Department of Crop and Forest Sciences-AGROTECNIO, Universitat de Lleida, Av. Alcalde Rovira Roure 191, 25198 Lleida, Spain.
Unidad de Recursos Forestales, CITA de Aragón, Av. Montañana, 930, 50059 Zaragoza, Spain.
Tree Physiol. 2017 Nov 1;37(11):1478-1492. doi: 10.1093/treephys/tpx101.
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 (δ18O and δ2H), leaves for carbon isotope composition (δ13C) 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.
次地中海森林被认为是地中海和欧亚大陆生态系统之间的生态过渡带，对全球变化非常敏感。在西班牙比利牛斯山脉前地区，报道了苏格兰松（Pinus sylvestris L.）的减少和相关的橡树物种（Quercus spp.）的扩张。尽管这与干旱胁迫的增加有关，但潜在机制尚不完全清楚，也缺乏合适的监测方案。本研究的目的是深入了解预测松树选择性衰退的生理机制，特别是关注碳和水关系。为此，我们在一个混合的苏格兰松和 Quercus subpyrenaica E.H del Villar 林分中进行了一个跨越两个生长季节的采样活动。我们季节性地采集了嫩枝木质部和土壤的水分同位素组成（δ18O 和 δ2H）、叶片的碳同位素组成（δ13C）和茎干来量化非结构性碳水化合物（NSC）浓度，并测量了水势和叶片气体交换。第一个夏季干旱对两个物种都很严重，达到了很低的黎明前水势（-2.2 MPa）、非常低的气孔导度（12 ± 1.0 mmol m-2 s-1）和接近零或甚至负的净光合作用，特别是在苏格兰松中（橡树为-0.6 ± 0.34 μmol m-2 s-1，松树为-1.3 ± 0.16 μmol m-2 s-1）。因此，苏格兰松更紧密的气孔控制和等水策略导致了对碳同化的更大限制，这也反映在碳储存中，橡树的总 NSC 浓度是松树的两倍（分别为 7.8 ± 2.4%和 4.0 ± 1.3%）。与观测到的碳同化结果一致，我们观察到 Q. subpyrenaica 在夏季干旱后的黎明前水势恢复速度比 P. sylvestris 快（分别为-0.8 MPa 和-1.1 MPa）。正如同位素数据所支持的那样，这可能与 Q. subpyrenaica 更深、更可靠的供水有关。与这些短期观测结果一致，我们发现，与橡树相比，不断增加的干旱胁迫对松树长期生长的负面影响更为明显。所有这些观察结果都为苏格兰松衰退的早期预警提供了证据，并表明 Q. subpyrenaica 在该地区具有适应性优势。
