The forest floor provides an important interface of soil-atmosphere CO₂ exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO₂ exchange (NFFE) and the entire net ecosystem CO₂ exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO₂ fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (R_ff) was higher and photosynthesis (GPP_ff) was lower, leading to a 1.4 times stronger net CO₂ source compared to the pine stand. We further found that drought enhanced R_ff more than GPP_ff, leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (R_eco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting R_eco, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO₂ fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.

森林地面提供了土壤-大气CO ₂交换的重要接口，但是由于测量限制，它们的控制和对生态系统规模碳预算的贡献尚不确定。在这项研究中，我们在树冠之下和树冠之上部署了涡旋协方差系统，以测量空间整合的林地净CO ₂交换量（NFFE）和整个净生态系统CO _2的量。瑞典北方两个非常成熟的对比展位上进行交换（NEE）。我们首先开发了一种改进的共谱模型，以校正冠层以下的通量数据。我们的经验性冠层下共谱模型显示，相比于冠层上湍流共谱中的分布，干线空间中大型和小型涡流的贡献更大。我们发现，应用冠层以上共谱模型不会影响稀疏松树林下冠层以下的年度CO ₂通量，但会严重低估茂密的云杉-松混交林通量。在云杉-松树混交林中，林地呼吸（R _ff）较高，光合作用（GPP _ff）较低，导致净CO ₂增强1.4倍来源相比松林。我们进一步发现干旱比GPP _ff增加的R _ff更大，导致NFFE增加。在六个地点年内，森林地表通量对生态系统规模呼吸（R _eco）的贡献为82％，对初级生产总值（GPP）的贡献为12％。由于两个展台之间的年度GPP相似，因此其年度NEE的显着差异是由于R _eco的差异，后者主要是由NFFE的变化所驱动。这意味着NFFE充当了这两个对比立场之间NEE差异的驱动因素。因此，本研究强调了林地CO ₂的重要作用。通量调节北方森林碳平衡。它还要求在获取高时空分辨率的林底通量数据方面作出更大的努力，以改善对全球变化对森林碳循环影响的预测。