Double-stranded RNA-mediated recovery of American chestnut populations: A demographic analysis

Cryphonectria parasitica, the chestnut blight pathogen, can cause serious reductions of the survival and reproduction in American chestnuts (Castanea dentata). This pathogenic fungus can be infected with a cytoplasmic hyperparasite (a double-stranded (ds) RNA) which debilitates the pathogen and reduces its virulence. In host populations infected with hyperparasitized fungi, trees can respond to infection and recover. However, no work has determined if recovery of individual trees translates into recovery at the population level. The main objectives of this study are: (1) evaluate the effects of C. parasitica on trees of varying size; (2) determine how disease alters host demographics; and (3) evaluate the extent of dsRNA mediated recovery in American chestnut populations.; Inoculation and natural infection studies revealed that small individuals succumb to infection regardless of the virulence of the inoculant. The presence of dsRNA appears to delay death for medium to large sized branches.; Matrix projection models compared the finite rate of population increase (lambda) and size distributions of healthy, recovering, and non-recovering populations of the American chestnut. Disease reduced population growth rates of non-recovering. Further, retrogressions in size of large individuals are observed in diseased populations but not in healthy ones resulting in an increased frequency of small to mid-sized trees in non-recovering populations. Transition matrices for recovering populations contained characteristics of both healthy and non-recovering populations, and population growth rates tended to be slightly lower than growth rates found in healthy populations.; Sensitivity and elasticity analyses indicate that dsRNA should be introduced onto 1--10 cm dbh trees in non-recovering populations to have the largest impact on population growth rates. The G/L/ F (Growth/Longevity/Fecundity) elasticity ratio, used to detect stressed populations in conservation biology, did not detect the effects of chestnut blight epidemics. Since pathogen infections do not materially affect survivorship, epidemics have little effect on the G/L/ F portrayal of a population.; Studies on the effects of disease on seedling survival and growth revealed that the disease status of the adult population generally affects emergence and survival of seedlings with a trend for recovering populations to perform best. Disease status does not influence final seedling size; rather, final seedlings size was influenced by population.; This study emphasizes the need to examine not only effects of infection on individuals but also on populations. Matrix projection models are an effective tool for predicting population growth rates and examining the relative contributions of different life history stages to population growth rates. This information allows effective management strategies to be developed.