Silvicultural treatments may reduce impact of hemlock woolly adelgid
Eastern hemlock (Tsuga canadensis) is a foundational species in forests across the eastern United States. Hemlock trees provide critical ecosystem services, such as shading for streams, regulation of water temperature, and wildlife habitat for many species. Hemlocks are also economically important for the forest products industry, especially for paper and pulpwood.
The invasive insect, hemlock woolly adelgid (HWA, Adelges tsugae Annand), has killed millions of hemlocks across the eastern United States and altered forest dynamics since it was first discovered in 1950. In response, forest managers have sought proactive silvicultural strategies to strengthen hemlock resilience before infestation occurs.
Researchers at the U.S. Department of Agriculture, Forest Service, published a new study on improving hemlock resilience. Mary Ann Fajvan (Northern Research Station) and Andrea Hille (Allegheny National Forest) studied how thinning hemlock stands might be used as a proactive strategy against HWA establishment and spread by improving tree vigor and increasing light availability.
Study Overview
The researchers initiated a long-term study in 2005 on the Allegheny National Forest in Pennsylvania to determine whether thinning hemlock forests can improve individual hemlock tree health and growth. Specifically, they studied whether thinning could improve crown health, which they measured through changes in diameter growth, live crown ratio, and crown area.
The study comprised three mixed hemlock-hardwood stands with high hemlock densities. Half of each plot received thinning treatments. The other half served as a control (no thinning). Thinning treatments targeted trees for crown release by removing competing tree species on three to four sides.
Fajvan and Hille measured 96 hemlocks within each thinned plot and 90 hemlock trees in untreated plots. They took measurements of diameter, total height, live crown ratio, and crown area before treatment and at years 5 and 10.
Subject tree crown after thinning where crown was released on three sides in the Allegheny National Forest. USDA Forest Service photo by Mary Ann Fajvan.
Thinning Trees for Resilience
The authors reported three primary findings:
- Diameter changes—After 10 years, trees in thinned plots were, on average, about 5 centimeters larger in diameter at breast height (DBH) than those in the control plots. Although the diameter changes were not statistically significant between thinned and control plots, the finding suggests that thinning can enhance hemlock growth rates.
- Live crown ratio—After 5 years, live crown ratio was maintained in thinned stands and decreased significantly in control plots. Higher live crown ratio has been correlated with increased resistance to HWA, suggesting that maintaining live crown ratio through thinning could contribute to hemlock resilience.
- Crown area—Crown area increased significantly in thinned plots and decreased in control plots. A larger crown can improve a tree's ability to tolerate stresses, including insect infestations.
While recognizing more research is needed to fully understand the effects of thinning on HWA populations, the study found that preemptive thinning in hemlock stands may be a viable strategy to enhance hemlock resilience prior to HWA infestation. This information can be part of a strategy for forest managers to prepare for the arrival of HWA.