Data from: Does water shortage generate water stress? An ecohydrological approach across Mediterranean plant communities

The interactions between hydrological and ecological processes are key issues to improve our predictions of ecosystem responses to increasing droughts. However, predicting the dynamics and the impacts of vegetation water stress remains challenging because of complex ecohydrological feedbacks. The ecohydrological optimality approach proposes that functional adjustments within plant communities may buffer the increase in vegetation water stress despite local water shortage.
This study aimed to test whether vegetation water stress may be invariant across contrasting plant communities, reflecting possible optimality processes. We addressed the following question: does a lower soil water storage capacity under the same climate generate greater vegetation water stress over time? We hypothesized that vegetation water stress would be buffered around a low and constant level through the adjustment of vegetation biomass productivity net primary productivity (NPP), evapotranspiration (ET) and/or water-use efficiency (WUE) in relation with local soil water storage capacity.
We monitored 12 native plant communities distributed along a gradient of soil water storage capacity (ranging from 20 mm to 120 mm) during five successive years. Net primary productivity, ET, WUE as well as soil water dynamics were assessed and modelled for each plant community throughout the 5 years of study. Vegetation water stress was determined for each plant community as the deviation of between actual ET and their maximum ETm rate achieved under non-limiting conditions.
We found that NPP and ET were together proportionally related to local soil water storage capacity across the 5 years of study while WUE did not differ between plant communities. Vegetation water stress was found quite similar for all plant communities whatever the soil water storage capacity. These results suggested that vegetation water stress was strongly buffered by the community-level plant growth rates and total water use along the soil gradient, but not by WUE.
Our results suggest that stressful environments rarely exist for plant communities. A dynamic scaling relationship between NPP and ET may underpin the control of vegetation water stress over seasonal and pluriannual time-scales. Such results could contribute to better understanding processes associated with ecohydrological optimality and improve the predictions of vegetation dynamics under increasing droughts.

水文过程与生态过程之间的交互作用，是提升生态系统对日益加剧干旱事件响应预测能力的核心议题。然而，由于生态水文学反馈机制的复杂性，预测植被水分胁迫的动态及其影响仍极具挑战。生态水文学最优性范式（ecohydrological optimality）提出，即便在局地水资源短缺的情境下，植物群落内部的功能调控亦可缓冲植被水分胁迫的加剧态势。

本研究旨在验证：植被水分胁迫是否在不同类型的植物群落间保持恒定，以此反映潜在的最优性过程。我们提出如下科学问题：在气候条件一致的前提下，更低的土壤持水能力是否会随时间推移加剧植被水分胁迫？我们提出假说：植被水分胁迫可通过与局地土壤持水能力相关的植被生物量生产力（即净初级生产力（Net Primary Productivity, NPP））、蒸散量（Evapotranspiration, ET）及/或水分利用效率（Water-use Efficiency, WUE）的调控，维持在较低且恒定的水平。

本研究连续5年监测了沿土壤持水能力梯度（20 mm至120 mm）分布的12个原生植物群落。在整个5年研究周期内，我们对每个植物群落的净初级生产力、蒸散量、水分利用效率以及土壤水动态进行了评估与建模。对于每个植物群落，植被水分胁迫以实际蒸散量与非限制条件下达到的最大蒸散速率（Maximum Evapotranspiration, ETm）之间的偏差值进行量化。

研究结果显示，在整个5年监测周期内，净初级生产力与蒸散量均与局地土壤持水能力呈正比关系，而不同植物群落间的水分利用效率并无显著差异。无论土壤持水能力如何，所有植物群落的植被水分胁迫水平均较为相近。上述结果表明，植被水分胁迫主要通过群落尺度的植物生长速率与总用水量沿土壤梯度的协同调控实现缓冲，而非依赖水分利用效率。

本研究结果表明，植物群落几乎不存在极端胁迫环境。净初级生产力与蒸散量之间的动态缩放关系，或可支撑季节及多年时间尺度上植被水分胁迫的调控机制。上述研究结果有助于深化对生态水文学最优性相关过程的理解，并提升日益干旱情境下植被动态的预测精度。