Using plant hydraulic scaling to predict the drought vulnerability of the world's tallest tropical trees

热带雨林是地球上最重要的碳储存地之一，也是大规模水循环的必要条件。在热带森林中，直径超过70厘米的最大树木储存了25-45%的碳，但只占树木总数的不到4%。这些大树也比小树输送更多的水，这使它们成为未来的保护重点。大型热带树木可能非常古老，许多树的树龄在200-500年之间，有些树的树龄估计在100 - 1400年之间。因此，它们在历史上的极端气候事件中幸存下来，包括干旱。然而，最近的证据表明，水运的限制可能会使较大的树木更容易受到更极端、更频繁的干旱事件的影响，这是未来的预测。然而，我们仍然不明白大树是如何克服巨大的阻力，在如此大的垂直距离上运输水，对抗重力，这大大增加了树木在特定气候下所经历的水力压力。这些信息对于了解这些标志性的热带树木在预测的未来干旱频率和强度增加的情况下是多么脆弱至关重要。大树可以通过树干和树冠垂直改变多叶和茎的水力特性，从而最大限度地减少对高度的阻力增加的影响。然而，关于这些垂直变化的数据很少，而且没有热带树木的数据。因此，关于树木是否能够或不能补偿高对其水运输能力的负面影响，以及它们对未来干旱事件的脆弱性，我们的知识有限。在这个项目中，我们将结合世界上最高的热带树木的解剖学、结构和水力特性的垂直变化的新测量方法，在两个不同的热带地区——亚马逊和婆罗洲——实现以下目标：目标1：确定树木的水力和解剖学特征的垂直变化如何调节高大树木在不同环境条件下保持水分输送到叶子的能力。目的2：确定热带树木的关键结构和建筑特性是否控制植物水力和解剖特性的垂直梯度。目标3：确定考虑热带高大树木水力特性的垂直梯度如何改变热带森林水和碳循环的预测。为了实现这些目标，我们将研究世界上最高的热带树木。这将包括2019年在亚马逊发现的高达88.5米的树木，比新热带地区记录的任何其他树木都要高30米。我们将把这些树与婆罗洲同等大小的树进行比较，这些树来自龙脑科，龙脑科包含世界上最高的被子植物物种。在这些树木上，我们将测量60棵树木的水力和解剖学特征的垂直梯度，高度从20-90米不等。这些树木将来自巴西和婆罗洲的八个优势物种，使我们能够对比来自干燥，季节性气候（巴西）的树种与在潮湿，季节性气候（婆罗洲）中进化的树种的水力适应性。为了实现上述三个目标，我们的新型垂直水力特性测量将与全树水分运输和储存测量、基于最先进地面激光扫描的树木建筑数据和植被模型相结合。结合这些技术将使我们能够逐步改变我们目前对世界上最高的热带树木的水运限制的理解，以及这可能对未来气候情景下碳和水循环的影响。

Tropical rainforests are one of the planets most important stores of carbon, as well as being essential to water cycling at large scales. Within tropical forests the largest trees, with diameters exceeding 70 cm, store between 25-45% of the carbon, yet represent <4% of the total number of trees. These large trees also transport disproportionately more water than smaller individuals do, making them a conservation priority for the future. Large tropical trees are likely to be very old, with many between 200-500 years and some estimated to be >1400 years old. Therefore, they have survived historical extreme climate events, including drought. Yet, recent evidence suggests water transport limitations are likely to make larger trees more vulnerable to the more extreme, more frequent drought events, which are predicted for the future. However, we still do not understand how large trees manage to overcome the huge resistances associated with transporting water such large vertical distances, against gravity, which substantially increase the hydraulic stress the tree experiences in a given climate. This information is essential to understanding how vulnerable these iconic tropical trees will be to the predicted future increases in drought frequency and intensity. Large trees can minimise the effects of increasing resistance to water transport with height through changing multiple leaf and stem hydraulic traits vertically through their stem and canopy. However, data on these vertical changes are rare and do not exist for tropical trees. Consequently, there is limited knowledge concerning whether trees can or cannot compensate for the negative effects being taller has on their water transport capacity and therefore their vulnerability to future drought events.In this project we will combine novel measurements of vertical changes in tree anatomical, structural and hydraulic properties on the world's tallest tropical trees, in two different tropical regions - Amazonia and Borneo - to achieve the following aims:Aim 1: Determine how vertical changes in tree hydraulic and anatomical traits regulate the capacity of tall trees to maintain water transport to their leaves under different environmental conditions.Aim 2: Determine if key structural and architectural properties of tropical trees control the vertical gradients of plant hydraulic and anatomical properties. Aim 3: Determine how accounting for vertical gradients in hydraulic properties in tall tropical trees alters predictions of tropical forest water and carbon cycling.To achieve these aims we will study the tallest tropical trees in the world. This will include trees in Amazonia discovered in 2019 that reach 88.5 m tall, ~30m taller than any other tree recorded in the neotropics. We will compare these to equivalent sized trees in Borneo from the dipterocarp family, the family containing the tallest angiosperm species in the world. On these trees we will measure vertical gradients in hydraulic and anatomical traits on 60 trees varying in height from 20-90 m. These trees will come from eight dominant species in Brazil and Borneo, allowing us to contrast the hydraulic adaptations of trees species from drier, more seasonal climates (Brazil), to those of species that have evolved in wetter, a-seasonal climates (Borneo). To realise the three aims above, our novel vertical hydraulic trait measurements will be combined with measures of whole-tree water transport and storage, tree architectural data derived from state-of-the-art ground-based laser scanning and vegetation models. Combining these techniques will allow us to make a step-change in our current understanding of the limits to water transport in the world's tallest tropical trees and the impact this may have on carbon and water cycling under future climate scenarios.