Chestnuts in Virginia

historic range of the chestnut, before the blight
historic range of the chestnut, before the blight
Source: US Forest Service Atlas of United States Trees

A century ago, 20-25% of the trees in the Appalachian forests of Virginia were American chestnuts (Castanea dentata). It was the dominant "keystone" species that shaped the development of the other plants and animals around it.

The chestnut grew slowly as an understory tree, waiting for a storm or other event to create a natural clearing and provide needed sunlight. Once the sapling was in direct sunlight, it could put on a rapid growth spurt and exceed the height of nearby oaks and maples, then drop nuts for future chestnut saplings that would wait their turn and maintain the dominance of the species in the mountain forest.

Because the chestnut tree flowers develop late, long after spring frosts, Virginia's turkeys, deer, bears, and other forest wildlife could rely upon a steady supply of nuts for food. Vast amounts of protein covered the forest floor, after nuts fell each fall. The nuts of the chestnut tree were so rich in protein and oils, colonial settlers often left mature chestnut trees in their woodlots to produce annual nut crops for both people and pigs. Collecting nuts in the winter provided farmers a cash crop in a season when the farmers had were not already busy with planting, weeding, or harvesting.

When harvested, the bark was rich in tannins, used for making hides into leather. The chestnut wood was easy to split and highly-resistant to rot. Even today, there are skeletons of 100-year old chestnuts on the ground in the Blue Ridge, still resisting decay.

However, the living trees could not resist Cryphonectria parasitica (known until 1978 as Endothia parasitica), a fungus common in China and Japan. The chestnut blight was first discovered in chestnuts growing at the Bronx Zoo in 1904. William Murrill, a Virginia mycologist working at the New York Botanical Garden and known as "Mr. Mushroom," determined the cause. The blight spread throughout the eastern United States, with spores spread by wind and birds. It took about 20 years to reach Virginia.1

By 1914, the blight had reached Northern Virginia. By 1925, it was blazing through the Blue Ridge into North Carolina, killing virtually every overstory chestnut tree in its wake and spreading south and west at a rate of 24 miles per year. In a short 40 years, the tiny fungus killed between three and four billion trees and completely dismantled the forest landscape in more than 33 million acres of the southern Appalachians.

wild chestnut (Castanea dentata) on Warspur Trail near Mountain Lake (Giles County)
wild chestnut (Castanea dentata)
on Warspur Trail near Mountain Lake (Giles County)

Perhaps four billion chestnut trees were killed by the foreign fungus in 50 years. By 1950, nearly all mature chestnuts across the Eastern United States were dead.2

The American chestnut could cope with American forms of the fungus, growing "wound tissue" to isolate a part of the tree where local diseases had entered through a crack in the bark. Similarly, the Asian version of the chestnut tree (Castanea mollissima) could resist the Asian fungus. However, the Asian version of the fungus grows too fast for the American chestnut tree's defensive response.

The Asian fungus releases oxalic acid that kills the tree's living cells, just inside the bark layer. In American chestnut trees, the Cryphonectria parasitica fungus circles the entire trunk and girdles the tree, blocking the flow of water up to the leaves and blocking the flow of photosynthesized food from the leaves to the roots. A visible canker on the bark shows where the fungus is active inside the chestnut. The chestnut blight fungus grows through the tree's cambium cells faster than the tree can respond by creating callus (wound) tissue to wall off the disease.3

Above the site where the tree has been girdled by the fungus, an American chestnut will die. Below the infection site, the chestnut tree's roots usually survive because the fungus does not live in the soil. Below the root collar, the roots can resprout. A young chestnut tree matures until a crack in the bark allows a new infection, and the cycle repeats itself. As a result, we still see small chestnuts growing in Virginia forests today.

Fungal spores appear to be unable to get through the protective bark until the branches of the saplings reach a certain size. Near the age when a tree flowers and produces nuts, the expanding trunk naturally creates cracks in the bark. Also, after a certain number of years, a tree may experience naturally-caused wounds as branches rub against each other during a storm in the woods, or as something bumps against the tree. The cracks open up an avenue for infection. The fungus grows under the bark, blocking the flow of water and nutrients from the roots and killing the sapling... but if the sapling's leaves sent enough food down into the root system, the chestnut can sprout over and over again.

