Abstract
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 ndings 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.
Keywords: Depth-to-groundwater gradient, ecophysiological threshold, groundwater salinity, plant
functional traits, Rhamnaceae, Ziziphus lotus
