Effects of low thinning on carbon dioxide fluxes in a mixed hemiboreal forest

We used eddy-covariance (EC) measurements of net ecosystem exchange (NEE) above canopy to assess the effects of thinning on CO2 fluxes at the ICOS Sweden site Norunda in central Sweden. This forest site consists of mixed pine and spruce stands approx. 100 years old. The thinning during late autumn 2008, performed in a semi-circle from the mast extending 200 m outwards harvested about 25% of the volume. Measurements were conducted from 2007 to 2016 and thus, above canopy fluxes were recorded two years before and eight years after the thinning. We also measured the net flux from the forest floor with automatic chambers in three locations and with below-canopy EC during shorter periods before and after thinning. The chamber measurements during the first part of the growing season after thinning showed strongly enhanced effluxes in the order of 150–250% of the pre-thinning values. These chamber measurements were made on drier places within the thinned area because waterlogging made it impossible to use chambers at all available locations. The below-canopy EC measurements, which had a larger footprint as compared to the chambers, showed less enhanced fluxes (in the order of 35%). This footprint included also wetter areas. The above canopy EC measurements showed a reduction of daytime net flux by approx. 30% during the first summer after thinning. The median growing season fluxes then slowly increased but were not restored to the pre-thinning levels eight years after thinning. There was also a small decrease in growing season ecosystem respiration during the first summer after thinning and with a continued decreasing trend over time. It was concluded that this decrease in respiration was caused by successively decreasing decomposition of coarse organic substrates resulting from the thinning. This respiration decrease over time persisted even under gradual biomass increase, which otherwise would indicate increasing autotrophic respiration. The light-response and respiration models fitted to all data did not show any trends in daytime or nighttime fluxes so the conclusion was that the trends were caused by the thinning and not because of trends in meteorological drivers. The annual values contrasted with the summertime results since only a minor effect was observed on the annual NEE. Both ecosystem respiration and gross primary productivity were reduced as an effect of thinning. We explained the different summertime versus annual effects to be caused by the decrease in ecosystem respiration since respiration is dominating the NEE during non-growing season periods when photosynthesis is very low or even zero. Our results are a strong indication that the NEE of a forest could be maintained over time with harvesting practices that avoids clear-cutting and thereby enhance the total carbon uptake of forests