Abstract The boreal biome accounts for approximately one third of the terrestrial carbon (C) sink. However, estimates of its individual C pools remain uncertain. Here, focusing on the southern boreal forest, we quantified the magnitude and temporal dynamics of C allocation to aboveground tree growth at a mature black spruce (Picea mariana)-dominated forest stand in Saskatchewan, Canada. We reconstructed aboveground tree biomass increments (AGBi) using a biometric approach, i.e., species-specific allometry combined with forest stand characteristics and tree ring widths collected with a C-oriented sampling design. We explored the links between boreal tree growth and ecosystem C input by comparing AGBi with eddy-covariance-derived ecosystem C fluxes from 1999 to 2015 and we synthesized our findings with a refined meta-analysis of published values of boreal forest C use efficiency (CUE). Mean AGBi at the study site was decoupled from ecosystem C input and equal to 71 ± 7 g C m–2 (1999–2015), which is only a minor fraction of gross ecosystem production (GEP; i.e., AGBi / GEP ≈ 9 %). Moreover, C allocation to AGBi remained stable over time (AGBi / GEP; –0.0001 yr–1; p-value=0.775), contrary to significant trends in GEP (+5.72 g C m–2 yr–2; p-value=0.02) and CUE (–0.0041 yr–1, p-value=0.007). CUE was estimated as 0.50 ± 0.03 at the study area and 0.41 ± 0.12 across the reviewed boreal forests. These findings highlight the importance of belowground tree C investments, together with the substantial contribution of understory, ground cover and soil to the boreal forest C balance. Our quantitative insights into the dynamics of aboveground boreal tree C allocation offer additional observational constraints for terrestrial ecosystem models that are often biased in converting C input to biomass, and can guide forest-management strategies for mitigating carbon dioxide emissions.

中文翻译：

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地上树木生长是北方森林碳输入的一个次要且分离的部分

摘要 北方生物群落约占陆地碳 (C) 汇的三分之一。然而，对其个别 C 池的估计仍然不确定。在这里，我们以南部北方森林为重点，量化了加拿大萨斯喀彻温省成熟的黑云杉（Picea mariana）占主导地位的林分中 C 分配到地上树木生长的幅度和时间动态。我们使用生物特征方法重建地上树木生物量增量 (AGBi)，即物种特异性异速生长结合林分特征和使用面向 C 的采样设计收集的树木年轮宽度。我们通过比较 AGBi 与 1999 年至 2015 年间涡旋协方差衍生的生态系统碳通量来探索北方树木生长和生态系统碳输入之间的联系，我们将我们的发现与对北方森林碳利用效率 (CUE ）。研究地点的平均 AGBi 与生态系统 C 输入脱钩，等于 71 ± 7 g C m–2 (1999–2015)，这只是生态系统总产量 (GEP；即 AGBi / GEP ≈ 9 %) 的一小部分）。此外，与 GEP 的显着趋势相反（+5.72 g C m–2 yr–2；p 值 = 0.0001 yr​​–1；p 值 = 0.775） 0.02) 和 CUE (–0.0041 yr–1, p 值 = 0.007)。CUE 在研究区域估计为 0.50 ± 0.03，在审查的北方森林中估计为 0.41 ± 0.12。这些发现强调了地下树木碳投资的重要性，以及林下、地被植物和土壤对北方森林碳平衡的重大贡献。我们对地上北方树木碳分配动态的定量见解为陆地生态系统模型提供了额外的观察约束，这些模型通常在将碳输入转化为生物量方面存在偏差，并且可以指导森林管理策略以减少二氧化碳排放。