Cross-scale convergence in the carbon balance of managed boreal forests in Northern Sweden

• Multi-scale comparison of bottom-up and top-down estimates of boreal forest NEP. • Three-year mean NEP estimates converged across plot, landscape, and regional scales. • Inter-annual NEP trends varied across measurement approaches and spatial scales. • NEP response to 2018 drought differed between bottom-up and top-down estimates. • Results consolidate the strong C sink-strength of actively managed boreal forests. Boreal forests are globally important carbon (C) sinks, but strategies for maximising their climate benefit remain under debate. Major uncertainties in this discussion arise from contrasting sink-source estimates, which largely emanate from inherent limitations of standard measurement techniques to distinct spatio-temporal scales. Here, we use a spatially-nested measurement framework that integrates bottom-up (forest-plot inventory and chamber-based fluxes) and top-down (eddy-covariance; atmospheric observations and atmospheric transport modelling) approaches to reconcile the C balance of actively managed boreal forests in Northern Sweden across plot-, ecosystem-, landscape-, and regional scales during 2016–2018. We found that 3-year mean estimates of the net ecosystem production (NEP) across plot-, landscape-, and regional scales did not differ significantly, converging into a mean (± 95 % confidence interval) C sink of 118 ± 27 g C m -2 yr -1 . We also noted a convergence across these scales for the 3-means of the NEP components, i.e., gross primary production (908 ± 48 g C m -2 yr -1 ) and ecosystem respiration (790 ± 40 g C m -2 yr -1 ). However, estimates of the inter-annual variations in NEP and its components were inconsistent among most scales and measurement approaches. Furthermore, our results indicate a scale-dependency in the NEP response to the 2018 European summer drought, with a greater reduction of NEP observed in bottom-up compared to top-down estimates. Thus, this study consolidates the C sink-strength of managed boreal forests and advocates the need for cross-scale assessments to constrain forest C cycle-climate feedbacks.