The American chestnut is not an endangered species, or threatened with extinction. There are millions of small, young chestnut sprouts - and a few large, nut-producing trees - growing in the Virginia woods now. Resprouting chestnut trees die from deer browsing or the effects of the expanding fungal infection, but a few mature enough to produce nuts first. Those nuts carry the genes of the parents, and seedlings from the still-surviving chestnuts are just as susceptible to the blight. Today, the chestnut is a minor component of the understory, no longer a dominant species that affects the surrounding forest.

Many American chestnut trees survive as mature individuals because they have been planted outside their normal range, away from the fungus. The largest chestnut tree ever documented in North America was located in Sherwood, Oregon. Despite efforts of western states to impose plant quarantines, and the pattern of wind from west to east, the fungus may still spread across the continent to infect the chestnut trees now on the West Coast. No American chestnut tree is safe from infection.

canker damage canker damage canker damage
Source: USDA Forest Service, Bugwood.org (by Joseph O'Brien)

Prior to 1904, the blight from Asia may have been imported multiple times along with Asian chestnuts into San Francisco or Seattle - but since there were no native chestnuts on the West Coast, the disease could not spread from there. It is possible that the fungus arrived on chestnuts imported for food, or on 1,000 Japanese chestnut trees (Castanea crenata) sent to a New Jersey nursery in 1882. Asian trees stopped growing at about 40 feet, while American chestnuts grew to 80-100 feet, so orchardists were importing Asian species and trying to create commercial farms using the easier-to-harvest Chinese and Japanese chestnuts.4

Importing foreign chestnuts for hybridizing with the native American tree has a long history. Thomas Jefferson imported chestnuts from Europe to Monticello.5

An infected chestnut, or infested tree, reached Bedford, Virginia as early as by 1903. The logical place for an infection to appear at that time in Virginia would have been a port city, perhaps Norfolk or Alexandria. Bedford had a railroad connection to New York City, so perhaps a train from the Bronx brought the fungus into central Virginia before birds/wind brought it to the Blue Ridge.6

Today, the shipping terminals in Hampton Roads and an international airport such as Dulles are the likely "Ground Zero" for a new species introduction. However, the experience with the chestnut (and with white-nose syndrome in bats, and with snakehead fish in the Potomac River) demonstrates that a disease outbreak of foreign import could appear anywhere.

The original range of the chestnut tree species may have extended all the way to the Atlantic Ocean. Another fungus-like pathogen (a root rot, Phytophthora spp.) may have limited the ability of the chestnuts to grow east of the Blue Ridge in the 1800's. Phytophthora kills a tree completely, roots and all, whereas the Cryphonectria parasitica fungus kills only above the canker and allows the roots to resprout.7

Clearing a forest and grazing cattle on a pasture will also kill chestnut roots completely. Few American chestnut sprouts occur in open fields today. The Chinese chestnut thrives in the sun but grows poorly in shade. After the Phytophthora and the blight wiped out so many trees, odds are good that any healthy-looking chestnut growing today in an open field/subdivision lawn will be an introduced Asian species, while a young tree struggling in the forest will be a native American chestnut.

The Virginia forests are still transforming, after the loss of the chestnut. Chestnut oaks, red oaks, and red maples have been released by the removal of the chestnut overstory, becoming dominant in different places. However, gypsy moths may suppress the chestnut oaks, maples will be limited by their inability to grow in shade and replace themselves, and oak decline/oak wilt from various causes may constrain the capacity of the red oaks to replace the chestnut.

