Groundwater access does not ensure resilience: climate change effects in a groundwater-dependent ecosystem in the Mediterranean region

Climate change effects (i.e., rising temperatures and reduced rainfall) and land use changes are expected to affect dryland ecosystems worldwide, particularly those that rely on groundwater. These effects are already detected in southeastern Spain where we find one of the few groundwater-dependent ecosystems (GDEs) in European drylands. This GDE is dominated by the phreatophyte Ziziphus lotus (L.) Lam., a winter-deciduous arborescens shrub native to the Mediterranean region. We monitored spatio-temporal variation in groundwater depth and salinity, as well as climate (daily temperature, precipitation, and vapor pressure deficit) for 8 years (2018-2025) to assess the ecophysiological responses of Z. lotus to climate change effects. Hourly groundwater monitoring in 8 boreholes along a natural depth-to-groundwater gradient (2-25 m) was coupled with mid-summer (peak of plant physiological activity) measurements of water potential, photosynthetic capacity, transpiration rate, and water-use efficiency (WUE) of Z. lotus. Overall, annual trends indicated increased water stress and decreased photosynthetic activity of Z. lotus. Lower photosynthetic activity and stable transpiration rates reduced its WUE. Despite the absence of a clear long-term decline in groundwater levels, a sustained increase in air temperature without compensatory precipitation has led to increased atmospheric water demand, driving physiological stress and earlier defoliation in Z. lotus. Elevated groundwater salinity likely imposed additional osmotic stress. This decoupling between carbon assimilation and water loss suggests a progressive weakening of the ecosystem buffering capacity under combined atmospheric and osmotic stress. This long-term study shows that increasing climatic aridity reduces the productivity and weakens the resilience of GDEs, highlighting the vulnerability of these key dryland ecosystems.