School of Public and Environmental Affairs, Indiana University Bloomington, Bloomington, Indiana.
Department of Earth System Science, Stanford University, Stanford, California.
Plant Cell Environ. 2019 Jun;42(6):1802-1815. doi: 10.1111/pce.13517. Epub 2019 Feb 22.
Over the past decade, the concept of isohydry or anisohydry, which describes the link between soil water potential (Ψ ), leaf water potential (Ψ ), and stomatal conductance (g ), has soared in popularity. However, its utility has recently been questioned, and a surprising lack of coordination between the dynamics of Ψ and g across biomes has been reported. Here, we offer a more expanded view of the isohydricity concept that considers effects of vapour pressure deficit (VPD) and leaf area index (A ) on the apparent sensitivities of Ψ and g to drought. After validating the model with tree- and ecosystem-scale data, we find that within a site, isohydricity is a strong predictor of limitations to stomatal function, though variation in VPD and leaf area, among other factors, can challenge its diagnosis. Across sites, the theory predicts that the degree of isohydricity is a good predictor of the sensitivity of g to declining soil water in the absence of confounding effects from other drivers. However, if VPD effects are significant, they alone are sufficient to decouple the dynamics of Ψ and g entirely. We conclude with a set of practical recommendations for future applications of the isohydricity framework within and across sites.
在过去的十年中，描述土壤水势（Ψ）、叶片水势（Ψ）和气孔导度（g）之间关系的等水合或非等水合概念已经广受欢迎。然而，其效用最近受到了质疑，并且据报道，在生物群落之间，Ψ和 g 的动态之间存在惊人的不协调。在这里，我们提供了一个更扩展的等水合概念，该概念考虑了蒸气压亏缺（VPD）和叶面积指数（A）对Ψ和 g 对干旱的明显敏感性的影响。在用树木和生态系统尺度的数据验证模型后，我们发现，在一个站点内，等水合性是气孔功能限制的强预测因子，尽管 VPD 和叶片面积等其他因素的变化可能会对其诊断提出挑战。在不同的站点之间，该理论预测，在没有其他驱动因素的干扰效应的情况下，等水合性的程度是 g 对土壤水分下降的敏感性的良好预测因子。然而，如果 VPD 效应显著，它们本身就足以完全解耦 Ψ和 g 的动态。最后，我们提出了一套实用的建议，用于在站点内和站点之间应用等水合性框架。