At the same time, foresters are planning to restore the chestnut, by developing a blight-resistant strain and planting new stands throughout the old historic range of Castanea dentata. Three different options have been pursued:

  1. finding the few naturally-resistant trees still surviving in the woods, with just American chestnut genes, then growing new stock from their nuts (and grafts) to ensure the resistance is transmitted to future generations
  2. using the latest laboratory techniques to insert blight-resistant genes into the Castanea dentata genome, developing a genetically-modified organism (GMO) that could live in the forest and survive the fungus
  3. breeding a blight-resistant hybrid tree, mixing genes of the Chinese chestnut species (Castanea mollissima) with the American chestnut

chestnut canker
Chestnut canker
Source: Robert L. Anderson, USDA Forest Service, Bugwood.org
(Creative Commons Attribution 3.0 License)

First Option: Finding Naturally-Resistant "All American" Trees

The effort to find an all-American tree, naturally resistant to the blight, has been led by the American Chestnut Cooperators' Foundation based at Virginia Tech in Blacksburg. Seeds and grafts from relatively-healthy trees throughout the natural range of the chestnut have been planted near Mountain Lake, at the Blacksburg Airport, in Lesesne State Forest (Nelson County) and in other locations near Virginia Tech. In 2012, the foundation also had growers in 28 states and Canada.8

However, it appears those trees lack the ability to pass along their resistance to future generations. The largest "champion" chestnut in Virginia, located on a pasture fenceline in Amherst County, apparently lacks the ability to transmit its good fortune to its offspring. The healthy trees may have been located in areas that received fewer-than-average fungal spores, perhaps because of wind patterns. If that is the case, then trees may not be resistant, but instead are trees that by chance were not heavily infected.

An alternative explanation: the trees may have been infected by a hypovirulent (weakened) form of the fungus, so natural resistance was sufficient to fight off the blight. A virus does affect the fungus, weakening it - and chestnuts in Europe have recovered somewhat, as a result of this pattern. One dream of chestnut restoration specialists is that a hypovirulent form of the Cryphonectria parasitica fungus could out-compete the current form in the wild, allowing American chestnuts with pre-1904 genes time to grow wound tissue and resist the attack. In such a scenario, the native chestnut could recover naturally, without a massive re-introduction effort.9

A third possibility is that the healthy trees have some genetic pattern that provides resistance to the blight, but the genes are not expressed fully in the offspring (F1) generation and breeding healthy trees to each other will not create a new generation of blight-resistant chestnuts.

old chestnut tree, in full growth form
old chestnut tree, in full growth form
Source: USDA PLANTS Database Image by John Foley. Provided by National Agricultural Library. Originally from US Forest Service. United States, MD. 1901

Second Option: Creating a New Genome

The second option - direct intervention in the genetic pattern of the chestnut - is being pursued at the State University of New York, College of Environmental Science and Forestry and with numerous cooperators. National Science Foundation grants are supporting the research, with the expectation that the results will assist in tackling other diseases in other species within the Fagaceae plant family, including beeches and oaks.10

Genetic modification in theory should require far less time than standard plant breeding practices, since it takes at least seven years to grow a chestnut to maturity and collect pollen/nuts for a new generation. However, the chestnut tree has turned out to be a difficult species to manage in a laboratory setting. It is relatively challenging to generate a seedling from a mass of chestnut cells, genetically modified or not.

The genes that allow Asian trees to resist the fungus, and how they are expressed, are still not well understood. Instead of transferring those Asian genes into the American species, much chestnut GMO research is focused on transferring a well-understood gene from wheat that produces the oxalate oxidase enzyme. That enzyme could block the rapid growth of the Cryphonectria parasitica fungus, allowing the chestnut tree enough time to create a sufficient amount of wound tissue that would block further invasion of the tree by fungal hyphae. Other research is based on an artificially-created gene, mimicking the capability of frogs to survive despite all the fungi surrounding them in their moist habitats. The gene manufactures antimicrobial peptides, small proteins that kill the fungus.11

If this approach is successful in creating a genetically-modified chestnut that resists the blight, there is still a major constraint in planting this new version of the chestnut in the wild. All the laboratory products will share a common genome, and would lack much of the natural genetic diversity that allows a forest to survive attacks by other microbes. In a natural setting, genetic variations provide varying resistance to a number of threats. Some trees in a forest survive different attacks, while other trees succumb - but there are always survivors. However, if chestnuts from the laboratory share resistance to blight and almost all their other genes too, then some pathogen other than Cryphonectria parasitica could wipe out all of the chestnuts at once.

