NPP Boreal Forest: Consistent Worldwide Site Estimates, 1977-1994

The three objectives of this study were: (1) to summarize net primary productivity (NPP) and carbon allocation patterns for boreal forests, (2) to examine relationships between climatic and biological variables and NPP, and (3) to examine carbon allocation coefficients for all boreal forests or types of boreal forests that can be used to estimate NPP from easily measured components of NPP. Twenty-four Class I boreal forest stands (complete NPP budgets) and 45 Class II stands (aboveground NPP - ANPP - budget only) were identified. The geographic distribution of the Class I stands was not uniform; 46 percent of the stands were from two studies in North America and only one stand was from the important larch forests of Eurasia. Total (above- and below-ground) net primary productivity (NPP) ranged from 52 to 868 gC/m2/yr and averaged 424 gC/m2/yr. ANPP was consistently larger for deciduous than evergreen boreal forests in each of the major boreal regions, especially for boreal forests in Alaska. Ratios of belowground net primary productivity (BNPP) to total net primary productivity (BNPP:TNPP) were consistently larger for evergreen (0.36) than deciduous (0.19) boreal forests. NPP of different-aged stands in age sequence varied from 44 to 77 percent - a magnitude equal or greater to that of climatic factors or vegetation type. NPP and ANPP were positively correlated (r2 = 0.66 to 0.68) to mean annual aboveground increment for Class I stands and this empirical relationship explained 81 percent of the observed variation of ANPP for Class II stands. These robust relationships provide an approach for increasing the number and spatial coverage of boreal forest NPP data needed to evaluate NPP estimates from ecosystem models. Notable deficiencies of boreal forest NPP data were: ground layer vegetation and belowground NPP data, NPP data for boreal forest age sequences, and NPP data for boreal larch ecosystems in Eurasia.

本研究设定了三项核心目标：(1) 梳理寒带森林（boreal forests）的净初级生产力（Net Primary Productivity, NPP）与碳分配模式；(2) 探究气候变量、生物变量与NPP之间的关联关系；(3) 推导适用于所有寒带森林或各类寒带森林的碳分配系数，从而可通过易于测定的NPP组分估算NPP值。

本研究共识别出24个I类寒带林分（具备完整NPP收支数据）以及45个II类林分（仅包含地上净初级生产力（Aboveground Net Primary Productivity, ANPP）收支数据）。I类林分的地理分布并不均衡：其中46%的林分来自北美地区的两项研究，仅1个林分来自欧亚大陆重要的落叶松林生态系统。

地上与地下全量净初级生产力（Total Net Primary Productivity, TNPP）区间为52~868 gC/m²/yr，平均值达424 gC/m²/yr。在各主要寒带林区中，落叶型寒带森林的ANPP始终高于常绿型寒带森林，这一差异在阿拉斯加地区的寒带林中尤为突出。地下净初级生产力（Belowground Net Primary Productivity, BNPP）与TNPP的比值（BNPP:TNPP）在常绿寒带林中（均值0.36）始终高于落叶型寒带林（均值0.19）。

不同林龄林分的NPP占比跨度为44%~77%，该变化幅度不低于气候因子或植被类型带来的影响幅度。对于I类林分，NPP与ANPP均与年平均地上增量呈显著正相关（决定系数r²=0.66~0.68）；该经验关系可解释II类林分ANPP观测变异的81%。

上述稳健的关联关系为扩充寒带森林NPP数据的样本量与空间覆盖范围提供了可行路径，以助力评估生态系统模型输出的NPP估算结果。当前寒带森林NPP数据集仍存在明显不足：包括地表层植被与地下NPP数据、寒带森林林龄序列相关NPP数据，以及欧亚大陆寒带落叶松林生态系统的NPP数据。