Résumé
1. Plant–plant interactions shape the structure and composition of plant communities, but shifts in interaction outcomes might occur in the face of ongoing climate change.2. We assessed the influence of Pinus halepensis plantations on the long‐term ecophysiological performance of understorey vegetation, by conducting a retrospective comparison (1989–2007) of growth‐ring widths, δ13C and δ18O between Rhamnus lycioides shrubs from two contrasting vegetation types: P. halepensis plantations vs. open woodlands. We also measured the leaf δ13C, δ18O, δ15N, and nutrient concentrations of understorey R. lycioides shrubs growing at varying distances from planted trees within pine plantations.3. Dendroecological and stable isotope data revealed strong competitive effects of planted P. halepensis trees on R. lyciodies. Shrubs in pine plantations showed lower radial growth and higher growth ring δ13C and δ18O than those in open woodlands, indicating lower stomatal conductance and photosynthesis in the former. The strong competitive effects of P. halepensis on understorey R. lycioides were most evident in wet, productive years. Conversely, in very dry years, there were indications of a facilitative effect of planted P. halepensis canopies on understorey shrubs.4. Within pine plantations, understorey R. lycioides shrubs growing at shorter distances from planted trees were forced to rely on more superficial and ephemeral soil water sources, which reduced their stomatal conductance (higher leaf δ18O) and interfered with nutrient uptake (lower leaf N and P concentrations and more negative δ15N).5. The intrinsic water use efficiency of R. lycioides shrubs growing in open woodlands has increased during recent decades as a result of their ability to adjust their stomatal conductance in response to increasing temperature and atmospheric CO2. However, this adaptive response was much weaker or absent in the severely drought‐stressed shrubs from pine plantations.6. Pine afforestation strongly reduces water and nutrient availability for understorey shrubs in drylands, with potential long‐term consequences for ecosystem biodiversity, structure and functioning. Competition by P. halepensis on R. lycioides clearly outweighed facilitation in the long‐term, thus compromising the ability of understorey shrubs in semi‐arid pine plantations to cope with climate change.