Increasing drought effects on five European pines modulate Δ13C-growth coupling along a Mediterranean altitudinal gradient

1. Climate warming increases vulnerability to drought in Mediterranean water-limited forests. However, we still lack knowledge of the long-term physiological responses of coexisting pine species in these forests regarding their ability to cope with warming-induced drought stress. 2. We investigated spatiotemporal patterns of tree performance for five isohydric pines with partially overlapping ecological niches in the eastern Iberian Peninsula along an altitudinal gradient: Pinus halepensis = Pinus pinasterPinus nigraPinus sylvestrisPinus uncinata. 3. Using indexed tree-ring widths (TRWi), we assessed the changes in the temporal coherence of radial growth (growth synchrony, a(C)) over the period 1902-2011 across three elevation belts: low approximate to 1100m; mid=1615m; high=2020m. We also examined by mixed modelling whether TRWi showed an increased coupling with leaf-level gas exchange (inferred from indexed carbon isotope discrimination, Delta C-13(i)) by enhanced stomatal regulation in response to an amplified regional drought stress. 4. Increasingly negative annual water balances (decrease in annual precipitation minus evapotranspiration = -4.8mmyear(-1); 1970-2011) prompted more synchronous growth of coexisting pines between low- and mid-elevation belts, with a(C) rising from 0.25 +/- 0.04 (1902-1951) to 0.62 +/- 0.05 (1962-2011). This effect was coupled with tighter stomatal regulation at mid-elevation as indicated by high correlations between TRWi and Delta C-13(i) (>0.60 from the mid-1970s onwards) which resembled those found at low elevation. Simultaneously, TRWi vs. Delta C-13(i) uncoupling occurred at the high-elevation belt across species. 5. Weaker growth-climate relationships as elevation increased highlighted the major role of the altitude-dependent thermal gradient in growth responsiveness to drought; however, an intensified Delta C-13(i) response to spring water availability across elevation belts observed from mid-1970s onwards suggested regional shifts in tree physiological activity linked to earlier seasonal drought impacts. Warming-induced drought stress is spreading to higher altitudes in Iberian pinewoods as multispecies growth is linked to progressively tighter stomatal control of water losses reflected in wood Delta C-13.