Lethal Psi: Characterising critical embolism thresholds for Amazon tree survival

亚马逊南部地区面临着所有亚马逊地区中最大的气候威胁。这一地区比亚马逊的“核心”地区更干燥、更温暖，并且一直受到最明显的干旱和变暖趋势的影响。这也是亚马逊地区树木死亡率上升最明显的地区，大气测量表明，森林不再是碳汇，而是大气中碳的净来源。鉴于亚马逊南部处于应对气候变化的第一线，我们必须更好地了解其森林物种对气候压力的抵抗力。在致命的Psi中，我们将构建一个新的1公顷干旱实验，以更好地了解亚马逊南部树木的生理生存极限。越来越清楚的是，水力破坏过程在干旱诱导的树木死亡中起着重要的作用。在张力的作用下，水分从土壤输送到树冠。随之而来的是干旱和土壤干燥，运输水分的木质部导管中的紧张加剧，这可能导致木质部导管中形成气泡(栓塞)，扰乱向树冠的水分运输，最终导致树木死亡。虽然这一过程一般被理解，但目前一个关键的知识缺口是，我们不知道导致热带树木死亡的血栓形成的阈值。缺乏对导致死亡的生理阈值的了解，构成了准确模拟气候变化下树木死亡的关键不确定性。确定树木死亡的水力阈值并不是一件容易的事情，需要监测树木直到死亡的水力状态。在致死Psi中，我们跟踪水力功能的关键指标(例如，叶水势和树液通量)，从我们施加的干旱开始，一直到树木死亡，以量化木质部电导的损失如何转化为死亡风险。虽然在亚马逊地区进行了其他干旱实验，但这些实验没有监测死亡过程之前和过程中的血栓状态，因此无法深入了解生存的生理阈值。到目前为止，干旱实验只在亚马逊地区的东北部进行，那里的年降雨量几乎是我们研究地点的两倍，那里的气候变化远没有亚马逊南部明显。考虑到它们的生态区性质和亚马逊南部经历的快速气候变化，我们预计该地区的树木更接近它们的气候极限，在强制干旱下经历的死亡将比在东北部实验中观察到的要严重得多。最终，我们计划使用新获得的现场数据来开发改进的死亡率函数，我们将在亚马逊南部更广泛地应用这些函数，以更好地预测这一至关重要的地区的干旱死亡风险。这将通过更新一个独特的基于特征的模型来实现，该模型专门为模拟亚马逊森林及其对环境变化的反应而开发。

The Southern Amazon faces the greatest climatic threat of all Amazon regions. This region is drier and warmer than 'core' areas of the Amazon and has been subject to the most pronounced drying and warming trends. It is also the region of the Amazon where increases in tree mortality have been most marked and where atmospheric measurements suggest forests are no longer acting as a carbon sink but as a net source of carbon to the atmosphere. Given that Southern Amazon is at the front line of the Amazon's battle against climate change, it is essential that we better understand how resistant its forest species are to climate stress.In Lethal Psi, we will construct a new 1-hectare drought experiment to better understand the physiological survival limits of southern Amazon trees. It has become increasingly clear that the process of hydraulic failure plays an important role in drought-induced tree mortality. Water is transported from the soils to the canopy under tension. As drought ensues and the soil dries, the tension in the xylem vessels that transport water intensifies and this can lead to the formation of air bubbles (embolism) in xylem vessels, disrupting water transport to the canopy and ultimately resulting in tree death. While this process is understood in general terms, one critical current knowledge gap is that we don't know the thresholds in embolism formation that result in the death of tropical trees. This lack of understanding of the physiological thresholds that result in death constitutes a key uncertainty for accurately modelling tree mortality under climate change. Determining the hydraulic thresholds of tree death is not an easy task and requires monitoring tree hydraulic status up to the point of death. In Lethal Psi, we track key indicators of hydraulic function (e.g. leaf water potentials and sap flux) from the beginning of our imposed drought all the way to the death of the tree to quantify how loss of xylem conductance translates into mortality risk. While other drought experiments have been set up in Amazonia, these did not monitor embolism status before and during the mortality process and were thus unable to provide insights into physiological thresholds of survival. Up to now, drought experiments have only been set up northeastern Amazonia, where annual rainfall is almost twice that of our study site and where changes in climate have been much less pronounced than in southern Amazonia. Given their ecotonal nature and the rapid climate change experienced in southern Amazonia, we expect that trees in this region are much closer to their climatic limits and will experience much more accentuated mortality under imposed drought than observed in northeastern experiments. Ultimately, we plan to use the newly acquired field data to develop improved mortality functions that we will apply more broadly across southern Amazonia to better predict drought mortality risk of this critically important region. This will be done by updating a unique trait-based model specifically developed to simulate Amazon forests and their responses to environmental change.