In 2013, the Forest Health Initiative planted three experimental patches of chestnuts in the wild, including one location in Virginia. The three-year experiment will assess the resistance of the genetically modified chestnuts and the response of the ecosystem to the reintroduction. Experimenters are speculating that using chestnuts that are all-American, without Asian genes, will minimize surprises that might develop during a wide-scale introduction, where potential impacts of "alien genes" from Asia might creating problematic interactions with other species in an American forest.12

Third Option: Breeding a Chinese/American Hybrid

The American Chestnut Foundation, founded in 1983, is the leader in efforts to cross-breed the Chinese chestnut (Castanea mollissima) with the American chestnut (Castanea dentata). Two parent trees (named "Clappper" and "Graves") located at the Connecticut Agricultural Research Station have been the primary sources of pollen with Chinese chestnut genes that appear to provide resistance, though other parents (including "Nanking" and "Mahogany") are also involved in testing.13

Seven generations of chestnuts have been raised since the 1980's. Volunteers and staff working for the foundation climb mature trees (or get a lift in a bucket from a utility truck...) to gather the male pollen from specific trees and bag female flowers to isolate them from wild pollen. The volunteers go back up again to pollinate the female flowers with pollen from specific desired father trees, so the tree will grow a nut with genes from pre-determined parents. After a summer of growth, the nuts are harvested by volunteers/staff who go up the tree a third time - ideally, before squirrels decide the chestnuts are ripe.

By interbreeding (backcrossing) offspring of Clapper and Graves with various American chestnut parents, the American Chestnut Foundation has produced a final genome that is 15/16ths American and 1/16th Chinese, including the all-important genes for blight resistance from the Chinese ancestors but showing other characteristics (such as height at maturity) from the American ancestors. Resistance to Phytophthora pathogen is also desired.

Breeding a Chinese/American hybrid is a long-term (perhaps 100-year) project, requiring continued private support and cooperation from universities and government agencies. Creating the BC3F2 generation that is 15/16ths American and 1/16th Chinese took until 2005, when the "restoration chestnut" was finally born. Further efforts will include two more generations, one to be bred in 2014 and another in 2021, before reintroduction on a large scale starting in 2028.

Current plans are to establish numerous stands of "new" chestnuts widely throughout the Appalachians, by planting trees in each of the areas defined by the US Geological Survey 1:24,000 scale quadrangles. Planting of test plots began in 2008, with expansion to 14,000 trees planted on Appalachian Strip mines in 2014. Details of the genetics of planted samples are carefully planned and tracked. (When being grown, female flowers of key trees in the American Chestnut Foundation orchard were covered with bags - "chastity belts" - so researchers could fertilize with pollen from a specific father.) Wide distribution may lead to natural escape and growth of chestnut trees with American characteristics and blight resistance.14

The American Chestnut Foundation has distributed pollen and nuts to cooperators throughout the eastern United States, but their focus is on growing genetically-resistant trees on a research farm in Meadowview, Virginia (near Abingdon). There, the Wagner and Price research farms are stocked with thousands of chestnut trees that are watered, weeded, fertilized, studied. In many cases, the trees that have been planted are destroyed and replaced, once the evidence is clear that a particular mix of genes is not a success.

The US Forest Service has signed a Memorandum of Understanding with the American Chestnut Foundation, to govern the restoration in the forests. Starting in 2009, crossbred chestnuts were planted in National Forests in Virginia, Tennessee, and North Carolina. Though the locations are not made public in order to protect the planted seedlings, it is possible to guess at one character of each site: they are protected from deer. Excessive deer browsing will kill chestnuts faster than the blight, so it is likely that restocking efforts will be concentrated on rocky outcrops and fenced areas.

