Thinning turned boreal forest to a temporary carbon source- short term effects of partial harvest on carbon dioxide and water vapor fluxes

Even though the effect of thinning on CO 2 and H 2 O fluxes has been widely investigated, a holistic description of thinning -induced responses is yet to be provided. Here, we present a comprehensive study, investigating the impact of commercial thinning in an even -aged boreal forest in southern Finland using concurrent aboveand sub -canopy eddy -covariance measurements and a process -based ecosystem model. The thinning was done from below and removed ca. 40% of the basal area. The forest turned from a strong sink ( - 271 gCm -2 yr -1 ) to a moderate carbon source (+115 gCm -2 yr -1 ) during the year of thinning due to decreased ecosystem gross primary productivity (GPP eco ) and simultaneous increase in ecosystem respiration (R eco ). The reduced canopy density increased the light availability, near -ground air temperature and wind speed. This improved the photosynthetic efficiency of the remaining trees, resulting in only a moderate reduction in GPP eco (ca. 20%) compared to the foliage loss (ca. 45%). The decomposition of cutting residue likely increased the heterotrophic respiration that compensated for the reduced autotrophic respiration of removed trees, leading to R eco exceeding long-term average by ca. 10% during the year of thinning. Interestingly, thinning did not affect ecosystem evapotranspiration but changed its partitioning: both stand transpiration and interception evaporation decreased, whereas forest floor evapotranspiration increased. The inter -annual weather variability did not notably affect annual fluxes, which enabled robust quantification of thinning impacts. Our results show a strong qualitative resemblance with previously reported short-term responses of boreal forest to thinning. This is presumably due to similar management practices and species composition among the studies, and low variability of inter -annual weather and fluxes. Our study showed that sub -canopy eddy covariance measurements and process -based model can play a pivotal role in disentangling the confounding responses of forest floor and canopy to thinning.