Spatiotemporal variations in groundwater and evaporative demand drive ecophysiological functioning of a phreatophyte in drylands

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 jeopardize the structure and 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 phreatophyte that dominates one of the few terrestrial GDEs of semiarid regions in Europe. We assessed morpho-functional and hydraulic traits along a naturally occurring gradient of depth-to-groundwater (DTGW, 2–25 m) in a coastal aquifer, and throughout the growing season of the species. 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, higher atmospheric evaporative demand fostered higher transpiration rates and water stress. Differences in climatic conditions throughout the growing season drove temporal variability in Z. lotus responses along the DTGW gradient, with warmer and drier conditions promoting 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 DTGW and evaporative demand during the growing season. These findings highlight the existence of tipping points in the ecophysiological functioning of phreatophytic plants in drylands, which contribute to disentangle the functional responses of the related GDEs under groundwater detriment because of climate change effects.