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的交互作用，是提升生态系统对日益加剧干旱响应预测能力的核心议题。然而，受限于复杂的生态水文反馈机制，预测植被水分胁迫的动态及其影响仍颇具挑战。生态水文最优性理论指出，即便局地存在水分短缺，植物群落内的功能调整仍可缓解植被水分胁迫的加剧。

本研究旨在验证不同植物群落间的植被水分胁迫是否可保持恒定，以此反映潜在的最优性过程。我们提出如下科学问题：在相同气候条件下，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））、蒸散发（Evapotranspiration，ET）及/或水分利用效率（Water-use Efficiency，WUE）的调整，被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（范围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、蒸散发（ET）、水分利用效率（WUE）以及土壤水动态开展了评估与建模。植被水分胁迫通过实际蒸散发与非限制条件下达到的最大蒸散发速率（ETm）之间的偏差进行确定。

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发（ET）均与局地土壤持水能力呈比例相关，而不同植物群落间的水分利用效率（WUE）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程度均极为相近。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沿土壤梯度的总耗水量得到显著缓解，而非通过水分利用效率（WUE）。

本研究结果表明，植物群落几乎不会面临胁迫性环境。净初级生产力与蒸散发（ET）之间的动态比例关系，可能是调控季节及多年时间尺度上植被水分胁迫的核心机制。上述研究结果有助于深化对生态水文最优性相关过程的理解，并提升日益加剧干旱背景下植被动态的预测能力。