Instituto de Biodiversidade e Florestas, Programa de Pós-Graduação Sociedade, Natureza e Desenvolvimento, Universidade Federal do Oeste do Pará, Vera Paz, s/n, Salé, Santarém, Pará, 68040-255, Brazil.
Instituto de Engenharia e Geociências, Programa de Pós-Graduação em Recursos Naturais da Amazônia, Universidade Federal do Oeste do Pará, Vera Paz, s/n, Salé, Santarém, Pará, 68040-255, Brazil.
Tree Physiol. 2024 Aug 3;44(8). doi: 10.1093/treephys/tpae088.
Increases in hydrological extremes, including drought, are expected for Amazon forests. A fundamental challenge for predicting forest responses lies in identifying ecological strategies which underlie such responses. Characterization of species-specific hydraulic strategies for regulating water-use, thought to be arrayed along an 'isohydric-anisohydric' spectrum, is a widely used approach. However, recent studies have questioned the usefulness of this classification scheme, because its metrics are strongly influenced by environments, and hence can lead to divergent classifications even within the same species. Here, we propose an alternative approach positing that individual hydraulic regulation strategies emerge from the interaction of environments with traits. Specifically, we hypothesize that the vertical forest profile represents a key gradient in drought-related environments (atmospheric vapor pressure deficit, soil water availability) that drives divergent tree water-use strategies for coordinated regulation of stomatal conductance (gs) and leaf water potentials (ΨL) with tree rooting depth, a proxy for water availability. Testing this hypothesis in a seasonal eastern Amazon forest in Brazil, we found that hydraulic strategies indeed depend on height-associated environments. Upper canopy trees, experiencing high vapor pressure deficit (VPD), but stable soil water access through deep rooting, exhibited isohydric strategies, defined by little seasonal change in the diurnal pattern of gs and steady seasonal minimum ΨL. In contrast, understory trees, exposed to less variable VPD but highly variable soil water availability, exhibited anisohydric strategies, with fluctuations in diurnal gs that increased in the dry season along with increasing variation in ΨL. Our finding that canopy height structures the coordination between drought-related environmental stressors and hydraulic traits provides a basis for preserving the applicability of the isohydric-to-anisohydric spectrum, which we show here may consistently emerge from environmental context. Our work highlights the importance of understanding how environmental heterogeneity structures forest responses to climate change, providing a mechanistic basis for improving models of tropical ecosystems.
预计亚马逊森林的水文极端事件（包括干旱）将会增加。预测森林响应的一个基本挑战在于确定潜在的生态策略。用于调节水分利用的物种特异性水力策略的特征描述，被认为沿着“等水-非等水”谱排列，这是一种广泛使用的方法。然而，最近的研究质疑了这种分类方案的有用性，因为其指标强烈受到环境的影响，因此即使在同一物种内也会导致不同的分类。在这里，我们提出了一种替代方法，假设个体水力调节策略是由环境与特征相互作用产生的。具体来说，我们假设垂直的森林剖面代表了与干旱相关的环境（大气蒸气压亏缺、土壤水分供应）的关键梯度，这些环境驱动了不同的树木水分利用策略，以协调蒸腾作用（gs）和叶片水势（ΨL）与树木根系深度的关系，根系深度是水分供应的一个代理。在巴西季节性的东部亚马逊森林中对这一假设进行测试，我们发现水力策略确实取决于与高度相关的环境。上层树冠树木，经历高的蒸气压亏缺（VPD），但通过深根获得稳定的土壤水分供应，表现出等水策略，其 gs 的日变化模式和稳定的季节性最小 ΨL 几乎没有季节性变化。相比之下，林下树木，暴露在变化较小的 VPD 但土壤水分供应高度可变的环境中，表现出非等水策略，gs 的日波动在旱季随着 ΨL 的增加而增加。我们的发现表明，树冠高度结构了与干旱相关的环境胁迫因素和水力特征之间的协调，为保持等水到非等水谱的适用性提供了基础，我们在这里表明，这种谱可能始终从环境背景中出现。我们的工作强调了理解环境异质性如何构建森林对气候变化的响应的重要性，为改进热带生态系统模型提供了机制基础。
