Forest floor fluxes drive differences in the carbon balance of contrasting boreal forest stands

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 Rff 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 spatio-temporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.