Brownlee Annalis H, Sullivan Patrick F, Csank Adam Z, Sveinbjörnsson Bjartmar, Ellison Sarah B Z
Ecology. 2016 Jan;97(1):145-59. doi: 10.1890/15-0338.1.
Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as divergence. In the Brooks Range of northern Alaska, USA, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought stress. To investigate the hypothesis that drought-induced stomatal closure can explain divergence in the Brooks Range, we synthesized all of the Brooks Range white spruce data available in the International Tree Ring Data Bank (ITRDB) and collected increment cores from our primary sites in each of four watersheds along a west-to-east gradient near the Arctic treeline. For cores from our sites, we measured ring widths and calculated carbon isotope discrimination (δ13C), intrinsic water-use efficiency (iWUE), and needle intercellular CO2 concentration (C(i)) from δ13C in tree-ring alpha-cellulose. We hypothesized that trees exhibiting divergence would show a corresponding decline in δ13C, a decline in C(i), and a strong increase in iWUE. Consistent with the ITRDB data, trees at our western and central sites generally showed an increase in the strength of the temperature-growth correlation during late 20th century warming, while trees at our eastern site showed strong divergence. Divergent tree growth was not, however, associated with declining δ13C. Meanwhile, estimates of C(i) showed a strong increase at all of our study sites, indicating that more substrate was available for photosynthesis in the early 21st than in the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange, and measurements of growth and δ13C along moisture gradients within each watershed, suggest that drought-induced stomatal closure is probably not the cause of 20th century divergence in the Brooks Range.
长期以来，来自北方森林的树木年轮芯一直被用于重建过去的气候。然而，近年来，大量研究揭示了温度与树木生长之间的相关性恶化，这一现象通常被称为“背离”。在美国阿拉斯加北部的布鲁克斯山脉，对白云杉（Picea glauca）的研究表明，西部的树木总体上呈现出积极的生长趋势，而在20世纪后期变暖期间，布鲁克斯山脉中部和东部的树木则呈现出混合和消极的趋势。布鲁克斯山脉东部的生长季节气候被认为比西部更干燥。在此基础上，布鲁克斯山脉东部树木生长的背离被归因于干旱胁迫。为了研究干旱诱导的气孔关闭能否解释布鲁克斯山脉的背离现象，我们综合了国际树木年轮数据库（ITRDB）中所有可用的布鲁克斯山脉白云杉数据，并从北极树线附近四个流域中每个流域的主要地点沿西向东梯度采集了树木年轮芯。对于我们站点的芯样，我们测量了年轮宽度，并根据树木年轮α-纤维素中的δ13C计算了碳同位素分馏（δ13C）、内在水分利用效率（iWUE）和针叶细胞间CO2浓度（C(i)）。我们假设表现出背离现象的树木将显示出δ13C相应下降、C(i)下降以及iWUE大幅增加。与ITRDB数据一致，我们西部和中部站点的树木在20世纪后期变暖期间，温度与生长的相关性强度总体上有所增加，而我们东部站点的树木则表现出强烈的背离。然而，树木生长的背离与δ13C的下降并无关联。与此同时，C(i)的估计值在我们所有的研究站点都大幅增加，这表明21世纪初比20世纪初有更多的底物可用于光合作用。我们的结果得到了木质部液流通量密度测量、针叶气体交换以及每个流域内沿湿度梯度的生长和δ13C测量的证实，这表明干旱诱导的气孔关闭可能不是布鲁克斯山脉20世纪背离现象的原因。
