Ecosystem and Conservation Sciences, University of Montana, Missoula, Montana.
Division of Biological Sciences, University of Montana, Missoula, Montana.
Glob Chang Biol. 2018 Sep;24(9):3922-3937. doi: 10.1111/gcb.14165. Epub 2018 May 4.
Forests sequester large amounts of carbon annually and are integral in buffering against effects of global change. Increasing atmospheric CO may enhance photosynthesis and/or decrease stomatal conductance (g ) thereby enhancing intrinsic water-use efficiency (iWUE), having potential indirect and direct benefits to tree growth. While increasing iWUE has been observed in most trees globally, enhanced growth is not ubiquitous, possibly due to concurrent climatic constraints on growth. To investigate our incomplete understanding of interactions between climate and CO and their impacts on tree physiology and growth, we used an environmental gradient approach. We combined dendrochronology with carbon isotope analysis (δ C) to assess the covariation of basal area increment (BAI) and iWUE over time in lodgepole pine. Trees were sampled at 18 sites spanning two climatically distinct elevation transects on the lee and windward sides of the Continental Divide, encompassing the majority of lodgepole pine's northern Rocky Mountain elevational range. We analyzed BAI and iWUE from 1950 to 2015, and explored correlations with monthly climate variables. As expected, iWUE increased at all sites. However, concurrent growth trends depended on site climatic water deficit (CWD). Significant growth increases occurred only at the driest sites, where increases in iWUE were strongest, while growth decreases were greatest at sites where CWD has been historically lowest. Late summer drought of the previous year negatively affected growth across sites. These results suggest that increasing iWUE, if strong enough, may indirectly benefit growth at drier sites by effectively extending the growing season via reductions in g . Strong growth decreases at high elevation windward sites may reflect increasing water stress as a result of decreasing snowpack, which was not offset by greater iWUE. Our results imply that increasing iWUE driven by decreasing g may benefit tree growth in limited scenarios, having implications for future carbon uptake potential of semiarid ecosystems.
森林每年固定大量的碳，在缓冲全球变化的影响方面发挥着重要作用。大气中 CO 浓度的增加可能会增强光合作用和/或降低气孔导度（g），从而提高内在水分利用效率（iWUE），这对树木生长具有潜在的间接和直接益处。虽然在大多数全球树木中都观察到了 iWUE 的提高，但增强的生长并非普遍存在，这可能是由于生长同时受到气候的限制。为了研究我们对 CO 与气候之间相互作用及其对树木生理和生长影响的理解不完整，我们采用了环境梯度方法。我们结合树木年代学和碳同位素分析（δ C）来评估在背风侧和迎风侧大陆分水岭上两个气候差异明显的海拔梯度上的 18 个地点的黑云杉基面积增量（BAI）和 iWUE 的时间协变。我们分析了 1950 年至 2015 年的 BAI 和 iWUE，并探索了与每月气候变量的相关性。正如预期的那样，所有地点的 iWUE 都在增加。然而，同期的生长趋势取决于地点的气候水分亏缺（CWD）。只有在最干旱的地点才会出现显著的生长增加，而在 CWD 历史最低的地点，生长减少最大。前一年夏末干旱对所有地点的生长都有负面影响。这些结果表明，如果 iWUE 增强足够强，通过减少 g 有效延长生长季，可能会间接地使较干燥地点的生长受益。高海拔迎风地点的强烈生长减少可能反映出由于积雪减少而导致的水分胁迫增加，而这并没有被更高的 iWUE 所抵消。我们的结果表明，由 g 减少驱动的 iWUE 增加可能在有限的情况下有利于树木生长，这对半干旱生态系统未来的碳吸收潜力具有重要意义。
