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之一，也是大规模水循环的关键。在热带森林中，直径超过70厘米的最大的树木储存了25%-45%的碳，但占树木总数的4%。这些大树也比较小的个体运输不成比例的更多的水分，使它们成为未来的保护优先事项。高大的热带树木可能非常古老，许多树龄在200-500年之间，有些估计有1400年的树龄。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，水上运输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，这是对未来的预测。然而，我们仍然不知道大树是如何克服与向重力输送如此大的垂直距离的水相关的巨大阻力的，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时将有多大的脆弱性至关重要。大树可以通过通过树干和树冠垂直改变多个叶和茎的水力特性，来最大限度地减少水分运输阻力随高度增加的影响。然而，关于这些垂直变化的数据很少，而且不存在热带树木的数据。因此，关于树木是否能够补偿树木对其水分运输能力的负面影响，以及它们对未来干旱事件的脆弱性，人们知之甚少。在这个项目中，我们将结合对世界上最高的热带树木的树木解剖、结构和水力特性垂直变化的新测量，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：确定树木水力和解剖特征的垂直变化如何调节不同环境条件下高大树木维持水分向其叶片运输的能力。目标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：确定高大热带树木水力特性的垂直梯度如何改变对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年在亚马逊地区发现的高达88.5米的树，比新热带地区有记录的任何其他树高约30米。我们将把这些树与婆罗洲同等大小的树进行比较，这些树来自双翅果家族，该家族包含世界上最高的被子植物物种。在这些树上，我们将测量60棵树的水力和解剖学特征的垂直梯度，这些树的高度从20-90米不等。这些树将来自巴西和婆罗洲的8个优势树种，使我们能够对比来自更干燥、更季节性气候的树种(巴西)和在更潮湿、a-季节气候中进化的树种(婆罗洲)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传输和储存、从最先进的地面激光扫描获得的树木建筑数据和植被模型相结合。将这些技术结合在一起，将使我们能够改变目前对世界上最高的热带树木水分运输限制的理解，以及这可能对未来气候情景下的碳循环和水循环产生的影响。

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.