Eight-year field performance of backcross American chestnut (Castanea dentata) seedlings planted in the southern Appalachians, USA

Abstract

The American chestnut (Castanea dentata) is imperiled due primarily to two diseases caused by nonnative pathogens, notably chestnut blight and Phytophthora root rot (PRR) caused by Cryphonectria parasiticaand Phytophthora cinnamomi, respectively. Field reintroduction trials are important to test trees bred for blight resistance under forest conditions to quantify survival and field performance in managed stands that represent future restoration sites. We planted 513 pedigreed bare-root (1–0) nursery seedlings into an even-aged regeneration harvest in the Blue Ridge Mountains of eastern Tennessee, USA to test effects of breeding, genetics, and seedling size class on field performance, including survival, total growth, incremental growth (a measure of growth efficiency), and height growth patterns. Mortality was highest in the first two years after planting, and PRR symptoms coupled with soil and root assays that tested positive for P. cinnamomisuggested this disease was a significant contributor to mortality. Chinese chestnut (C. mollissima), known to be resistant to P. cinnamomiand C. parasitica, had the highest survival (96 percent) compared to American chestnut (34 percent) and hybrid generations, including the BC₃F₃generation (41 percent). Less advanced hybrid generations, such as the BC₁F₃and BC₂F₃, had the greatest total height and ground-line diameter (GLD) and height growth efficiencies. BC₃F₃generation seedlings were associated with higher-than-expected rates of nominal height growth patterns (i.e., flat growth rates over time or early mortality) compared to Chinese chestnut seedlings. Results, however, indicated the breeding program was successfully integrating desired American chestnut growth traits into the hybrid genome while transferring an intermediate level of resistance from the Chinese chestnut. We identified superior and inferior BC₃F₃families that can help inform breeding and restoration efforts. Visually grading seedlings before planting affected survival and growth of generations and genetic families differently, indicating that refinement of nursery production and restoration plantings should consider genetic effects. Across generations, seedling size grading improved total height and GLD growth that would save resources necessary for competition control and animal browsing protection, but large-size seedlings also exhibited lower C. parasiticaresistance rankings over time, probably owing to earlier bark fissuring. PRR probably affected outcomes, particularly for hybrid and American chestnut seedlings that have very low levels of P. cinnamomiresistance. Managers could expect advanced breeding generations to perform similarly to American chestnut in growth through the critical stage of early stand development. More durable resistance to C. parasiticaand mitigation of PRR through better site selection and/or resistance screening will improve future restoration efforts.

Citation

Clark, Stacy L.; Schlarbaum, Scott E.; Saxton, Arnold M.; Jeffers, Steven N.; Baird, Richard E. 2023. Eight-year field performance of backcross American chestnut (Castanea dentata) seedlings planted in the southern Appalachians, USA. Forest Ecology and Management. 532(10): 120820-. https://doi.org/10.1016/j.foreco.2023.120820.