Full scale testing of tree streamlining in wind (STREEM)

作为自然扰动动力学的一部分，风暴对树木造成经常性的重大破坏。在过去的几十年里，风暴造成了欧洲森林累积的50%以上的受损木材。由于它们具有相当大的、多层次的损害效应，因此人们对开发最小化其造成的损害的策略非常感兴趣。当前和未来的风暴灾害情况提出了一个问题，即是否以及如果是，如何积极应对风暴灾害，以尽量减少风暴对树木和森林的破坏风险。基于过程的风暴损害发展分析的必要前提是对风-树相互作用的全面理解。风与树的相互作用决定了地面附近气流动能向树木传递的动力学。动能传递的一个关键因素是树冠在风中的流线型。当树冠暴露在风荷载下时，它们会改变形状和空气动力学特性，以减少对周围空气流动的阻力。流线型的形状减少了作用在树木上的风荷载，从而减少了损坏的可能性。然而，迄今为止，关于野外真实树木树冠的风致流线形状动力学的知识和定量信息很少。以前的研究结果主要是通过对微型树木或单个树木部位的风洞调查获得的，这些研究结果只能在非常有限的程度上转移到真实的树木上。本文旨在研究自然流动动力学条件下真实树木树冠的风致流线形状对树木内部作用力的影响。在三个假设的基础上，我们想测试(1)风引起的树冠投影面积、阻力系数和树冠孔隙度的变化对风荷载动力学的影响，(2)风荷载对树木的作用如何依赖于近地面风速，以及(3)用于确定树木抗风荷载稳定性的树木拉拔试验是否适合近似实际风荷载。这些假设将在三个不同地点暴露于不同风气候的不同落叶和针叶树种上进行测试。对风致树冠流线动力学的进一步认识，将有助于更好地认识风暴灾害对树木的发展，为进一步开展风暴灾害研究提供重要的基础信息。

As part of the natural disturbance dynamics, storms cause recurrent major damage to trees. In the past decades, storms have produced more than 50% of the total amount of damaged wood accumulated in European forests. Due to their considerable, multi-layered damage effects, there is a great interest in the development of strategies to minimize the damage caused by them. The current and future storm hazard situation raises the question of whether and, if so, how to react proactively to the storm hazard to minimize the risk of storm damage to trees and forests.An essential prerequisite for the process-based analysis of storm damage development is a comprehensive understanding of wind-tree interactions. Wind-tree interactions determine the dynamics of the transfer of kinetic energy of the airflow near the ground to trees. A key element in the transfer of kinetic energy is the streamlining of tree crowns in wind. When tree crowns are exposed to wind loads, they change their shape and aerodynamic properties to reduce the resistance to the air flowing around them. The streamlined shape reduces the wind loads acting on trees and thus the probability of damage.However, very little knowledge and quantitative information are available to date on the dynamics of the wind-induced streamline shape of crowns of real trees in the field. Previous findings have mainly been obtained from wind tunnel investigations on miniature trees or on individual tree parts, which can only be transferred to real trees to a very limited extent. The present proposal aims to investigate how the wind-induced streamline shape of crowns of real trees under natural flow dynamics affects the forces occurring in trees. On the basis of three hypotheses, we want to test (1) what influence the wind-induced change of the crown projection area, drag coefficient, and porosity of tree crowns has on the wind load dynamics, (2) how wind loads acting on trees depend on the wind speed near the ground, and (3) whether tree pulling tests used to determine the stability of trees against wind loads are suitable to approximate real wind loads. The hypotheses will be tested on different deciduous and coniferous trees species exposed to different wind climates at three locations.An improved understanding of the wind-induced streamlining dynamics of tree crowns would lead to a better understanding of the development of storm damage to trees and provide important basic information for further storm damage research.