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.
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.
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 km2 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 (R2 = 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.