Population dynamics and conservation genetics of butternut in Great Smoky Mountains National Park
Over the last 50 years, butternut (Juglans cinerea L.) populations throughout eastern North America have been devastated by butternut canker disease, which is caused by the fungal pathogen Ophiognomonia clavigignenti-juglandacearum (Nair, Kostichka, & Kuntz) Broders & Boland, comb. nov. In addition to disease, butternut is also threatened by habitat loss, poor reproductive success in contemporary forests, and interspecific hybridization with non-native congeners, especially Japanese walnut ( Juglans ailantifolia Carr.), which has been widely planted throughout the range of butternut. Great Smoky Mountains National Park (GSMNP) represents an ecological reserve with a large tract of continuous forest, which serves as a refuge for many rare plant and animal species. Park managers who implement strategies to maintain and promote rare species require baseline information to develop management protocols. Within the Park, several remnant clusters of butternut trees are scattered across various watersheds. In this study, previously documented butternuts were surveyed within GSMNP in 2010, and tree health, vegetation community and genetic data for butternuts throughout the Park were collected. Chapter 2 focuses on the ecology of the butternut in GSMNP, including tree health, community composition, recruitment history and the occurrence of interspecific hybridization. Tree health data revealed that most butternut trees were in various states of decline. Some vigorous trees with few disease symptoms were observed, possibly reflecting disease resistance. Vegetation plots were installed and data from the plots indicate that butternut largely occurred in similar ecosystems, although a few populations persist on sites outside the typical ranges of elevation and composition. The abundance of butternut on a particular site showed an inverse relationship to the basal area of other early successional species, likely due to butternut’s shade intolerance. Recruitment of butternut was positively associated with old field succession after establishment of the Park. Recruitment was generally continuous across watersheds but declined severely with the closure of the young forest canopy and the arrival of butternut canker disease. Finally, interspecific hybrids were found within the Park, but they represented a small proportion of the trees sampled. Chapter 3 examines the population genetic structure and genetic diversity of the Park’s butternut population and the management implications of this information. The identification of barriers to dispersal, e.g., distance or landscape features, is useful in prioritizing sites to maintain gene flow and genetic diversity in order to preserve the adaptability of the species. Assessment of population genetic structure can reveal such barriers. In this study, statistically significant genetic differentiation was found at large scales (between the north and south halves of the Park). However, the magnitude of this difference was small, and its ecological importance is unknown. In addition to genetic differentiation based on spatial location, I determined that the butternuts in the older age class in the Park were not genetically distinct from a younger cohort. The majority of individuals within GSMNP appear to belong to a relatively genetically homogeneous population, probably representing a continuous panmictic population.
Woeste, Purdue University.
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