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1. Groundwater lowering can produce dramatic changes in the physiological performance and survival of plant species. The impact of decreasing water availability due to climate change and anthropogenic groundwater extraction on coastal dune ecosystems has become of increasing concern, with uncertainties about how vegetation will respond in both the short and long terms.

2. We aimed to evaluate the water-use responses of different plant functional types to increasing groundwater table depth and how this would affect their physiology in Mediterranean coastal dune systems differing in aridity.

3. We modelled water-table depth, quantified the contribution of different soil layers to plant water through Bayesian isotope mixing models and used a combination of spectral and isotope data to characterize plant ecophysiology. We found that increasing depth to groundwater triggered water uptake adjustments towards deeper soil layers only in the dry season. These adjustments in water source use were made by conifer trees (Pinus pinea, P. pinaster) and hygrophytic shrubs (Erica scoparia, Salix repens) but not by the xerophytic shrub Corema album. Moreover, we observed a greater use of groundwater under semi-arid conditions. Accompanying the greater use of water from deep soil layers as a response to increasing groundwater depth, the semi-arid dimorphic-rooted conifer tree P. pinea and hygrophytic shrub E. scoparia declined their water content (WI), without implications on photosynthetic parameters, such as chlorophyll content (CHL), photochemical index (PRI) and delta C-13. Unexpectedly, under semi-arid conditions, the shallow-rooted xerophytic shrub C. album, associated with an absence of water source use adjustments, showed a decline in WI, CHL and PRI with groundwater table lowering.

4. We provide insight into how different species, belonging to different functional types, are acclimating to groundwater changes in a region experiencing climatic drought and a scarcity in groundwater due to anthropogenic exploitation. Greater depth to groundwater combined with limited precipitation can have a significant effect on plants’ water source use and ecophysiology in semi-arid coastal dune ecosystems.