Genetics and silvicultural treatment influence the growth and winter shoot dieback of American and Chinese chestnut seedlings grown in Vermont, USA

Background/Question/Methods Approximately 100 years ago, American chestnut (Castanea dentata [Marsh.] Borkh.) was functionally removed as an overstory tree by mortality caused by the fungal pathogen Cryphonectria parasitica (chestnut blight). Multiple efforts of species restoration have been attempted, and the hybridization of American chestnut with the highly blight-resistant Chinese chestnut (Castanea mollissima Blume) followed by the sequential backcross of progeny with American chestnuts shows great promise in producing blight-resistant trees with 94% American chestnut genes. Yet, for full restoration to its entire former range, breeding should also integrate adaptation to region-specific stresses such as the greater cold tolerance needed in higher elevations and northern environs. Two factors that can significantly influence cold tolerance are genetics and the environmental surroundings (e.g., cold exposure) of plant tissues. We examined first-year growth and winter shoot dieback of 13 genetic sources of American chestnut along with two sources each of Chinese chestnut and of red oak (Quercus rubra L. – a native competitor) under three silvicultural treatments (open, partial- and full-canopy overstories) in the Green Mountain National Forest, Vermont, USA. Differences in canopy cover among silvicultural treatments were measured using hemispherical photography. Differences in winter air temperature lows associated with silvicultural treatments were also assessed.  

Results/Conclusions Seedlings grown under open canopies that provided greater access to light and other resources exhibited greater growth than seedlings grown under partial and closed canopies. However, open canopies also resulted in lower winter temperatures that increased winter shoot injury. Chinese chestnut seedlings had significantly greater growth but also experienced greater winter injury than American chestnut and red oak seedlings. There were also significant differences in growth and winter injury among American chestnut sources. Seedlings from sources from warmer low-elevation, southern and central locations grew more but experienced greater winter injury than those from sources from the colder north. Separating seed sources into temperature zones (warm, moderate and cold) may provide a reliable index for assessing the tradeoffs between growth and vulnerability to winter shoot injury, and could help to identify those sources that best bridge those tradeoffs (have good growth and elevated tolerance to the cold). Our results after one year of outplanting suggest that both silvicultural treatment and genetic selection influence growth and winter injury at the northern limit of American chestnut’s range. However, additional analyses are needed to evaluate the tradeoff between growth and winter injury over time, including years of variable/extreme climate stress.