In recent decades, butternut populations throughout its range in the eastern North America have been decimated by butternut canker, a disease caused by the fungal pathogen Ophiognomonia clavigignenti-juglandacearum (=Sirrococcusclavigignenti-juglandacearum). Disease mortality, poor regeneration in contemporary forests, and hybridization with Japanese walnut threaten the survival of butternut throughout its historic range. Successful conservation of this species requires a better understanding of its current survival, conditions under which regeneration occurs, and its genetic integrity where it co-occurs with Japanese walnut. We assessed butternut populations within Great Smoky Mountains National Park (GSMNP) to determine recruitment history of populations, post-canker survivorship and health, and degree of hybridization with Japanese walnut. We used National Park Service monitoring records to locate and collect health and survivorship data for 210 butternut trees across 19 watersheds within GSMNP. Tree core samples were taken throughout the Park to determine the maximum tree age within populations. Within five watersheds, we intensively cored populations, collecting samples from 8-20 butternuts, along with the two nearest competing overstory trees. Tree cores were aged to determine recruitment date and the competitive environment in which individual butternut regenerated. Leaf samples were collected for genetic analysis to determine the frequency of hybridization.
Results/Conclusions
Based upon the percentage of individuals from the NPS monitoring database that were not found in our survey, we estimate survivorship of butternut over the past two decades to be approximately 68%. Of the surviving trees, less than 1% were canker-free and most were in various stages of decline due to girdling by fungal cankers. A small proportion of trees scattered across multiple watersheds remained vigorous, with only mild disease symptoms, suggesting some natural resistance occurs within these populations. Tree core samples revealed continuous recruitment from the time of Park establishment (1934) until around 1980, after which regeneration declined drastically across all watersheds. Individual watersheds experienced peaks in recruitment between 1960 and 1980. Maximum tree age across all watersheds was 85 years, but most trees were less than 60 years old. Initial genetic analyses indicate that butternut-Japanese walnut hybrids comprise a small portion of the total trees sampled. Based upon our results, butternut populations in GSMNP have declined dramatically due to disease mortality and thirty years of minimal regeneration. The presence of healthy trees and low rates of hybridization offer hope that the trees in GSMNP may contribute to efforts to develop and reestablish disease-resistant populations of this threatened tree species.