, that the presence of one pest, the elongate hemlock scale, may discourage another pest, the woolly adelgid, from colonizing a hemlock tree. Furthermore, the density of elongate hemlock scale is strongly influenced by the presence of a third pest, the circular hemlock scale. These results highlight the importance of considering multiple pests when assessing the impacts of biological invasions in a community.

A third study published in Ecosphere explored the impacts of the hemlock woolly adelgid in intact hemlock stands and in post-adelgid hemlock stands now dominated by young black birch trees, including the Harvard Forest's long-term Hemlock Removal Experiment. Their data on carbon fluxes, tree growth, foliage production, soil microbe activity, and nitrogen content in plant tissues and soils gave an ecosystem picture of post-adelgid forests. Results included that the young birch stands showed higher primary production than the hemlock stands they replaced, and no difference in the amount of carbon being released from soils. These results suggest good news: if allowed to re-forest naturally, stands infested by the woolly adelgid in southern New England should remain a "sink" for atmospheric carbon dioxide, despite reorganization of stand structure and species composition. 

• Learn more about hemlock studies at the Harvard Forest.

• Read the scientific papers:
• Potential social and economic impacts of the hemlock woolly adegid in Southern New England, by Li et al. in Southeastern Naturalist
• Interactions between invasive herbivores and their long-term impacts on New England hemlock forests, by Gomez et al. in Biological Invasions
• Net primary production and soil respiration in New England hemlock forests affected by the hemlock woolly adelgid, by Finzi et al. in Ecosphere

(Photo of hemlock sapling by David Foster.)