Large tree crowns in closed forest canopies: Measuring structure and estimating light

Trees compete for light. Crown traits are the result of an evolutionary history dominated by this fact, and species exhibit a range of strategies including characteristic shapes and light-foraging abilities in response to this competition. Shape plasticity and crown asymmetry result from the growth and death of branches over long time scales. It is impossible to track every branch in stand-scale forest models, and there are no good approaches that accurately capture the emergent tree-level properties of this branch-scale process. Most forest models therefore ignore tree shape and asymmetry.;Models of tree size and shape are important in both scientific research and in evaluating policy questions. Light absorption in large canopy trees determines their own demographic rates and sets the template of light levels that drives understory growth and mortality, driving both community and ecosystem processes. Models that ignore crown shape and asymmetry could lead to faulty inferences and predictions.;Our work attempts to overcome some of the difficulties in both measuring and modeling large crown shape and light availability. We develop a new approach to extracting three-dimensional crown structural information from high resolution digital stereo imagery to accurately measure crown structure of over nine hundred well-studied large canopy trees. We also present a statistical model that integrates multiple data sources into estimates of the "true" but unmeasurable light available to individual trees. Third, we develop two crown models for forest simulations that capture their space-filling nature with minimum detail, and we parameterize these models from data; one models crown shape, the other crown location. Fourth, we investigate the relationship between light availability and growth.;We extract extensive fine-scale structural detail from the imagery, and generate detailed crown envelopes. We find that light availability predicts the growth rates of large trees primarily for shade intolerant species, while shade tolerant species show little correlation between light and growth. We dramatically improve the ability of forest models to predict exposed canopy area and canopy status from a small number of commonly available inputs. Finally, we successfully model the direction of crown movement and the population-level distribution of crown asymmetry.