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

强风会给森林带来巨大的破坏力。例如，英国1987年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，因为它们具有社会和经济价值，但人们对风对热带雨林的影响知之甚少。我们最近使用了沙巴州(婆罗洲岛上的一个马来西亚州)的调查森林，使用机载激光扫描，作为NERC资助的人类改良热带雨林项目的一部分。我们在激光扫描中发现了热带地区迄今发现的最高的树：龙脑香科树种中一棵89米高的个体，该树种以出产巨人而闻名。这一发现引发了人们对热带森林高度的机械极限以及为什么沙巴州的低地森林比亚马逊和非洲的森林高得多的原因提出了质疑。一种想法是，在沙巴州有遮挡的山坡上，树木特别高，因为它们很少经历强风。另一种想法是，龙脑果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抵御强风。这两种理论都需要严格的评估。该项目解决了这些想法，通过研究位于沙巴州的40平方公里古老丘陵森林中的风流和树木响应，NERC的空中研究设施已经通过激光扫描(ALS)和成像光谱学绘制了那里的地面海拔、树冠高度和叶片养分浓度。热带最高的森林与这些山丘上矮小的石南林并列在一起，使这一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。我们将比较婆罗洲高大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)矮得多的森林中的风流，包括分析风是如何穿过丘陵景观导致暴露地点的森林受到强风影响的。我们还将通过一种新的方法来量化个别树木对狂风的易感性，该方法包括使用地面激光扫描对数百棵热带雨林树木进行3D重建，并模拟树干在暴露在阵风中时的应力。将风流图与树木的易感性结合起来，将使我们能够制作出地图，显示我们认为每个热带地区的树木最有可能被风吹走的地方。关键的问题是，预计处于高风险的地点是否被发现受到了风的破坏。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)的生长和死亡情况。以这种史无前例的规模测量树木人口统计学将提供一个独特而强大的数据集，用来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、弹性模数和树木结构将与法国圭亚那和泛热带数据集的值进行比较，以评估异常高大的树木是否在机械上是独特的。将利用其他热带地区的风况进行模拟，以评估极风事件的缺乏是否可能对婆罗洲如此高大的森林的发展起到了作用。ALS测量的树冠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.