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

Abstract

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. 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 = P. pinaster ≤ P. nigra ≤ P. sylvestris ≤ P. uncinata. Using indexed tree-ring widths (TRWi) we assessed changes in temporal coherence of radial growth (growth synchrony, âC) over the period 1902-2011 across three elevation belts: low ≈ 1100 m; mid = 1615 m; high = 2020 m. We also examined by mixed modelling whether TRWi showed an increased coupling with leaf-level gas exchange (inferred from indexed carbon isotope discrimination, Δ13Ci) by enhanced stomatal regulation in response to an amplified regional drought stress. Increasingly negative annual water balances (decrease in annual precipitation minus evapotranspiration = -4.8 mm year−1; 1970-2011) prompted more synchronous growth of coexisting pines between low- and mid-elevation belts, with âC 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 Δ13Ci (>0.60 from the mid-1970s onwards) which resembled those found at low-elevation. Simultaneously, TRWi vs. Δ13Ci uncoupling occurred at the high-elevation belt across species. 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 Δ13Ci 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 Δ13C.