Hemorrhaging Hemlocks

Hemorrhaging Hemlocks

A recent study has discovered that hemlocks in New York’s Catskill region are declining rapidly. Research conducted collaboratively by the U.S. Forest Service and the University of Vermont utilized ground-based research in addition to high-tech aerial imaging to survey 274 square miles of the Catskill mountains in 2012. Data suggested that only 16% of the region’s hemlock population was healthy, a 43% drop from data collected a decade prior. The decline can be almost exclusively attributed to the Hemlock Woolly Adelgid (HWA), Adelges tsugae.

Hemlock populations in the Catskill Mountains are suffering severe decline due to HWA.

Hemlock populations in the Catskill Mountains are suffering severe decline due to HWA.

 

Hemlock Woolly Adelgid is an invasive insect and a major forest pest of concern in the United States. Native to Japan, HWA was first detected in the 1950s in Virginia but was primarily found in urban settings. By the 1980s the infestation had spread to hemlock’s native range which forms a triangle from eastern Canada west to Minnesota, south through the Appalachian mountains to northern Georgia and South Carolina and back northeast to Pennsylvania. HWA targets the eastern and Carolina hemlock (Tsuga canadensis and Tsuga caroliniana, respectively) two important species of eastern forests. In the United States the woolly adelgid lacks natural predators and native hemlocks also lack natural defenses, this unfortunate pairing has resulted in the high mortality in the 18 states where this pest has made its home. The great volume of hemlock mortality has affected water quality and soil erosion in the ecosystems in which they are a critical member.

Elongate hemlock scale, Fiorinia externa, was also found in the study area, but was considered only to minimally contribute to the decline. Forest Service Forest Entomologist Ryan Hanavan said about the research methodology, “The hyperspectral imaging system helps us to identify tree species and detect stress related to insect and disease activity. It uses vegetation-specific spectral band combinations in the visible and near-infrared spectrum.” By using the spectral band combinations, they were able to construct ‘vegetation indices’ which can measure specific health indicators which allowed researchers to categorize hemlock damage. Research was conducted in plot-level measurements and then used in building larger-scale health assessments covering larger parcels of the landscape by utilizing hyperspectral imagery.

The HWA initiative is being collectively attacked by the forest service in conjunction with state forestry and agricultural agencies. Significant Forest Service resources are being focused on establishing biocontrol agents to manage HWA-impacted areas, including in New York.