Chestnuts used chemicals to dominate southern Appalachian forests

USDA Forest Service research confirms that chemicals in the leaves of the American chestnut suppress the growth of other trees and shrubs-and probably played a part in the species’ past dominance of the southern Appalachian forest.

Southern Research Station ecologist Barry Clinton (Coweeta Hydrologic Laboratory)-with fellow researchers from Clemson University and the University of North Carolina-Chapel Hill-tested the effects of fallen chestnut leaves on five tree species that historically competed with the American chestnut before chestnut blight destroyed almost all of the great trees.

“American chestnut reached its greatest size and stand density in the southern Appalachians, where it may have taken up almost 50 percent of the forest canopy,” said Clinton. “Chestnut’s dominance has traditionally been attributed to its rapid growth rate, resistance to rot and fire, and ability to thrive on poor soil. Our experiments show that allelopathy may also have contributed to its dominance.”

Allelopathy is the secretion by plants of chemicals that inhibit the growth or reproduction of competing plant species. Black walnut is a prime example of allelopathy: the tree produces the chemical Juglone, which suppresses the growth of trees, shrubs, and other vegetation. Other allelopathic trees include sycamore, eucalyptus, and hackberry.

Clinton and his fellow researchers tested the effects of an extract made from the leaves of young American chestnut trees on the seeds of red maple, eastern white pine, eastern hemlock, yellow-poplar, and the native shrub rosebay rhododendron. Under controlled laboratory conditions, the researchers found that the extracts inhibited the germination of eastern hemlock and rosebay rhododendron. Eastern hemlock is a major species along the mountain streams of the Southern Appalachians. Rosebay rhododendron has become the dominant shrub on moist sites, where it interferes with hardwood regeneration and threatens the diversity of cove forests.

“Our results suggest that chestnut may have had a controlling effect on rhododendron germination and growth in the past,” said Clinton, “and that the rapid encroachment of this shrub in the 20th century may be largely due to the end of the tree’s allopathic influence.”

One of the most interesting results of the experiment was the ability of chestnut leaf extract to suppress the germination of eastern hemlock, which has steadily migrated into the chestnut-blighted areas of southern Appalachian forests.

“We have anecdotal evidence that eastern hemlock, which also has allelopathic qualities, can inhibit chestnut seeds from sprouting,” said Clinton. “Historical accounts show that these trees rarely occurred together in pre-blight forests. We are starting to get a picture of the dynamic competitive relationship that once existed between these two important southern Appalachian tree species.”

Results from the experiment on chestnut allelopathy are published in the July 15, 2002 issue of Forest Ecology and Management, which is available from the SRS website in full text formats at http://www.srs.fs.fed.us/pubs/viewpub.jsp?index=4723.

For more information: Barry Clinton at (828-524-2128) or bclinton@fs.fed.us