Warming and increased CO2 levels are generally believed to enhance photosynthetic rates (A) and tree growth, especially in alpine treelines. However, this positive effect may be limited by moisture availability. Here we compare the radial growth and intrinsic water use efficiency (iWUE) of two widely-distributed evergreen needleleaf species, Tibetan juniper (Juniperus tibetica) and Balfour spruce (Picea balfouriana), at the same site near the treeline on the southeastern Tibetan Plateau using tree-ring width and dual isotopes (δ13C and δ18O). We observed that with a 70-ppm increase in atmospheric CO2 from 1954 to 2007, juniper radial growth was significantly stimulated, whereas spruce growth remained constant. Correlation analysis between tree-ring width and climate factors revealed that spruce was more moisture-sensitive than juniper. Although the increases in iWUE for juniper (22%) and spruce (26%) were similar, iWUE changes in spruce were more influenced by stomatal conductance (gs) rather than A, supporting that moisture availability inhibits spruce growth. Our study implies that predicting growth dynamics in alpine forests based on the growth-temperature relationship may be inadequate, as they are likely moisture-limited. It also suggests that increases in A may not be as widespread as previously thought, which helps improve forest productivity modeling.

中文翻译：

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水分利用策略决定了青藏高原东南部云杉和杜松的不同生长趋势


通常认为变暖和 CO2 水平增加会提高光合速率 （A） 和树木生长，尤其是在高山林线中。然而，这种积极影响可能会受到水分供应的限制。在这里，我们比较了两种分布广泛的常绿针叶物种——藏柏 （Juniperus tibetica） 和巴尔福云杉 （Picea balfouriana） 的径向生长和内在水分利用效率 （iWUE），在青藏高原东南部林线附近的同一地点使用树木年轮宽度和双同位素 （δ13C 和 δ18O）。我们观察到，从 1954 年到 2007 年，大气中 CO2 增加了 70 ppm，杜松径向生长受到显着刺激，而云杉生长保持不变。树木年轮宽度与气候因素之间的相关性分析表明，云杉比杜松对水分更敏感。尽管杜松 （22%） 和云杉 （26%） 的 iWUE 增加相似，但云杉的 iWUE 变化更多地受气孔导度 （gs） 而不是 A 的影响，这支持水分可用性抑制云杉生长。我们的研究表明，基于生长-温度关系预测高山森林的生长动态可能是不充分的，因为它们可能受到水分限制。它还表明 A 的增加可能并不像以前认为的那样普遍，这有助于改进森林生产力建模。