Cryphonectria parasitica:
Its effect on
Southern Appalachian landscapes
The forest landscapes of the Southern Appalachians have existed primarily of the same vegetation components in varying mixtures and distributions for roughly 85 million years. Nonetheless, change is the only factor that has remained constant over this period of time. People are probably the most important factors of change since arriving to the region over 9000 years ago. Since the middle of the 19th century human disturbance, in the particular the use of fire, land clearing and agriculture, logging, industrialization, urbanization, and the suppression of fire, has resulted in Southern Appalachian forest landscapes that bear little to no resemblance to the forest landscapes of the past. Possibly the most extreme case of ecological disturbance in the Southern Appalachians occurred with the introduction of the chestnut blight fungus (Cryphonectria parasitica) (Oak, 2002).
Cryphonectria parasitica, (formerly Endothia parasitica) entered the Unites States on imported Japanese chestnut trees (Castanea crenata) sometime during the late 1800s. After its importation the disease was able to spread from its native Japanese host to the American chestnut tree (Castanea dentata) when the Japanese trees were sold as nursery stock through local merchants and mail-order supply companies. The first case of the fungus was reported in 1904 in New York City and within 50 years the disease had spread through the native range of C. dentata killing nearly all of the mature trees (Anagnostakis, 2000). Even though the disease was mainly spread from the vicinity of New York City in 1904 there is evidence that it occurred in other localities at earlier dates. For example, observations in Newark, Delaware (1902) and in an orchard in Bedford County, Virginia (1903) indicate that the disease may have already been present (Anagnostakis, 1997). The chestnut pathogen was able to spread quickly throughout the chestnut’s native range since the tree lacked co-evolved disease resistance (Oak, 2002). Today, only sprouts from the base of the trunk enable the species to evade extinction. However, C. parasitica usually kills the sprouts before they are sexually mature and able to cross pollinate and produce seeds. Consequently, C. dentata has been unable to evolve any real resistance to the disease (Anagnostakis, 2000).
The Oak has since been elevated to the most dominant tree species group in the Southern Appalachians with the elimination of American Chestnut trees as a canopy species. The history of disturbances, in particular the fire regime of light ground fire followed by suppression of fire by Native Americans and European settlers to reduce understory vegetation and promote browse for game, allowed aggressive sprouters like the American chestnut and oaks to build up large reserves in the understory. As the chestnuts died off due to the blight and aggressive fire suppression was implemented, oaks which already occupied the mid- and understory quickly occupied the newly available growing space (Oak, 2002).
The change from a predominately American chestnut dominated forest to an Oak dominated forest occurred over a relatively short time span on millions of acres in the Southern Appalachian Mountains. Due to state-federal cooperative fire control programs, public land acquisition to form national forests and parks, and lower rates of harvest, Oak forests have been able to age for 70 to 90 years relatively free of disturbance. Only when the American chestnut is re-introduce as a functioning component of the ecosystem, i.e. reinstating a once native biotic resource at its original position thus providing habitat for native species, can the Southern Appalachian forest ecosystem be considered healthy (Oak, 2002).
Comparison of Southern Appalachian forest composition: structure, disturbance characteristics, and values perspective; pre-1900 vs. current |
|
PRE-1900 |
CURRENT |
Composition American Chestnut |
Composition Oak |
Relatively Young and More Complex Age Structure |
Cohorts 80-100 Years Old |
Sparse Understory |
Dense Understory |
Widely Spaced, Large Diameter Overstory |
Dense, Small Diameter Overstory |
High Disturbance (Fire, Farming, Logging) |
Low Disturbance (Fire Suppression) |
Small, Dispersed Human Population |
Large, Urbanized Human Population |
Forest Utilization Perspective |
Ecosystem Protection Perspective |
(Oak, 2002)
Before the introduction of C. parasitica the native range of C. dentata was greater than 800,000 km2. It is believed that C. dentata made up 40-50% of the forest canopy in portions of its range in Appalachia. Its native range extended from southern Maine and Ontario in the north to Georgia, Alabama and Mississippi in the South. The tree now exists primarily as stump sprouts due to the fact that the blight does not infect the root system. Since sexual reproduction is rare, the species is likely to face serious erosion of its gene pool when old shoot systems fail to send up new sprouts and perish (Kubisiak, 2003).
The fungal disease is spread from tree to tree via spores on the feet, fur, and feathers of various insects and animals that walk across the cankers. Likewise, sexual spores that shoot into the air after rain storms serve as another means of infection (Anagnostakis, 2000). The fungus first enters the tree through wounds whereupon it grows in and under the bark. In an attempt to control the infection the tree eventually girdles itself. In time the cambium all the way around the twig, branch, or trunk is killed and everything distal to the canker dies. Since the fungus does not enter into the root collar the tree is able to survive by sending up sprouts, which in time are infected by the fungus and the cycle begins anew (Anagnostakis, 1997). The blight cycle of sprouting-infection-death-sprouting varies depending on the location of the tree. The cycle takes approximately 10 years for trees in full sun of a clear cut and up to 40 years for trees in the understory of a forest (Anagnostakis, 2000).
