Crown plasticity and canopy dynamics in northern hardwood forests

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The canopy plays a central role in the population, community and ecosystem dynamics of forests. Yet, forest canopies remain poorly understood because the difficulty of accessing tall trees has limited the number and scope of studies conducted directly in forest canopies. This dearth of information has in turn limited the development of models that can be used to simulate and elucidate the dynamics of forest canopies. I propose an integrated program of empirical and modeling studies, to be conducted in the forests of central Ontario, that will elucidate how crown plasticity and canopy structure influence the dynamics of northern hardwood forests. A mobile aerial lift will be used to conduct in-situ studies of crown plasticity, as measured by the lateral growth and recession of tree crowns. The empirical studies will be designed for two purposes: 1) to examine how crown plasticity enables trees of different sizes, ages, and species to compete for canopy space, and 2) to calibrate a model that realistically simulates crown plasticity and competition for canopy space. The key state variable of the model will be exposed crown area (the projected area of crown exposed to direct sunlight), a metric that reflects the outcome of competition and can be used to predict the subsequent growth and mortality of trees. Once calibrated, the model will be used to address the following questions. How does crown plasticity influence the development of size structure and the rate of self-thinning? Does crown plasticity decrease with age and thereby increase canopy turnover? Can productivity and timber yield be enhanced by management practices that optimize the allocation of canopy space among trees? While crown plasticity is widely acknowledged to be an important driver of forest canopy dynamics, it has not been thoroughly studied and has rarely been included in forest simulation models. The proposed field studies will provide new insights into the causes and consequences of crown plasticity, and the proposed modeling studies will provide a tool for exploring the interplay between crown and canopy dynamics and predicting the outcome of alternative management regimes.