Wood Jeffrey D, Gu Lianhong, Hanson Paul J, Frankenberg Christian, Sack Lawren
School of Natural Resources, University of Missouri, Columbia, Missouri, USA.
Environmental Sciences Division and Climate Change Institute, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.
Glob Chang Biol. 2023 Apr;29(7):2015-2029. doi: 10.1111/gcb.16582. Epub 2023 Jan 15.
Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem-scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (Ψ ), a property that integrates the drought response of an ecosystem's plant community across the soil-plant-atmosphere continuum. Specifically, Ψ defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the Ψ of a Quercus-Carya forest from an "ecosystem pressure-volume (PV) curve," which is analogous to the tissue-level technique. When community predawn leaf water potential (Ψ ) was above Ψ (=-2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψ fell below Ψ , the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, Ψ is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall-induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψ observations fell below Ψ , the forest is commonly only 2-4 weeks of intense drought away from reaching Ψ , and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom-up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of Ψ , and species in an ecosystem require compatible leaf-level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.
土壤干旱和大气干旱日益威胁着全球植物的生存和生产力。然而，概念上的差距限制了我们预测气候变化下生态系统尺度干旱影响的能力。在此，我们引入了生态系统萎蔫点（Ψ），这一特性整合了生态系统植物群落跨越土壤-植物-大气连续体的干旱响应。具体而言，Ψ定义了一个阈值，低于该阈值，根系提取土壤水分的能力和叶片维持气孔功能的能力会大幅下降。我们结合生态系统通量和叶片水势测量，从一条“生态系统压力-容积（PV）曲线”推导出了栎属-山核桃属森林的Ψ，该曲线类似于组织水平的技术。当群落黎明前叶片水势（Ψ）高于Ψ（=-2.0兆帕）时，森林对环境动态变化高度敏感。当Ψ降至Ψ以下时，森林对环境变化变得不敏感，并且在近两个月的时间里成为二氧化碳的净排放源。因此，Ψ是一个定义生态系统功能状态显著转变的阈值。尽管暴雨后降雨促使生态系统气体交换有所恢复，但结构和生理损伤的遗留影响抑制了冠层光合能力。在超过16个生长季中，只有10%的Ψ观测值低于Ψ，但该森林通常只需2 - 4周的严重干旱就会达到Ψ，因此高度依赖频繁降雨来补充土壤水分供应。基于对根系密度剖面和土壤水分特征曲线的自下而上分析，我们提出土壤水分获取能力是Ψ的主要决定因素，并且生态系统中的物种需要具有如膨压丧失点等兼容的叶片水平性状，以便叶片萎蔫与无法从土壤中进一步提取水分相协调。
