Natascha Kljun

The future role of boreal forests in the global carbon cycle is uncertain given the rapid climate change in high latitudes. At the landscape scale, heterogeneity in stand age and land cover, contributions from terrestrial and aquatic fluxes, and harvest export may create complex carbon cycle-climate interactions. However, the integrated response of the net landscape carbon balance (NLCB) to inter-annual variations (IAVs) in environmental conditions is poorly understood. Here, we used tall-tower eddy covariance and stream monitoring to integrate terrestrial and aquatic carbon fluxes with harvest export for a 68 km² boreal catchment in Sweden during 2016–2020. This actively managed forest landscape acted as a net carbon sink with a 5-year mean (± standard deviation) NLCB of 128±55 g C m^-2 yr^-1. The NLCB IAV included a reduced sink (36 g C m^-2 yr^-1) during the cool/cloudy year 2017. In the other four years, featuring a drought summer (2018) and an exceptionally warm/wet winter (2020), the landscape acted as a significant sink (127–180 g C m^-2 yr^-1). The NLCB IAV corresponded primarily to variations in landscape respiration, followed by GPP and harvest export, with negligible contributions from landscape CH₄ and aquatic carbon fluxes. The NLCB IAV was not correlated to any single environmental factor. However, daily NLCB contrastingly responded to key environmental factors as a function of forest aboveground biomass and mire contributions. Overall, our study indicates that the annual carbon sink-strength of the managed boreal forest landscape may be resilient to a wide range of IAVs in environmental conditions.