The Downfall of Hemlock

The recent diminishing of eastern hemlock is the result of the introduction of a non-native pest, the hemlock woolly adelgid (HWA) which extracts nutrients out of the buds of the tree (Webster et al. 2012). Though they are also found in the western United States, HWA does not have as devastating an impact on the hemlock in western North America as they do in the East. Findings of a wider variety of haplotypes in western hemlock compared to eastern hemlock suggest a coevolutionary process has taken place in the West (Havill et al. 2006). The deterioration of the hemlocks has caused consequential changes in canopy structure, understory composition, stream properties, and species make-up. As the hemlocks disappear, new species taking advantage of the open canopy emerge, such as maple and birch. With this shift in species, the biodiversity of the stream is also experiencing changes. Diversity of fish species are greater in hemlock-dominated streams than hardwood-dominated streams in the Delaware River basin (Ross et al. 2003). Also, piscivorous fish, such as the Brook Trout, were found more often in hemlock streams while hardwood streams experience greater numbers in insectivorous fish. This greatly alters the success of various species and vegetation in the stream. Furthermore, hemlock provides dense shade which maintains cool temperatures and limits NPP in the stream. Birds who have relied on hemlock, such as the black-throated green warbler and the winter wren, have left the area as maple trees take over the overstory (Tingley et al. 2002). The removal of hemlock also promotes an expansion of the understory species, rhododendron. Rhododendron inhibits the seed germination of many species besides hemlock (Webster et al. 2012). 

This mass disappearance of hemlock resembles the mass disappearance of chestnut trees in the same area 100 years ago. However, while the hemlock fell to an insect, the chestnut trees died from a fungus that causes chestnut blight (Elliott and Swank 2008). Similar to the coevolution found between HWA and hemlock in some parts of the world, chestnut blight in chestnut’s original habitat in Japan have developed a coevolutionary process outside of the southern Appalachia. The blight enters through wounds in the tree and grows beneath the bark, eventually killing the tree. As the trees died off, the rhododendron grew more with the increasing availability of sunlight and further inhibited the re-growth of chestnut trees. Some shorter chestnut trees are still visible in the forest with their height limited by the persistent blight. The debris of the chestnut trees are still visible in the understory of the forest due to their incredibly slow decay rate. The varying decay rates of woody debris and litter explain the slow changes observed. Before the blight was introduced, the forest was home to species who preferred to eat the fruit of the tree, such as blue jays. There was an absence of hemlock due to the allelopathic characteristics of chestnut (Vandermust et al. 2002). The leaf litter of the chestnut controlled what could grow in the forest as it suppressed seed germination in other species such as hemlock. Once the chestnut trees succumbed to blight, however, this suppression was no longer there and created an environment for the hemlock to thrive.

Elliott, Katherine J., and Wayne T. Swank. “Long-Term Changes in Forest Composition and Diversity Following Early Logging (1919–1923) and the Decline of American Chestnut (Castanea Dentata).” Plant Ecology 197, no. 2 (August 2008): 155–72.

Havill, Nathan P., Michael E. Montgomery, Guoyue Yu, Shigehiko Shiyake, and Adalgisa Caccone. “Mitochondrial DNA from Hemlock Woolly Adelgid (Hemiptera: Adelgidae) Suggests Cryptic Speciation and Pinpoints the Source of the Introduction to Eastern North America.” Annals of the Entomological Society of America 99, no. 2 (March 1, 2006): 195–203.[0195:MDFHWA]2.0.CO;2.

Ross, R. M., R. M. Bennett, C. D. Snyder, J. A. Young, D. R. Smith, and D. P. Lemarie. “Influence of Eastern Hemlock (Tsuga Canadensis L.) on Fish Community Structure and Function in Headwater Streams of the Delaware River Basin.” Ecology of Freshwater Fish 12, no. 1 (March 1, 2003): 60–65.

Tingley, Morgan W., David A. Orwig, Rebecca Field, and Glenn Motzkin. “Avian Response to Removal of a Forest Dominant: Consequences of Hemlock Woolly Adelgid Infestations.” Journal of Biogeography 29, no. 10/11 (2002): 1505–16.

Vandermast, D. B., D. H. Van Lear, and B. D. Clinton. “American Chestnut as an Allelopath in the Southern Appalachians.” Forest Ecology and Management 165, no. 1 (July 15, 2002): 173–81.

Webster, J. R., K. Morkeski, C. A. Wojculewski, B. R. Niederlehner, E. F. Benfield, and K. J. Elliott. “Effects of Hemlock Mortality on Streams in the Southern Appalachian Mountains.” The American Midland Naturalist 168, no. 1 (2012): 112–31.