The American Chestnut Foundation has a regional adaptation program designed to capture the potential diversity exhibited throughout the range of the tree in the Foundation’s back-cross breeding program.   The basic premise is that Chinese chestnut trees (Castanea mollisima) and others of asian origin have exhibited resistance to the fungus (Cryphonectira parasitica) that attacks the American chestnut (Castanea dentata).   Breeding the Chinese chestnut with the American chestnut can impart the resistance of the Chinese to the American.  Further crosses to American parents can result in a tree that still has resistance to blight, but that looks and ecologically functions like the American chestnut.   There have been several studies that looked at the amount of breeding required to recover the American character.  The current understanding is about four generations.  Additional breeding is required to concentrate the resistance and  get the trees to breed true for resistance.

The understanding of ecologists is that the tree would require the American chestnut characteristics to compete successfully in the forest setting.  And that trees with inferior apical dominance for instance, would be at a distinct disadvantage and would not get their genes successfully into the next generation.

The regional adaptability program was conceived by Dr. Larry Inman from the University of Minnesota and written about as early as 1987.  The basic premise was that trees in different regions had evolved and that it would be necessary to capture the genes from these adapted ecotypes to provide the project with trees of the exhibited pheno and genotypical variety.  This might include flowering time, disease tolerance, cold tolerance and certainly morphological variety.  The practical approach to implementing a regional adaptability program was to create state chapters where the chapters could identify their significant remaining trees.  The genes would be captured by pollinating these trees (if flowering) with advanced breeding pollen and then growing their progeny for selection based on resistance and ecological “timber” form.  This one local generation potentially captures about half the local genes.  The selection may drive that in one direction or another.  There has been talk about trying to increase that to 75% with additional local pollinations.

This approach has many interesting components, but one which greatly interests those of us active in the program.   Finding local American chestnut of a size that they might flower, and in a location that they can be successfully pollinated. In Connecticut, this proves especially challenging. Connecticut was the site of many early chestnut resistance experiments and the location where Dr. Arthur Graves and others have performed many breeding experiments. The result is that many of the trees found throughout the state are in fact trees that Graves or his associates planted forty or more years ago. They are a variety of crosses of Chinese chestnut and Japanese chestnut and even perhaps some back-crosses.  Locations of only a small percentage of these are known.   They are generally discovered (rediscovered) when someone receives a telephone call with the report of flowering American chestnuts. This sighting is evaluated with a site visit, and often samples taken for analysis.

A training session with summer intern Christine Cadigan provided an opportunity to compare chestnuts in their native setting to with other hardwood species.  Several species are challenging to distinguish from American chestnut at a distance.  American beech (Fagus grandifolia), Black birch (Betula lenta) and even Hickory (Carya sp.) leaves can all be very similar in appearance to American chestnut especially early in the season befor the chestnut leaves have achieved full size and their darker color.  Our walk led us past at least thirty American chestnut trees – of which unfortunately – only perhaps two are accessible and have a good chance of flowering this year.

After the session I had the opportunity to stop and visit several other trees that had been reported as surviving American chestnut.  Disappintingly, these trees were hybrids or even Chinese chestnut.  Clearly someone had decided to plant them in the specific location and while I didn’t take a core, I would estimate their age at 30 years plus.  There was evidence of heavy fruiting (spiked husks on the ground) and little evidence of blight.

Leaves from below.

detail of the leaf back

leaf closeup

These can be compared to the leaf samples (below) I found in Wilton, CT just a few days earlier.

Wilton American chestnut back detail.
Wilton American chestnut back detail.

Using the table in figure 2 of the recovery of American character paper by Hebard, Stiener and Diskin we can compare the above samples.

Morphological comparison

Comparing the leaves, we can see that the Wilton sample has a more oblong shape with an acute versus obtuse base. The twigs are reddish in color and thin as compared to the heftier twig of green color of the unknown sample from New Hartford. The stipules are barely visible in the American sample from Wilton and almost overwhelm the unknown sample from New Hartford.
It is certainly worth noting that the New Hartford tree was within a quarter mile of the vast number of American chestnut samples growing wild. The trees are out there – we just have to try harder to find them.