If plantings are successful over the next 50 years and the chestnut is able to regain its once-dominant position in the Appalachian forest, then there will be substantial ecological effects. Restoration may be a good thing for the chestnut species in particular and perhaps for the southern forest ecosystem as a whole, but other species will be diminished in their significance. One concern is that a genetically-improved chestnut would become an invasive species comparable to the Bradford pear, or some insects that normally feed on chestnuts may have a toxic response to the hybrid Chinese/American chestnut trees.

oak forest on Bull Run Mountain in Prince William County, where American Chestnut Foundation planted new chestnut seedlings in 2011-12 to test ability to survive deer, rodents, and storms (before resistant seedlings are re-introduced and exposed to risks)
oak forest on Bull Run Mountain in Prince William County, where American Chestnut Foundation planted new chestnut seedlings in 2011-12 to test ability to survive deer, rodents, and storms (before resistant seedlings are re-introduced and exposed to risks)

Links

References

1. Virginia Shepherd, "Pursuing an American Dream," Virginia Wildlife, Volume 70 Issue Number 2 (February 2009), Virginia Department of Game and Inland Fisheries, http://www.dgif.virginia.gov/virginia-wildlife/chestnutstory.pdf (last checked July 17, 2013)
2. US Forest Service, "American Chestnut Restoration Project," http://www.fs.fed.us/r8/chestnut/qa.php (last checked July 17, 2013)
3. Sandra L. Anagnostakis, "Revitalization of the Majestic Chestnut: Chestnut Blight Disease," American Phytopathological Society, 2000, http://dx.doi.org//10.1094/APSnetFeature-2000-1200 (last checked July 17, 2013)
4. Dr. Sandra L. Anagnostakis, "Chestnuts and the Introduction of Chestnut Blight," Connecticut Agricultural Experiment Station, New Haven Connecticut, http://www.ct.gov/caes/cwp/view.asp?a=2815&q=376754 (last checked September 4, 2010)
5. Dr. Sandra L. Anagnostakis, "Chestnut Breeding in the United States," Connecticut Agricultural Experiment Station, New Haven Connecticut, http://www.ct.gov/caes/cwp/view.asp?a=2815&q=376752 (last checked September 4, 2010)
6. Anagnostakis, "Chestnuts and the Introduction of Chestnut Blight" (last checked September 4, 2010)
7. Emily W. B. Russell, "Pre-Blight Distribution of Castanea dentata (Marsh.) Borkh," in Bulletin of the Torrey Botanical Club, Vol. 114, No. 2 (Apr.-Jun., 1987), pp. 183-190, http://www.jstor.org/stable/2996129 (last checked September 4, 2010)
8. "2013 Newsletter," American Chestnut Cooperators' Foundation, 2013, http://ipm.ppws.vt.edu/griffin/news.html (last checked July 17, 2013)
9. "Hypovirulence-Infecting the infection," American Chestnut Foundation - Virginia Chapter, http://www.vatacf.org/Hypovirulence.shtml (last checked July 17, 2013)
10. Susan Freinkel, American chestnut: the life, death, and rebirth of a perfect tree, University of California Press, 2009, pp.156-163; "Genomic tool development for the Fagaceae," Award Abstract #0605135, National Science Foundation, http://www.nsf.gov/awardsearch/showAward?AWD_ID=0605135 (last checked July 17, 2013)
11. Hebard, F. V. "The Backcross Breeding Program of the American Chestnut Foundation," in Proceedings of the Conference on Restoration of American Chestnut to Forest Lands, 2005, http://sfr.psu.edu/public/chestnut/information/conference-2004/conference/hebard(last checked September 4, 2010)
12. "Into the wildwood," The Economist, May 4 2013, http://www.economist.com/news/science-and-technology/21577033-gm-species-may-soon-be-liberated-deliberately-wildwood/print; "Like-Minded Rivals Race to Bring Back the Chestnut Tree," The New York Times, July 13, 2013, http://www.nytimes.com/2013/07/14/us/like-minded-rivals-race-to-bring-back-an-american-icon.html (last checked July 17, 2013)
13. "The Chestnut Story," American Chestnut Foundation presentation to the Prince William Conservation Alliance in Manassas, Virginia on September 2, 2010
14. "Chestnut restoration effort has roots in Fauquier," Fauquier NOW, January 29, 2013, http://www.fauquiernow.com/index.php/fauquier_news/article/chestnut-restoration-effort-has-roots-in-fauquier-2013; Helen Thompson, "Plant science: The chestnut resurrection," Nature, Volume 490, Issue 7418 (October 3, 2012), http://www.nature.com/news/plant-science-the-chestnut-resurrection-1.11504 (last checked July 17, 2013)


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