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Submediterranean forests are considered an ecotone between Mediterranean and Eurosiberian ecosystems, and are very sensitive to global change. A decline of Scots pine ( Pinus sylvestris  L.) and a related expansion of oak species ( Quercus  spp.) have been reported in the Spanish Pre-Pyrenees. Although this has been associated with increasing drought stress, the underlying mechanisms are not fully understood, and suitable monitoring protocols are lacking. The aim of this study is to bring insight into the physiological mechanisms anticipating selective decline of the pines, with particular focus on carbon and water relations. For this purpose, we performed a sampling campaign covering two growing seasons in a mixed stand of  P. sylvestris  and  Quercus subpyrenaica  E.H del Villar. We sampled seasonally twig xylem and soil for water isotope composition (δ 18 O and δ 2 H), leaves for carbon isotope composition (δ 13 C) and stems to quantify non-structural carbohydrates (NSC) concentration, and measured water potential and leaf gas exchange. The first summer drought was severe for both species, reaching low predawn water potential (−2.2 MPa), very low stomatal conductance (12 ± 1.0 mmol m −2  s −1 ) and near-zero or even negative net photosynthesis, particularly in  P. sylvestris  (−0.6 ± 0.34 μmol m −2  s −1  in oaks, −1.3 ± 0.16 μmol m −2  s −1  in pines). Hence, the tighter stomatal control and more isohydric strategy of  P. sylvestris  resulted in larger limitations on carbon assimilation, and this was also reflected in carbon storage, showing twofold larger total NSC concentration in oaks than in pines (7.8 ± 2.4% and 4.0 ± 1.3%, respectively). We observed a faster recovery of predawn water potential after summer drought in  Q. subpyrenaica  than in  P. sylvestris  (−0.8 MPa and −1.1 MPa, respectively). As supported by the isotopic data, this was probably associated with a deeper and more reliable water supply in  Q. subpyrenaica . In line with these short-term observations, we found a more pronounced negative effect of steadily increasing drought stress on long-term growth in pines compared with oaks. All these observations confer evidence of early warning of  P. sylvestris  decline and indicate the adaptive advantage of  Q. subpyrenaica  in the area.

tree physiology

Contrasting ecophysiological strategies related to drought: the case of a mixed stand of Scots pine ( Pinus sylvestris ) and a submediterranean oak ( Quercus subpyrenaica )