A 3D perspective on the effects of topography and wind on forest height and dynamics

强风会对森林产生巨大的破坏力。例如，1987年英国的大风暴吹倒了大约1500万棵树。大多数关于风害的研究都发生在温带种植园，因为它们的社会和经济价值，但对风对热带雨林的影响知之甚少。我们最近使用机载激光扫描技术对沙巴（马来西亚婆罗洲岛上的一个州）的森林进行了调查，这是NERC资助的人类改造热带森林计划的一部分。我们在激光扫描中发现了热带地区有史以来最高的树：一棵89米高的龙脑香科树木，以生产巨人而闻名。这一发现提出了关于热带森林高度的机械限制的问题，以及为什么沙巴的低地森林比亚马逊和非洲的森林高得多的原因。一种想法是，树木在沙巴有遮蔽的山坡上特别高，因为它们很少经历大风。另一种观点是，龙脑香树进化出了一套独特的机械特性，使它们比其他热带树木更能承受强风。这两种理论都需要严格的评估。该项目通过研究位于沙巴的40平方公里的古老山地森林中的气流和树木反应来解决这些想法，NERC的空中研究设施已经通过激光扫描（ALS）和成像光谱绘制了地面海拔、树冠高度和叶面营养浓度。热带地区最高的森林与这些丘陵中发育不良的石楠林并列，使景观成为评估风在决定森林高度和动态方面的作用的理想选择。我们将比较婆罗洲极高森林中的风流与南美洲（法属圭亚那）和受飓风影响的加勒比海（波多黎各）中较矮森林中的风流，包括分析风如何通过丘陵景观，导致暴露地点森林上的强风。我们还将通过一种新的方法来量化个别树木对风吹的敏感性，该方法涉及使用地面激光扫描和模拟建模的方法对数百棵热带雨林树木进行三维重建，当它们暴露在阵风中时，它们的树干内的应力。结合风的流动地图与树木的敏感性将使我们能够产生地图，显示我们认为树木在每个热带地区最有可能受到风吹的风险。关键的问题是，预测的高风险地点是否被风破坏。为了验证这一点，我们将使用重复调查ALS的最新进展来跟踪大约20万棵高大树木（>30米）的生长和死亡。在这种前所未有的规模上测量树木的数量将提供一个独特而强大的数据集，以测试风是否是树木死亡和树冠高度的主要决定因素。最后，我们将把这项工作放在全球范围内。婆罗洲树木的木材密度，弹性模量和树木结构将与法属圭亚那和泛热带数据集的值进行比较，以评估特殊的高大树木是否具有机械特性。模拟将使用来自其他热带地区的风况，以评估缺乏极端风事件是否可能导致婆罗洲如此高大的森林的发展。ALS测量的冠层高度与地上碳密度密切相关，因此这些分析将提供气候变化背景下碳动态的全面视角。

Strong winds can unleash huge destructive power on forests. For example, Britain's Great Storm of 1987 blew over about 15 million trees. Most research into wind damage occurs in temperate plantations, because of their social and economic value, but much less is known about the impact of wind on tropical rainforest. We recently used survey forests in Sabah (a Malaysian state on the island of Borneo) using airborne laser scanning, as part of a NERC-funded programme into human-modified tropical forests. We spotted the tallest tree ever found in the tropics in the laser scans: an 89-m tall individual in the Dipterocarpaceae family of trees, which are renowned for producing giants. This discovery raises questions about the mechanical limits of height in tropical forests and the reason why Sabah's lowland forests grow so much taller than those of Amazonia and Africa. One idea is that trees are particularly tall in sheltered hillsides in Sabah because they seldom experience strong winds. Another idea is that Dipterocarps have evolved a unique set of mechanical properties that enable them to withstand strong wind better than other tropical trees. Both of these theories need critical evaluation.The project addresses these ideas, by studying wind flow and tree responses in 40 square kilometres of old-growth hill forest situated in Sabah, where NERC's airborne research facility has already mapped ground elevation, canopy height and foliar nutrient concentrations by laser scanning (ALS) and imaging spectroscopy. The tallest forests in the tropics juxtapose with stunted heath forests in these hills, making the landscape ideal for evaluating the role of wind in determining forest height and dynamics. We will compare wind flows in the immensely tall forests of Borneo with those found in much shorter forests of South America (French Guiana) and the hurricane-impacted Caribbean (Puerto Rico), including an analysis of how wind is funneled through hilly landscapes resulting in strong wind forces on forests in exposed sites. We will also quantify the susceptibility of individual trees to windthrow by a novel approach involving 3D reconstruction of hundreds of rainforest trees using terrestrial laser scanning and simulation modelling of stresses within their trunks when exposed to wind gusts. Combining the wind flow maps with the tree susceptibility will allow us to produce maps showing where we think trees are most at risk of windthrow in each tropical region. The key question is whether sites predicted to be at high risk and found to be wind damaged. To test this, we will use the latest advances in repeat-survey ALS to track growth and mortality of about 200,000 tall trees (>30 m). Measuring tree demography at this unprecedented scale will provide a unique and powerful data set with which to test whether wind is a major determinant of tree death and canopy height. Finally, we will put this work in a global context. Wood density, modulus of elasticity and tree architecture of Bornean trees will be compared with values from French Guianan and pan-tropical datasets, to evaluate whether the exceptional tall trees are mechanically distinct. Simulations will be run using wind regimes from other tropical regions to assess whether a lack of extreme wind events may have contributed to the development of such tall forests in Borneo. ALS-measured canopy height is closely related to the aboveground carbon density, so these analyses will provide a wall-to-wall perspective on carbon dynamics in the context of climate change.