The evolution of form and function of trees of diverse species has taken place over hundreds of millions of years, while urban environments are relatively new on an evolutionary time scale, representing a novel set of environmental constraints for trees to respond to. It is important to understand how trees of different species, planted in these anthropogenically-structured urban ecosystems, are responding to them. Many theories have been advanced to understand tree form and function, including several that suggest the fractal-like geometry of trees is a direct reflection of inherent and plastic morphological and physiological traits that govern tree growth and survival. In this research, we analyzed the “fractal dimension” of thousands of tree crowns of many different tree species, growing in different urban environments across the United States, to learn more about the nature of trees and their responses to urban environments at different scales. Our results provide new insights regarding how tree crown fractal dimension relates to balances between hydraulic- and light-capture-related functions (e.g., drought and shade tolerance). Our findings indicate that trees exhibit reduced crown fractal dimension primarily to reduce water loss in hotter cities. More specifically, the intrinsic drought tolerance of the studied species arises from lower surface to volume ratios at both whole-crown and leaf scales, preadapting them to drought stress in urban ecosystems. Needle-leaved species showed a clear trade-off between optimizing the fractal dimension of their crowns for drought vs. shade tolerance. Broad-leaved species showed a fractal crown architecture that responded principally to inherent drought tolerance. Adjusting for the temperature of cities and intrinsic species effects, the fractal dimension of tree crowns was lower in more heavily urbanized areas (with greater paved area or buildings) and due to crowns conflicting with utility wires. With expectations for more urbanization and generally hotter future climates, worldwide, our results add new insights into the physiological ecology of trees in urban environments, which may help humans to provide more hospitable habitats for trees in urbanized areas and to make better decisions about tree selection in urban forest management.

中文翻译：

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树冠的分形维数解释了不同尺度下物种对城市环境的功能特征响应

不同物种树木的形态和功能的进化已经发生了数亿年，而城市环境在进化时间尺度上相对较新，代表了树木应对的一组新环境约束。了解种植在这些人为结构的城市生态系统中的不同物种的树木如何对它们做出反应非常重要。已经提出了许多理论来理解树木的形式和功能，其中一些理论表明树木的分形几何形状是控制树木生长和生存的固有和可塑性形态和生理特征的直接反映。在这项研究中，我们分析了在美国不同城市环境中生长的许多不同树种的数千个树冠的“分形维数”，了解更多关于树木的性质及其对不同尺度的城市环境的反应。我们的结果提供了关于树冠分形维数如何与水力和光捕获相关功能（例如，耐旱和耐阴）之间的平衡相关的新见解。我们的研究结果表明，树木表现出减少的树冠分形维数，主要是为了减少炎热城市的水分流失。更具体地说，所研究物种的内在耐旱性源于整个冠部和叶片尺度上较低的表面积与体积比，使它们预先适应城市生态系统中的干旱胁迫。针叶树种在优化其树冠分形维数的干旱与耐荫性之间表现出明显的权衡。阔叶物种表现出分形冠结构，主要响应固有的耐旱性。调整城市温度和内在物种效应后，在城市化程度较高的地区（铺砌面积或建筑物更大），由于树冠与公用电线发生冲突，树冠的分形维数较低。随着全球范围内对更多城市化和普遍更热的未来气候的期望，我们的研究结果为城​​市环境中树木的生理生态学增加了新的见解，这可能有助于人类为城市化地区的树木提供更宜居的栖息地，并做出更好的树木选择决策在城市森林管理方面。树冠的分形维数在城市化程度更高的地区（铺砌面积或建筑物更大）中较低，并且由于树冠与公用电线发生冲突。随着全球范围内对更多城市化和普遍更热的未来气候的期望，我们的研究结果为城​​市环境中树木的生理生态学增加了新的见解，这可能有助于人类为城市化地区的树木提供更宜居的栖息地，并做出更好的树木选择决策在城市森林管理方面。树冠的分形维数在城市化程度更高的地区（铺砌面积或建筑物更大）中较低，并且由于树冠与公用电线发生冲突。随着全球范围内对更多城市化和普遍更热的未来气候的期望，我们的研究结果为城​​市环境中树木的生理生态学增加了新的见解，这可能有助于人类为城市化地区的树木提供更宜居的栖息地，并做出更好的树木选择决策在城市森林管理方面。