Some may be surprised to learn that the Horse Chestnut is not really a chestnut. They are actually a member of a completely different family, Hippocastanaceae, which contains horse chestnuts and buckeyes. The Horse chestnut (image below) is not vulnerable to the chestnut blight, so we are not breeding Horse chestnut nor looking for examples of the tree to breed.
By browsing the following sites, one will also find that there are at least five different species of chestnuts that can be found in the eastern United States, but there are several other species and subspecies that can be found worldwide. Connecticut, because of long interest in ornamental plants and also the work of the CT Agricultural Experiment Station, has a significant amount of pure and hybrid chestnuts specimens throughout the state. In fact, less than one-quarter of the samples we are sent validate as pure American chestnut.
There are two specific and very different approaches to chestnut identification.
In this approach, leaf and twig samples are examined by a scientist – are the currently accepted identification approach for the American Chestnut Foundation tree breeding program. Morphological review is a process where a trained scientist reviews a set of known morphological attributes that can be seen in a well preserved twig and leaf sample, to identify the sample. The approach may follow the steps outlined in a dichotomous tree – or more generally jump directly to the characteristics that scientist has determined are defining for the validation.
The photo at right shows a branch of an American chestnut (or BC4) that has strong American chestnut character – including pronounced leaf hooks, reddish brown stem, “boat shaped” leaves, small or non-existent petiole. The hand is holding a leaf from a Chinese chestnut for comparison.
In the photo to the right several distinctive features are evident on one of the Chinese chestnut controls in the Great Mountain Forest Orchard. Ignore that the leaves are covered with rain, but note the strong alternate but regular leaf arrangement on the green stems as well as leaf shape, color, shine and prominent petiole at base of leaf. These are all notable features of Chinese chestnut.
Perhaps the most comprehensive review of these characteristics can be seen in the paper entitled Recovery of American Chestnut Character By Matt Diskin, Dr. Kim Stiener and Dr. Fred Hebard. For this research they developed an Index of Species Identity comprised of twenty one characteristics that show contrasting difference in the American chestnut and Chinese chestnut. These characteristics were derived from some of the dichotomous keys (described below) as well as other known differentiating characteristics.
While most people can quickly learn to distinguish a tree that is clearly not of native origin, it is fairly difficult to learn the subtle variation that distinguishes the trickier samples. Many of the samples supplied to us are of trees that appear to be American, but when examined closely, have signs that they are of hybrid origin. We have found that sending the samples to the same scientists helps us to provide consistent results.
Web sites and keys for identification
It is anticipated that development of genomic tools will facilitate isolation of genes that provide resistance in chestnut. This information is eagerly sought by the breeding programs, and is expected to dramatically improve the efficiency of back-cross breeding both by reducing generation time (no need to inoculate) but also by concentrating on offspring with the best known genetic characteristics. More can be read about using genetic analysis to differentiate American chestnut from Chinese chestnut, and also identifying specific marker regions identifying key attributes such as blight resistance – at the Fagaceae Project web-site.
Do you have a tree you think may be American chestnut?
Please place your sample of leaves and twig between two sheets of thin cardboard (to keep flat) and mail to the address below.
It would be especially helpful if you could take the time to download and complete a Tree Locator Form. [*Adobe PDF format] to accompany the sample. Just download and print out, fill in and mail to address below.
TACF New England Regional Breeding Program
USDA Forest Service Northern Research Station
705 Spear Street
South Burlington, VT 05403
Photos courtesy of Bill Adamsen