Amy is a second-year M.S. student studying forest biology under the direction of Dr. Doug Jacobs. Her future plans include farming chestnuts in Appalachian Ohio while focusing on crop research and regional economic development. She is tangentially involved with the shale gas boom in Appalachian Ohio and is interested in the environmental and economic impacts of that industry as well.
Amy collecting increment cores to analyze stand history (photo by Amy Miller).
Welcome Immigrants: The Role of chinese Chestnut in American Chestnut Restoration
Compass Spring 2013/Forestry & Natural Resources
by: Amy Miller
Chestnuts, the Castanea genus, consist of seven species, all of which have ecological, economic, and cultural importance. They are popular world-wide, including Europe, the Caucasus Mountains, and eastern Asia. Historically, American chestnut (Castanea dentata) was an ecologically and economically important tree species in the eastern North American forest until it was essentially wiped out in the early 20th century by chestnut blight, caused by the fungus Cryphonectria parasitica. All chestnut species regularly bear sweet nuts, which is a unique feature among their relatives, the oaks and beeches. Chestnuts have traditionally been an important food source for people in remote, mountainous areas and are still staples in the cuisine of several cultures around the world. In North America, Castanea has also been useful to humans because of its extremely rot-resistant wood, which has been used for building bars and other structures, and because of high levels of tannins in the bark, which were once important for tanning leather. Additionally, the delicious and abundant nuts are an important food source for wildlife.
Three chestnut species (C. mollissima, C. henryi, and C. seguinii) are indigenous to China; C. crenata is native to Japan; C. sativa is the European chestnut; and C. dentata and C. pumila are native to North America. Despite separation by physical barriers (e.g., oceans and continents), chestnut species have similar site requirements and climatic limits. General biological traits of these species are also similar, and they easily interbreed when cultivated together. In their native forests, most chestnut species are canopy trees with upright growth forms.
American and European chestnut species are all susceptible to chestnut blight, but because the blight fungus is native to Asia, all Asiatic chestnut species are naturally resistant to varying degrees. The American Chestnut Foundation (TACF) has been working since the 1980s to breed a blight-resistant hybrid chestnut through a backcross breeding program, combining American chestnut and a Chinese chestnut (C.mollissima). These hybrids are now being produced and marketed under the name of ‘Restoration Chestnut’, and it is the intent of TACF to restore these trees to the Appalachian forest. Research efforts have focused on characteristics of American chestnut, but success of Restoration Chestnuts will also depend on the Chinese chestnut characteristics that have been incorporated. While Chinese chestnut has been studied in its native range and in plantation settings around the world, little is known about how it might grow and compete in North American forests.
The Big Shot! A slingshot was used to collect chestnut leaves for DNA extraction (photo by Amy Miller).
Ideally, Restoration Chestnuts will resemble American chestnut with respect to growth habit and morphology but incorporate Chinese chestnut genes for blight resistance. However, details of the inheritance and mechanisms of blight resistance are still largely unknown. The breeding process is intend- ed to reduce the Chinese portion of the hybrid genome; however, some Chinese chestnut traits will remain because of genes that may be linked to blight resis- tance and other fitness traits. With the goal of restoring American chestnut via hybridization, it is important that the Chinese source of resistance be well adapted to the American chestnut’s entire native range and growing conditions.
On a Connecticut property known as the Sarah Cunningham estate, there is an orchard of Chinese chestnut trees whose offspring have become established in an adjacent forest. This indicates that Chinese chestnut can regenerate and compete with American native oak (Quercus spp.), cherry (Prunus spp.), and other common species under suitable conditions, and bodes well for the ability of Restoration Chestnuts to populate North American hardwood forests. Researchers wonder how these Chinese chestnut trees were able to establish, survive, and compete so well, given that such an extensive naturalization of the species hasn’t been seen elsewhere in the United States. I am investigating this unique stand by attempting to reconstruct its development and by analyzing genetic relatedness of the naturalized Chinese chestnut trees. This may give chestnut researchers valuable insight into the ecology of chestnuts and the suitability of certain characteristics of Chinese chestnut in North American forests. It will also complement a large project led by Dr. Douglass Jacobs, which involves modeling carbon sequestration in eastern forests in anticipation of future chestnut reintroduction.
In April 1926, 67 Chinese chestnut seedlings were planted in a private orchard owned by Miss Sarah Cunningham in Dayville, CT. In 1965, researchers found 37 of the original orchard trees still alive and documented small trees that had grown up in adjacent fields from seed shed by the original trees. The site was visited again in 1992, and 28 of the original trees were still alive. Many offspring were naturalized and growing, turning the adjacent field into a forest where chestnut was a dominant canopy component alongside native North American species. I visited the trees in May 2012 and found those 28 original parent trees still alive, and the mature offspring were still plentiful in the adjacent approximately one-hectare forest.
Mature Chinese chestnut in the Sarah Cunningham Forest in Connecticut (photo by Amy Miller).
During this visit to CT, my crew and I mapped the forest. We measured and gave health ratings to all naturalized chestnut found. The map we created is being used to gather several pieces of ecological information, including: basal area of chestnut, the spatial distribution of all species, size-class dis- tributions of chestnut and other forest-canopy species, and the size and health of the chestnut trees relative to slope and soil conditions. In addition to creating the map, increment cores were taken from a large subset of the chestnut with an increment borer. The cores are being analyzed to reconstruct 5 the growth history of the chestnut trees by looking at growth responses to climate, insect outbreaks, and interactions with other trees in the forest. Closely-spaced rings may indicate suppression, or competition from nearby trees, and a series of wide rings may indicate release, or the sudden removal of that competition.
Little is known about the genetics of the naturalized trees. The original orchard trees show variation in growth habit, nut size, blight resistance, and many other traits, so I am using modern genetic tools to determine how much of that variation is maintained and how much selection pressure is evident in the forest offspring. Leaves were collected from all of the orchard and forest trees with the aid of a slingshot, and DNA was extracted to analyze relatedness among the parents, among the offspring, and between the parents and offspring. An array of microsatellite markers are currently being used to show overlap in genotypes between the individuals, and the computer program CERVUS will be used to statistically determine relatedness. If the results show that most of the offspring come from a small number of parents, we might surmise that certain Chinese chestnut trees would make better parents than others for Restoration Chestnuts. If the offspring are from a variety of parents, or appear to be from random parents, we may conclude that various Chinese chestnut trees could make suitable Restoration Chestnut parents, as long as they are blight resistant.
Taken as a whole, the history, ecology, and genetic information for these Chinese chestnut trees will help us understand factors that may contribute to the growth, survival, and overall success of this introduced species in North America. It may also provide valuable ecological information for Restoration Chestnut introduction and may better enable us to select parents for future hybrid breeding. With this knowledge and the continued efforts to produce and introduce blight-resistant, well-adapted chestnut trees to eastern North American, we hope to restore an important component of the ecosystem, boost our natural-resource economy, and a recover a lost gem of our culture and history.