Department of Forestry, Michigan State University, East Lansing, Michigan, 48824, USA.
Ecol Appl. 2021 Jun;31(4):e02297. doi: 10.1002/eap.2297. Epub 2021 Feb 25.
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.
不同物种的树木在形态和功能上的进化历经了数亿年，而城市环境在进化时间尺度上相对较新，代表了树木需要应对的一组新的环境约束。了解不同物种的树木在这些人为构建的城市生态系统中是如何做出反应是很重要的。许多理论已经被提出用于理解树木的形态和功能，其中一些理论表明，树木的分形几何形状是内在的和可塑的形态和生理特征直接反映，这些特征决定了树木的生长和生存。在这项研究中，我们分析了在美国不同城市环境中生长的数千种不同树种的树冠的“分形维数”，以了解更多关于树木的本质及其对不同尺度城市环境的反应。我们的研究结果提供了新的见解，即树冠分形维数与与水力和光捕获相关功能之间的平衡（例如，干旱和耐荫性）有关。我们的发现表明，树木表现出降低树冠分形维数的趋势，主要是为了减少在较热城市中的水分流失。更具体地说，所研究物种的内在耐旱性源于整个树冠和叶片尺度上更低的表面积与体积比，使它们适应城市生态系统中的干旱胁迫。针叶树种在优化树冠分形维数以适应干旱和耐荫性之间表现出明显的权衡。阔叶树种的树冠分形结构主要响应其内在耐旱性。在调整城市温度和内在物种效应后，树冠分形维数在城市化程度更高的地区（具有更多的铺砌面积或建筑物）以及由于树冠与公用事业电线冲突而降低。考虑到全球范围内城市人口的增长和未来气候普遍变热，我们的研究结果为城市环境中的树木生理生态学提供了新的见解，这可能有助于人类为城市地区的树木提供更适宜的栖息地，并在城市森林管理中做出更好的树种选择决策。
