Thinning turned boreal forest to a temporary carbon source - short term effects of partial harvest on carbon dioxide and water vapor fluxes
Toprak Aslan, Samuli Launiainen, Pasi Kolari, Olli Peltola, Juho Aalto, Jaana Bäck, Timo Vesala, Ivan Mammarella
Abstract
Even though the effect of thinning on CO2 and H2O 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 above- and 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−2yr−1) to a moderate carbon source (+115 gCm−2yr−1) during the year of thinning due to decreased ecosystem gross primary productivity (GPPeco) and simultaneous increase in ecosystem respiration (Reco). 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 GPPeco (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 Reco 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.