Full scale testing of tree streamlining in wind (STREEM)

作为自然干扰动力的一部分，风暴会反复对树木造成重大损害。在过去的几十年里，风暴造成的破坏木材占欧洲森林累积破坏木材总量的50%以上。由于其巨大的、多层次的损害影响，人们对制定将其造成的损害降至最低的战略非常感兴趣。当前和未来的风暴灾害形势提出了一个问题，即是否以及如果是这样的话，如何对风暴灾害做出积极反应，以将风暴对树木和森林的破坏风险降至最低。基于过程的风暴破坏发展分析的一个基本前提是全面了解风与树木的相互作用。风与树的相互作用决定了近地面气流向树传递动能的动力学。动能传递的一个关键要素是树冠在风中的流线型。当树冠暴露在风荷载下时，它们会改变自己的形状和空气动力学特性，以减少对周围空气流动的阻力。这种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，到目前为止，关于田间真实树木树冠的风诱导流线型形状的动态变化的知识和定量信息很少。以前的发现主要是通过对微型树木或单个树木部分进行风洞调查获得的，这些研究成果只能在非常有限的程度上转移到真正的树木上。本提案旨在研究在自然流动动力学下，真实树木树冠的风致流线形状如何影响树木中发生的力。在三个假设的基础上，我们想要检验(1)树冠投影面积、阻力系数和孔隙率的风致变化对风荷载动态的影响，(2)作用于树木的风荷载如何依赖于地面附近的风速，以及(3)用于确定树木对风荷载的稳定性的拔树试验是否适用于接近真实的风荷载。这些假设将在三个地点暴露在不同风气候下的不同落叶和针叶树种上进行验证。更好地了解树冠的风致精简动态将有助于更好地理解风暴对树木的破坏发展，并为进一步的风暴破坏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.