The American chestnut (Castanea dentata) has been driven to functional extinction in eastern deciduous forest by chestnut blight (causal agent Cryphonectria parasitica). The American chestnut was hybridized with Chinese chestnut (Castanea mollissima) to increase blight resistance. The hybrids were then back-crossed and intercrossed to the BC3F3 generation to retain the American morphology with the goal being to reintroduce the species to its native range. However, physiological data to accurately predict responses of these hybrids to environmental conditions are lacking. Here we address two questions: How does simulated climate change affect the growth and carbon fixation of chestnuts? Do hybrid genotypes of the BC3F3 generation respond to simulated climate change similarly to their pure American counterparts? Our experiment grew chestnuts from seed in environmental chambers for the equivalent of one growing season. Plants were randomly assigned to one of four treatments in a factorial design of elevated CO2 and elevated temperature. We replicated this experiment with different genotypes the following year. In total nine hybrid genotypes (four D and five W) and four pure American varieties (denoted as the state they were obtained from) were grown.
Results/Conclusions
We found that biomass was similar among the four treatments but not among genotypes in year one. D-genotypes showed greater biomass than W-genotypes which showed biomass similar to the pure Americans. Regardless of treatments, average seed mass of D-genotypes was greater than other genotypes (p <0.001), which we conclude to be the central factor explaining hybrid chestnut growth in the first growing season. When we replicated the experiment, elevated CO2 increased biomass when genotype and seed mass were included in the model (p = 0.02). Hybrid genotypes increased in biomass 20% to 26%. Overall, Americans increased in biomass by 23% but some varieties shifted biomass allocation and the smallest seed variety responded negatively. The influence of seed mass was consistent the second year with D-genotypes having larger seeds than W-genotypes and pure Americans. Findings indicate that increased CO2 under future climate scenarios will influence hybrid reintroduction, but not as much as the initial seed mass of the chestnuts. These larger seeds of the D-genotype are likely a residual Chinese chestnut trait. Even if the larger seeds are not an American trait, they may help for reintroduction purposes as large seeded genotypes will be able to quickly establish when reintroduced.