Water is the main limiting factor for groundwater-dependent ecosystems (GDEs) in drylands. Predicted climate change (precipitation reductions and temperature increases) and anthropogenic activities such as groundwater drawdown jeopardise the functioning of these ecosystems, presenting new challenges for their management. We developed a trait-based analysis to examine the spatiotemporal variability in the ecophysiology of Ziziphus lotus, a long-lived phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional traits and stem water potential along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the species-growing season. Increasing DTGW and salinity negatively affected photosynthetic and transpiration rates, increasing plant water stress (lower predawn and midday water potential), and positively affected Huber value (sapwood cross-sectional area per leaf area), reducing leaf area and likely, plant hydraulic demand. However, the species showed greater salt-tolerance at shallow depths. Despite groundwater characteristics, higher atmospheric evaporative demand in the study area, which occurred in summer, fostered higher transpiration rates and water stress, and promoted carbon assimilation and water loss more intensively at shallow water tables. This multiple-trait analysis allowed us to identify plant ecophysiological thresholds related to the increase in salinity, but mostly in DTGW (13 m), and in the evaporative demand during the growing season. These findings highlight the existence of tipping points in the functioning of a long-lived phreatophyte in drylands and can contribute to the sustainable management of GDEs in southern Europe, paving the way for further studies on phreatophytic species.

水是旱地依赖地下水的生态系统 (GDE) 的主要限制因素。预测的气候变化（降水减少和温度升高）和地下水下降等人为活动危及这些生态系统的功能，对其管理提出了新的挑战。我们开发了一种基于性状的分析，以检查Ziziphus Lotus生态生理学的时空变异性，一种长寿的浮生植物，在欧洲半干旱地区为数不多的陆地 GDE 之一占主导地位。我们评估了沿海含水层中自然发生的深度到地下水（DTGW，2-25 m）梯度以及整个物种生长季节的形态功能特征和茎水势。增加 DTGW 和盐度对光合和蒸腾速率产生负面影响，增加植物水分胁迫（黎明前和正午水势降低），并积极影响 Huber 值（每叶面积的边材横截面积），减少叶面积和可能的植物水力需求。然而，该物种在浅水处表现出更大的耐盐性。尽管存在地下水特征，但研究区夏季发生的较高的大气蒸发需求促进了较高的蒸腾速率和水分胁迫，在浅水位更强烈地促进碳同化和水分流失。这种多性状分析使我们能够确定与盐度增加相关的植物生态生理阈值，但主要在 DTGW (13 m) 和生长季节的蒸发需求中。这些发现强调了旱地长寿命浮生植物功能存在临界点，并有助于南欧 GDE 的可持续管理，为进一步研究浮生植物物种铺平道路。