The length of time that carbon remains in forest biomass is one of the largest uncertainties in the global carbon cycle, with both recent historical baselines and future responses to environmental change poorly constrained by available observations. In the absence of large-scale observations, models used for global assessments tend to fall back on simplified assumptions of the turnover rates of biomass and soil carbon pools. In this study, the biomass carbon turnover times calculated by an ensemble of contemporary terrestrial biosphere models (TBMs) are analysed to assess their current capability to accurately estimate biomass carbon turnover times in forests and how these times are anticipated to change in the future. Modelled baseline 1985–2014 global average forest biomass turnover times vary from 12.2 to 23.5 years between TBMs. TBM differences in phenological processes, which control allocation to, and turnover rate of, leaves and fine roots, are as important as tree mortality with regard to explaining the variation in total turnover among TBMs. The different governing mechanisms exhibited by each TBM result in a wide range of plausible turnover time projections for the end of the century. Based on these simulations, it is not possible to draw robust conclusions regarding likely future changes in turnover time, and thus biomass change, for different regions. Both spatial and temporal uncertainty in turnover time are strongly linked to model assumptions concerning plant functional type distributions and their controls. Thirteen model-based hypotheses of controls on turnover time are identified, along with recommendations for pragmatic steps to test them using existing and novel observations. Efforts to resolve uncertainty in turnover time, and thus its impacts on the future evolution of biomass carbon stocks across the world's forests, will need to address both mortality and establishment components of forest demography, as well as allocation of carbon to woody versus non-woody biomass growth.

中文翻译：

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了解全球森林碳交易量的不确定性

碳在森林生物量中保留的时间长度是全球碳循环中最大的不确定性之一，现有的观测值对近期的历史基线和对环境变化的未来响应均没有很好的限制。在缺乏大规模观测的情况下，用于全球评估的模型倾向于依靠生物质和土壤碳库周转率的简化假设。在这项研究中，分析了由当代陆地生物圈模型（TBM）集合计算出的生物质碳周转时间，以评估其当前准确估算森林中生物质碳周转时间的能力，以及预计这些时间在未来如何变化。以1985年至2014年为基准的全球平均森林生物量周转时间在TBM之间为12.2至23.5年。在解释TBM之间总周转的变化方面，物候过程中的TBM差异控制着叶子和细根的分配和周转率，与树木死亡率一样重要。每个TBM表现出的不同管理机制导致了本世纪末广泛的合理周转时间预测。基于这些模拟，不可能就不同地区未来周转时间的可能未来变化以及生物量变化得出可靠的结论。周转时间的时空不确定性都与有关植物功能类型分布及其控制的模型假设紧密相关。确定了十三种基于模型的周转时间控制假说，以及针对实用步骤的建议，以便使用现有和新颖的观察结果对其进行测试。解决周转时间不确定性及其对未来世界森林中生物质碳储量未来演变的影响的努力，将需要解决森林人口统计学的死亡率和人为因素，以及将碳分配给木质和非木质木材的问题。生物量增长。