Interactive effects of animals and silviculture on the early life history of oak
Oak (Quercus) is a dominant component of the forest canopy in many deciduous forests of the eastern United States. In the past century, a pattern of anthropogenic fire suppression throughout the eastern United States has created conditions that favor more shade-tolerant tree species at the expense of oak regeneration. Loss of oak as dominant canopy component would have a powerful impact on forest ecosystems; oak acorns are a keystone food resource for greater than 50 species of mammals and birds in these forests, due to their ubiquity and nutritional content. In response, forest managers have developed and applied silvicultural techniques, including timber harvesting, to emulate forest disturbance and regenerate oak. Timber harvesting can impact key trophic interactions with animals during early oak life history, thus potentially impacting the oak regeneration process indirectly; however, these impacts are not well understood. To address this research gap, I examined the effects of clearcut and shelterwood harvesting approaches on key early life history parameters involved in acorn predation, dispersal, seedling survival, and seedling growth for black oak (Q. velutina) and white oak (Q. alba) in southern Indiana. I found that the initial phase of shelterwood harvesting (midstory removal) resulted in increased acorn dispersal and reduced survival but that these changes were dependent on dispersal agent and acorn species. At the seedling life stage, growth was highest but survival was lowest (for black oak) in clearcut openings. The lower survival in clearcuts was likely due to drought during two consecutive growing seasons. Herbivory by white-tailed deer (Odocoileus virginianus) impacted seedling growth but not survival; rates of browsing were low throughout the study. With this field data, I developed an individual-based, spatially-explicit simulation model of early oak life history connecting early life history parameters with consequences for oak regeneration. Simulation results indicated that the observed differences in acorn predation and dispersal parameters in shelterwoods had a small, but significant, negative impact on accumulation of oak seedlings (<1.4 m height) in the period following harvest. However, this effect was not carried through to density of oak saplings (?1.4 m). Greater disturbance in the second and third phases of the shelterwood harvest may result in stronger impacts on seed predation and dispersal. Increasing the frequency of drought years in the simulation model had a powerful negative effect on both oak seedlings and saplings, and the effect increased with intensity of harvest disturbance. The impact of drought is concerning given predictions that climate change will bring more frequent drought to the Midwestern United States; the ultimate impacts on oak regeneration success are unclear since drought may affect oak competitors even more strongly. The simulation model I developed is a flexible tool that could be used to examine a wide range of animal impacts on early oak life history and project their impacts on oak regeneration success.
Swihart, Purdue University.
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