How calcium (Ca) uptake by trees in eastern Canadian forests has responded to decades of atmospheric acid deposition and its sharp decrease that started three decades ago is still largely unknown. Here, we tested a novel approach based on the measurements of Ca and strontium (Sr) concentrations and Sr isotope signature (⁸⁷Sr/⁸⁶Sr ratio) in sequential extracts of sugar maple tree rings and soils to assess changes in Ca availability and sources through time. The study was conducted at three sites showing a gradient of one order of magnitude in soil mineral weathering fluxes and exchangeable pools of Ca. We found that wood Ca and Sr concentrations increased with Ca and Sr concentrations in the exchangeable fraction (NH₄Cl extracts) of the top mineral soil across sites. In addition, wood ⁸⁷Sr/⁸⁶Sr ratio was strongly correlated with ⁸⁷Sr/⁸⁶Sr ratio of this soil fraction. Both Ca and Sr concentrations in the exchangeable fraction of the wood decreased from pith to bark at all sites, whereas no trend was observed for the wood residual fraction. This was interpreted as a result of radial mobility of Ca in sapwood rather than a decline in the uptake of Ca and Sr by tree roots over time. The ⁸⁷Sr/⁸⁶Sr ratio of tree rings slightly increased over time only at the site with the higher soil exchangeable Ca pool, suggesting an increase in the contribution of soil mineral weathering relative to atmospheric deposition. In contrast with our hypothesis, our results show i) no evidence of pronounced changes in the relative contribution of the atmospheric vs. soil mineral weathering Ca components over time at the site with the smallest soil exchangeable Ca pools; and ii) the difficulty of assessing the temporal changes in the availability and the sources of Ca from sequential extractions of the wood due to a dynamic exchange process of Sr (and therefore Ca) between the exchangeable and residual fractions of the wood.

在加拿大东部森林中树木对钙（Ca）的吸收如何响应数十年来的大气酸沉降及其三十年前开始的急剧下降一直是未知的。在这里，我们测试了一种新方法，该方法基于对糖枫树年轮和土壤的连续提取物中的钙和锶（Sr）浓度以及Sr同位素特征（⁸⁷ Sr / ⁸⁶ Sr比）的测量，以评估Ca的有效性和来源的变化，时间。该研究是在三个地点进行的，这些地点显示出土壤矿物风化通量和可交换的Ca池中一个数量级的梯度。我们发现木材中的Ca和Sr浓度随可交换部分（NH ₄Cl提取物）跨站点的顶级矿物土壤。此外，木材⁸⁷的Sr / ⁸⁶ Sr比值是紧密联系在一起相关⁸⁷ SR / ⁸⁶这种土壤馏分Sr比值。木材可交换部分中的Ca和Sr浓度在所有部位都从髓减少到树皮，而木材中残留部分没有任何趋势。这被解释为钙在边材中的径向迁移的结果，而不是由于树根随时间推移对钙和锶的吸收下降的结果。在⁸⁷ SR / ⁸⁶仅在土壤交换性钙池较高的地方，树木年轮的Sr比随时间的推移略有增加，表明土壤矿物风化对大气沉积的贡献增加。与我们的假设相反，我们的结果表明：i）没有证据表明在土壤交换性钙池最小的地点，大气与土壤矿物风化钙组分的相对贡献随时间的变化显着；ii）由于木材的可交换部分和剩余部分之间Sr（以及因此的Ca）的动态交换过程，难以评估木材的连续提取过程中Ca的可用性和来源的时间变化。