Grant F. Walton Center for Remote Sensing and Spatial Analysis, Rutgers University
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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.

Photo of severely defoliated hemlock canopy in Sparta Glen Municipal Park, Sparta, NJ (March 1995)

Adelgid infestation map as of 1995 (source: US Forest Service)

Infestation by hemlock woolly adelgid (HWA) is thought to have begun in Richmond, Virginia around 1953.  Dispersed by wind, birds, mammals, and even humans, HWA has spread into hemlock forest stands in eleven states from Massachussettes to North Carolina.  The hemlock woolly adelgid infestations appeared in New Jersey during the mid-1980's and caused significant mortality to some of the more noted stands in the state by the early 1990's.  The map illustrates the spread of HWA infestation (red) as of 1995 (map source: US Forest Service)..



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.


  A. This is a 1984 Landsat TM image of Sparta Glen Park, in Sparta, NJ. Each pixel has a ground resolution of 30 meters. Healthy hemlock forest pixels appear red in color. This is the NIR/red reflectance pattern for healthy hemlock forest. Neighboring deciduous forest pixels appear blue-green or gray in color. Water appears black.  

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.






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


 Rutgers University
Grant F. Walton Center for Remote Sensing and Spatial Analysis
14 College Farm Road
Cook College, Rutgers University
New Brunswick, New Jersey 08901-8551
732 932 1582

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