Crown plasticity and canopy dynamics in northern hardwood forests

冠层在森林的种群、群落和生态系统动态中起着核心作用。然而，对森林冠层的了解仍然很少，因为获取高大树木的困难限制了直接在森林冠层进行的研究的数量和范围。这种信息的缺乏反过来又限制了可用于模拟和阐明森林冠层动态的模式的发展。我建议在安大略省中部的森林中进行一个经验和模型研究的综合计划，这将阐明树冠可塑性和冠层结构如何影响北部阔叶林的动态。将使用移动式空中升降机进行树冠可塑性的现场研究，通过树冠的横向生长和衰退来测量树冠的可塑性。实证研究的目的有两个：1)研究树冠可塑性如何使不同大小、年龄和物种的树木竞争冠层空间；2)校准一个真实模拟树冠可塑性和冠层空间竞争的模型。该模型的关键状态变量将是暴露树冠面积（暴露在阳光直射下的树冠投影面积），这是一个反映竞争结果的指标，可用于预测树木的后续生长和死亡。一旦校准，该模型将用于解决以下问题。冠塑性如何影响尺寸结构的发展和自疏速率？树冠可塑性是否随年龄增长而降低，从而增加冠层周转率？生产力和木材产量能否通过优化树木间冠层空间分配的管理措施得到提高？虽然树冠可塑性被广泛认为是森林冠层动态的重要驱动因素，但它尚未被深入研究，也很少被纳入森林模拟模型。本文提出的野外研究将为研究林冠可塑性的原因和结果提供新的见解，并提出建模研究将为探索林冠和冠层动态之间的相互作用以及预测替代管理制度的结果提供工具。

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.