Disturbance History and Decline of American Chestnut

Invasive diseases can interact with other disturbances and the environment to affect host population decline, realized niche, and interspecific interactions. In my dissertation, I sampled oak-chestnut forests first sampled for vegetation composition 66-89 years ago and conducted a field experiment to evaluate the effects of variance in environment, in invasive disease prevalence, in logging history, and in deer browsing on the abundance, realized niche, and survival of a declining host tree in southwestern Virginia, USA. The disease and host I studied were American chestnut (Castanea dentata) and chestnut blight (Cryphonectria parasitica), a disease introduced in the early 1900's that received national attention due to chestnut's timber value and ubiquity. Two sources of mortality studied here, in addition to chestnut blight, had significant effects on chestnut abundance and survival. Current chestnut abundance was significantly lower in areas where preblight chestnuts were logged than in areas where they were not logged. Since chestnut is highly palatable to deer, high levels of deer browsing significantly increased chestnut mortality rate, which affects its interactions with other understory species. Pre-blight chestnut abundance was not predictive of post-blight chestnut abundance. Multiple regressions of principal components representing the environment with pre-blight and current chestnut abundance demonstrated that chestnuts are now more restricted to iii areas with slopes and aspects receiving high light (southern to western facing slopes) and xeric species than pre-blight chestnut was. When assessing a study area with larger variation in elevation, chestnut abundance was also related to elevation and moderately acidic soil pH. Although chestnut blight was predicted to be less prevalent in environments where chestnut abundance was higher, this prediction was not supported. Chestnut blight prevalence was not significantly related to any measured environmental variable, was independent of chestnut density, and infected an average of 150f chestnuts, regardless of chestnut abundance. This work highlights the drastic change in chestnut's interactions with the environment now that chestnut blight is endemic across its range and the need for a better understanding of chestnut blight's spatiotemporal dynamics.

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