Overview of hemlock health

Abstract

Although many insects and diseases are associated with hemlock, we will, ironically, draw our first conclusion, that hemlocks are very sensitive to the stress of insect defoliation, from a tale of gypsy moth defoliation. The unprecedented gypsy moth (Lymantria dispar L.) outbreak of 1981, when nearly 13 million acres were defoliated in the Northeast (Fig. 1), resulted in hemlock snags throughout the hardwood forest at West Point, New York (and many other locations), in 1986. There were so many larvae in May 1981 that all the preferred hardwood leaves were eaten and the caterpillars still needed more foliage. They turned their hungry attention to the white pine (Pinus strobus L.) and eastern hemlock (Tsuga canadensis (L.) Carriere) trees that grew in association with the hardwoods and, in many cases, defoliated these conifers completely. Hemlock and white pine responded to this defoliation in very different ways (Stephens 1984). Hemlock trees died within 1 year and most of these were dominant or codominant trees. In contrast, many fewer white pine trees died and the trees that did die succumbed slowly-within 5 years. In addition, these white pine were mostly intermediate or suppressed trees. Stephens (1984) hypothesized that the species impacts were caused by differential bud development patterns and by how much bud (1982 growth) damage each species sustained.

Parent Publication

• Proceedings: symposium on sustainable management of hemlock ecosystems in eastern North America

Citation

Souto, Dennis R.; Shields, Kathleen S. 2000. Overview of hemlock health. In: McManus, Katherine A.; Shields, Kathleen S.; Souto, Dennis R., eds. Proceedings: Symposium on sustainable management of hemlock ecosystems in eastern North America. Gen. Tech. Rep. NE-267. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 76-80.