Predicting Light Transmittance and Seedling Growth Across a Gradient of Riparian Forest Canopy Structures
Type of DegreeDissertation
Forestry and Wildlife Sciences
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In response to shifts in forest management that have favored continued retention of forest cover, the objective of this study was to explore how forest structure influences the understory light environment and seedling growth. The study was conducted within western Georgia along a created gradient of mixed-hardwood riparian forest structures ranging from 50 to 100 percent cover. This gradient was created by random assignment of four levels of midstory removal to fifty 0.05 ha plots. Forest structure, light transmittance, and development of planted seedlings were quantified within each plot. A modeling approach was then used to construct equations predicting understory light levels using metrics describing the forest canopy. Mortality and growth models were also fit to explore the relationship between seedling development and forest structure over two growing seasons (2004, 2005). Results suggest that the vertical sighting method of estimating canopy cover was the best evaluated measure for predicting light transmittance. Addition of stand density and top height to this model increased the variance explained to nearly eighty percent. Analysis of seedling growth data resulted in models with limited predictive power. However, models do provide some general inferences about the relation between structure and seedling growth. Logistic regression showed that the probability of seedling mortality was related to the interaction between basal area (m2 ha-1) and canopy depth as well as percent canopy closure estimated using hemispherical photography. Growth models found that seedling diameter and height increments were most strongly related to canopy height, canopy closure, and seedling basal diameter. Above all, this research highlights the potential importance of quantifying vertical and horizontal canopy characteristics when evaluating effects of forest structure on seedling growth and the understory environment. Moreover, structural relationships provided by this research may assist in the formulation of riparian forest management strategies that promote the development of seedling reproduction under an existing forest canopy.