Department of Environmental Sciences, Utrecht University, Utrecht, The Netherlands.
Department of Geography, Swansea University, Swansea, UK.
Tree Physiol. 2019 Jun 1;39(6):983-999. doi: 10.1093/treephys/tpz018.
Altitudinally separated bristlecone pine populations in the White Mountains (California, USA) exhibit differential climate-growth responses as temperature and tree-water relations change with altitude. These populations provide a natural experiment to explore the ecophysiological adaptations of this unique tree species to the twentieth century climate variability. We developed absolutely dated annual ring-width chronologies, and cellulose stable carbon and oxygen isotope chronologies from bristlecone pine growing at the treeline (~3500 m) and ~200 m below for the period AD 1710-2010. These chronologies were interpreted in terms of ecophysiological adaptations to climate variability with a dual-isotope model and a leaf gas exchange model. Ring widths show positive tree growth anomalies at treeline and consistent slower growth below treeline in relation to the twentieth century warming and associated atmospheric drying until the 1980s. Growth rates of both populations declined during and after the 1980s when growing-season temperature and atmospheric vapour pressure deficit continued to increase. Our model-based interpretations of the cellulose stable isotopes indicate that positive treeline growth anomalies prior to the 1980s were related to increased stomatal conductance and leaf-level transpiration and photosynthesis. Reduced growth since the 1980s occurred with a shift to more conservative leaf gas exchange in both the treeline and below-treeline populations, whereas leaf-level photosynthesis continued to increase in response to rising atmospheric CO2 concentrations. Our results suggest that warming-induced atmospheric drying confounds positive growth responses of apparent temperature-limited bristlecone pine populations at treeline. In addition, the observed ecophysiological responses of attitudinally separated bristlecone pine populations illustrate the sensitivity of conifers to climate change.
高海拔分离的怀特山脉（美国加利福尼亚州）的狐尾松种群，随着温度和树木水分关系随海拔的变化，表现出不同的气候-生长响应。这些种群为探索这种独特树种对 20 世纪气候变异性的生态生理适应提供了一个自然实验。我们从生长在林线（3500 米）和林线下200 米处的狐尾松中建立了绝对日期的年轮宽度年表，以及纤维素稳定的碳和氧同位素年表，时间范围为公元 1710 年至 2010 年。我们根据双同位素模型和叶片气体交换模型，根据气候变异性对这些年表进行了解释。年轮宽度显示林线处的树木生长异常为正，与 20 世纪的变暖以及相关的大气干燥相比，林线下的生长速度一直较慢，直到 20 世纪 80 年代。两个种群的生长速度都在 20 世纪 80 年代期间和之后下降，当时生长季节的温度和大气蒸气压亏缺继续增加。我们对纤维素稳定同位素的模型解释表明，20 世纪 80 年代之前林线处的正生长异常与气孔导度和叶片水平蒸腾和光合作用增加有关。自 20 世纪 80 年代以来，由于在林线和林线下种群中，叶片气体交换向更保守的方向转变，生长速度减缓，而叶片水平的光合作用继续增加，以应对大气中不断增加的二氧化碳浓度。我们的研究结果表明，变暖引起的大气干燥使林线处明显受温度限制的狐尾松种群的正生长反应变得复杂。此外，分离的高海拔狐尾松种群的生态生理响应表明了针叶树对气候变化的敏感性。
