Colorado State University, Department of Forest and Rangeland Stewardship, Campus Delivery 1472, Fort Collins, CO 80523, USA; National Park Service, Water Resources Division, 1201 Oakridge Drive #250, Fort Collins, CO 80525, USA.
U.S. Geological Survey, 2150 Centre Avenue, Building C, Fort Collins, CO 80526, USA.
Sci Total Environ. 2020 Sep 15;735:139523. doi: 10.1016/j.scitotenv.2020.139523. Epub 2020 May 20.
Long duration tree-ring records with annual precision allow for the reconstruction of past growing conditions. Investigations limited to the most common tree-ring proxy of ring width can be difficult to interpret, however, because radial growth is affected by multiple environmental processes. Furthermore, studies of living trees may miss important effects of drought on tree survival and forest changes. Stable carbon isotopes can help distinguish drought from other environmental factors that influence tree-ring width and forest stand condition. We quantified tree-ring radial expansion and stable carbon isotope ratios (δC) in riparian cottonwoods (Populus angustifolia and P. angustifolia x P.trichocarpa) along Snake Creek in Nevada, USA. We investigated how hydrological drought affected tree growth and death at annual to half-century scales in a partially dewatered reach (DW) compared to reference reaches immediately upstream and downstream. A gradual decline in tree-ring basal area increment (BAI) began at DW concurrent to streamflow diversion in 1961. BAI at DW diverged from one reference reach immediately but not from the other until nearly 50 years later. In contrast, tree-ring δC had a rapid and sustained increase following diversion at DW only, providing the stronger and clearer drought signal. BAI and δC were not significantly correlated prior to diversion; after diversion they both reflected drought and were correlated for DW trees only. Cluster analyses distinguished all trees in DW from those in reference reaches based on δC, but BAI patterns left trees intermixed across reaches. Branch and tree mortality were also highest and canopy vigor was lowest in DW. Results indicate that water scarcity strongly limited cottonwood photosynthesis following flow diversion, thus reducing carbon assimilation, basal growth and survival. The dieback was not sudden, but occurred over decades as carbon deficits mounted and depleted streamflow left trees increasingly vulnerable to local meteorological drought.
长时段、具有年度精度的树木年轮记录可用于重建过去的生长条件。然而，仅对最常见的树木年轮代理——树轮宽度进行研究可能难以解释，因为径向生长受到多种环境过程的影响。此外，对活树的研究可能会错过干旱对树木存活和森林变化的重要影响。稳定的碳同位素可以帮助区分干旱与影响树木年轮宽度和森林林分状况的其他环境因素。我们量化了美国内华达州蛇溪沿岸河岸棉白杨（Populus angustifolia 和 P. angustifolia x P. trichocarpa）的树木年轮径向扩张和稳定的碳同位素比值（δC）。我们研究了在部分脱水段（DW）与上游和下游参考段相比，水文干旱如何在年际到半世纪的时间尺度上影响树木生长和死亡。自 1961 年溪流改道以来，DW 的树木年轮基面积增量（BAI）逐渐下降。BAI 在 DW 与一个参考段同时偏离，但直到近 50 年后才与另一个参考段偏离。相比之下，只有在 DW 分流后，树木年轮 δC 才会迅速且持续增加，从而提供更强、更清晰的干旱信号。在分流之前，BAI 和 δC 没有显著相关性；分流后，它们都反映了干旱，仅与 DW 树木相关。聚类分析根据 δC 将 DW 中的所有树木与参考段中的树木区分开来，但 BAI 模式使树木在流域内相互混合。DW 中的树枝和树木死亡率也最高，树冠活力最低。结果表明，水流改道后，水资源短缺强烈限制了棉白杨的光合作用，从而减少了碳同化、基础生长和存活。衰退不是突然发生的，而是在几十年的时间里逐渐发生的，因为碳亏空增加，耗竭的溪流使树木越来越容易受到当地气象干旱的影响。