To control the spread of the fungus various methods were employed including chemical treatments and clearing and burning chestnut trees around infection sites. These early attempts proved to be less than successful (Anagnostakis, 2000). In 1965 Jene Grente discovered Hypovirus-infected strains of C. parasitica in recovering stands of European chestnut trees (Castanea sativa) in Italy. Grente proposed that these strains, which he named hypovirulent, be used as a biological control agent against the chestnut pathogen. Hypovirulence is a virus of the fungus that allows the defense systems of the tree to restrict the fungus to the outer bark due to the reduced virulence of the fungus by the virus (Anagnostakis, 2000). Hypovirulence has dramatically reduced the severity of the chestnut blight in European chestnut stands. However, little progress has been made in North America despite numerous attempts of releasing virus-infected strains into C. parasitica populations (Milgroom, 1994).
In an attempt to turn the tide, various breeding programs have been developed with the goal of reintroducing blight resistant American chestnut back into its native range (Jacobs, 2004). Initial breeding efforts were begun in 1922 by the USDA followed by breeding programs at the Connecticut Agricultural Experimental Station in 1930, which were sponsored by the Brooklyn Botanical Garden. Breeding programs at the Station consisted of crossing American chestnut trees that were thought to have some blight resistance with resistant Chinese and Japanese chestnut trees. These early efforts proved to be unsuccessful with the offspring having poor form and often dieing when planted in forest plots. Between 1969 and 1975 the largest planting of hybrids (10,000) occurred in the Lesesne State Forest in Virginia. However, a 1981 survey of 5000 of the original hybrids found only eight that had desirable growth, form, and blight resistance (http://ipm.ppws.vt.edu/griffin/breed.html).
More recently the American Chestnut Foundation (ACF) and the American Cooperators Foundation (ACCF) both began their own breeding programs. The ACCF in cooperation with Virginia Tech has developed what they call “all-American intercrosses” that combine the resistance from several native American chestnut trees all of which survived the original blight and have tested positive for blight resistance. These all-American intercrosses have since been grafted into selected aging clearcuts in the Jefferson and George Washington Forests in Virginia and several locations in West Virginia, North Carolina and Maryland (http://ipm.ppws.vt.edu/griffin/manage.html). In 1983 the ACF was founded with the goal of developing “a blight-resistant American chestnut tree via scientific research and breeding…” and for restoring “the tree to its native forests along the eastern United States”. In 1989 the ACF established the Wagner Research Fame in Meadowview, Virginia. The purpose of the facility is to “breed blight resistance from the Chinese chestnut tree into the American chestnut tree, while maintaining the American chestnut’s characteristics”. To date the Wagner Research Farm has been able to produce an American-Chinese hybrid that contains at least 94% American genes. The ultimate goal is to produce an American chestnut tree that contains no Chinese characteristics except blight resistance. The ACF believes it will have seeds of this American-Chinese hybrid ready to plant sometime during 2006 (http://www/acf.org/About.htm).
Once an appropriate chestnut tree is developed, identification of a suitable reintroduction site is critically for its survival. A reintroduction site should be one that avoids environmental stresses such as drought, frost, and cold air drainage routes due to the fact that the trees will be challenged by the blight. It’s believed that the best sites will include those that contain as many of the following characteristics as possible: presence of large chestnut stumps or snags, shallow coves, slopes facing north to east, elevations of less than 2000 feet to reduce stress caused by winter temperatures, full morning sun, well-drained land, soils with pH 5 to 6 (http://ipm.ppws.vt.edu/griffin/accfhab.html)
Only time can tell if the American Chestnut will regain the dominant stature it once had some 50-100 years ago.
References
Anagnostakis, S. L. (2000). American chestnut sprout
survival with biological control
of the chestnut-blight fungus population.
Forest Ecology and Management.
152: 225-233.
Anagnostakis, S. L. (1997). Chestnuts
and the Introduction of Chestnut Blight.
http://www.caes.state.ct.us/FactSheetFiles/PlantPathology/fspp008f.htm. The
Connecticut Agricultural Experimental
Station.
PP008 (7/2004)
Anagnostakis, S. L. (2000). Revitalization of the Majestic
Chestnut: Chestnut Blight
Disease. http://www.apsnet.org/online/feature/chestnut/top.html. ASPnet.
(6/2004)
Griffin, L. American Chestnut Cooperators’ Foundation:
American Chestnut Habitat.
http://ipm.ppws.vt.edu/griffin/accfhab.html.
(6/2004)
Griffin, L. American Chestnut Cooperators’ Foundation: Breeding For Blight
Resistance. http://ipm.ppws.vt.edu/griffin/breed.html. (6/2004)
Griffin, L. American Chestnut Cooperators’ Foundation:
Restoration Efforts –
Managing Aging Clearcuts For
American Chestnut Revival.
http://ipm.ppws.vt.edu/griffin/manage.html. (6/2004)
Jacobs, D. F. & Severeid, L. R. (2004). Dominance of interplanted American
chestnut (Castanea dentata) in southwestern Wisconsin, USA. Forest
Ecology and Management. 191: 111-120.
Kubisiak, T. L. & Roberds, J. H. (2003). Genetic Variation In Natural Populations
Of American Chestnut. Journal of the American Chestnut Foundation.
XVI (2): 42-48.
Milgroom, M. G. (1994). Population biology of the chestnut blight fungus,
Cryphonectria parasitica. Canadian Journal of Biology. 73:
S311-S319.
Oak, S. W. (2002). From The Bronx To Birmingham: Impact of Chestnut Blight
and Management Practices on Forest Health Risks in the Southern
Appalachian Mountains. Journal of the American Chestnut Foundation.
XVI (1): 32-41.
The American Chestnut Foundation. History of the American
Chestnut Foundation.
http://www.acf.org/About.htm (7/2004)