Limited cold tolerance could complicate the restoration of American chestnut to the northeastern United States

Background/Question/Methods The American chestnut (Castanea dentata (Marshall) Borkh.), once a major component of eastern forests, was functionally removed from forest ecosystems by chestnut blight (caused by the fungus Cryphonectria parasitica (Murr.) Barr), first identified in 1904.  Backcross breeding for blight resistance has shown real promise for near-term, comprehensive species restoration.  However, it is uncertain if limitations in tissue cold tolerance might restrict restoration of the species at the northern limits of American chestnut’s historic range.  Of particular interest is the ability of shoots and seeds of American and backcross chestnut to withstand the temperature lows often experienced by native species in these northern locations. We used relative electrolyte leakage of tissues following controlled freezing tests to assess cold tolerance.  Shoots of American chestnut and backcross chestnut saplings growing in two plantings in Vermont were tested during November 2006, February 2007 and April 2007 to assess their cold tolerance relative to ambient low temperatures.  Shoots of two potential native competitors, northern red oak (Quercus rubra L.) and sugar maple (Acer saccharum Marsh), were also sampled for comparison.  A separate study was conducted to compare the cold tolerance of the nuts of American chestnut, backcross chestnut and northern red oak.  Results/Conclusions During the winter, the shoots of American and backcross chestnuts were approximately 5°C less cold tolerant than the shoots of  red oak and sugar maple, with a tendency for American chestnut to be more cold tolerant than the backcross chestnut.  Terminal shoots of American and backcross chestnut also showed significantly more freezing damage in the field than nearby red oak and sugar maple shoots, which showed no visible injury.  Field results confirm that the shoots of American and backcross chestnut are vulnerable to damage at ambient temperatures experienced in the north.  A comparison of nuts representing pure American chestnut, backcross chestnut and red oak showed that red oak was approximately 2°C more cold tolerant than American chestnut, with backcross chestnut being intermediate and statistically indistinguishable from the other two species.  Analysis of pure American chestnuts identified a significant difference in cold tolerance among sources, with an almost 5°C spread between the most and least cold tolerant sources.  This variation in cold tolerance among sources suggests the existence of genetic variation of nut cold tolerance within existing American chestnut populations, and thus the possibility of positive selection for nut cold tolerance within a breeding program.