Natural Resources Institute Finland, Helsinki, Finland.
Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina, USA.
Glob Chang Biol. 2022 May;28(9):2910-2929. doi: 10.1111/gcb.16117. Epub 2022 Feb 22.
The terrestrial net ecosystem productivity (NEP) has increased during the past three decades, but the mechanisms responsible are still unclear. We analyzed 17 years (2001-2017) of eddy-covariance measurements of NEP, evapotranspiration (ET) and light and water use efficiency from a boreal coniferous forest in Southern Finland for trends and inter-annual variability (IAV). The forest was a mean annual carbon sink (252 [ 42] gC ), and NEP increased at rate +6.4-7.0 gC (or ca. +2.5% ) during the period. This was attributed to the increasing gross-primary productivity GPP and occurred without detectable change in ET. The start of annual carbon uptake period was advanced by 0.7 d , and increase in GPP and NEP outside the main growing season contributed ca. one-third and one-fourth of the annual trend, respectively. Meteorological factors were responsible for the IAV of fluxes but did not explain the long-term trends. The growing season GPP trend was strongest in ample light during the peak growing season. Using a multi-layer ecosystem model, we showed that direct fertilization effect diminishes when moving from leaf to ecosystem, and only 30-40% of the observed ecosystem GPP increase could be attributed to . The increasing trend in leaf-area index (LAI), stimulated by forest thinning in 2002, was the main driver of the enhanced GPP and NEP of the mid-rotation managed forest. It also compensated for the decrease of mean leaf stomatal conductance with increasing and LAI, explaining the apparent proportionality between observed GPP and trends. The results emphasize that attributing trends to their physical and physiological drivers is challenged by strong IAV, and uncertainty of LAI and species composition changes due to the dynamic flux footprint. The results enlighten the underlying mechanisms responsible for the increasing terrestrial carbon uptake in the boreal zone.
过去三十年来，陆地净生态系统生产力（NEP）一直在增加，但负责的机制仍不清楚。我们分析了 2001-2017 年芬兰南部一个北方针叶林的涡度协方差测量的 17 年 NEP、蒸散量（ET）和光水利用效率的趋势和年际变异性（IAV）。该森林是一个年均碳汇（252[42]gC），在此期间，NEP 以+6.4-7.0gC（或约 2.5%）的速度增加。这归因于总初级生产力（GPP）的增加，并且在 ET 没有可检测到的变化的情况下发生。每年碳吸收期的开始提前了 0.7 天，主生长季外的 GPP 和 NEP 增加分别约占年趋势的三分之一和四分之一。气象因素是通量的年际变异性的原因，但不能解释长期趋势。在充足的光照下，生长季 GPP 趋势在生长高峰期最强。使用多层生态系统模型，我们表明，从叶片到生态系统，直接施肥效应会减弱，只有观察到的生态系统 GPP 增加的 30-40%可以归因于。2002 年森林疏伐刺激的叶面积指数（LAI）的增加是增强的 GPP 和 NEP 的主要驱动因素，中轮作管理森林。它还补偿了随着 和 LAI 的增加而导致的平均叶片气孔导度的降低，解释了观察到的 GPP 和 的趋势之间的明显比例关系。结果强调，由于强烈的 IAV，以及由于动态通量足迹导致的 LAI 和物种组成变化的不确定性，将趋势归因于其物理和生理驱动因素具有挑战性。结果启发了负责北方地区陆地碳吸收增加的潜在机制。
