Hemlock Decline in the New Jersey Highlands Region

Mapping and Monitoring Eastern Hemlock Defoliation Due to the Hemlock Woolly Adelgid

A Research Effort by Dr. Denise Royle and Dr. Richard Lathrop at the Grant F. Walton Center for Remote Sensing and Spatial Analysis, Cook College, Rutgers University


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.

Using Landsat Thematic Mapper Data and Change Detection Techniques to Monitor Hemlock Forest Health (Forest Science 43(3): 327-335)

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.




Denise Royle, , 732 932 1582
Grant F. Walton Center for Remote Sensing and Spatial Analysis, Rutgers University

Web site by Denise Royle, Rick Lathrop, John Bognar. Last modified 2 October 2002.