Over the past four decades, the eastern hemlock (Tsuga canadensis) has been declining in health and vigor in eastern North America.  Although other factors may be involved, the major cause of hemlock decline is infestation by an introduced, sap-feeding insect, the hemlock woolly adelgid (Adelges tsugae).  Feeding by the adelgid causes hemlock needles to dry up and fall off the tree, resulting in death to the tree within 2-4 years.  Infested hemlock branches appear to have tiny, cottony masses on the undersides of the twigs where the needle attaches to the twig.  This cottony mass is the egg sac produced by the adelgid.

Defoliation of eastern hemlock (Tsuga canadensis Carriere) forest caused mainly by the hemlock woolly adelgid (Adelges tsugae Annand) was detected, quantified, and mapped for a 1,267 square kilometer study area in the New Jersey Highlands using anniversary dates of Landsat Thematic Mapper data (1984 and 1994). A model relating estimates of canopy condition to the temporal difference in near infrared/red reflectance (i.e., the vegetative index difference) was developed to predict and map four classes of hemlock condition across the study area.

Data from 105 circular ground plots (90 meter diameter) were used to develop the regression model, while data from 50 plots were reserved for accuracy assessment. The vegetative index difference was highly correlated to hemlock damage as measured on the ground (R squared = 0.73). Lightly defoliated hemlock canopy did not differ spectrally from healthy hemlock, thus these two classes were joined together.

Accuracy assessment showed that hemlock condition can be predicted within one-half damage class with an overall accuracy of 64% for four damage classes, 70-72% for three classes, and 78-92% for two classes. Of the 7,735 hectares of hemlock forest in 1984, 47% remained healthy to lightly defoliated, 44% had experienced moderate to severe defoliation, and 9% were dead by 1994.

B. This is a 1994 Landsat image of Sparta Glen Park, in Sparta, NJ. Many hemlock trees on the western half of the park have died from HWA infestation. Defoliated and dead hemlock forest appears as a decrease in the intensity and number of red pixels.

• Aerial oblique view of dead hemlock stand on Sparta Mountain in Ogdensburg, NJ (July 1995)
• Close-up of hemlock woolly adelgid on hemlock branch
  
  

• USDA Forest Service: Northeast Area Hemlock Decline Research Information
• VA Tech Photos and characteristics of the eastern hemlock (expired link)
• Factsheet VA Tech Entomology Department (expired link)
• In Southern Connecticut, Yale University School of Forestry & Environmental Studies
• CRSSA Participation in the 2001-02 NY/NJ Highlands Regional Assessment Update
• Sterling Forest: A Landscape Ecological Analysis (CRSSA past project within the Highlands region)
  

Denise Royle, royle@crssa.rutgers.edu, 732 932 1582
Grant F. Walton Center for Remote Sensing and Spatial Analysis, Rutgers University