Greenhouse gas and energy fluxes in a boreal peatland forest after clearcutting
Abstract:
The most common forest management method in Fennoscandia is rotation forestry including clearcutting and forest regeneration. In clearcutting, stem wood is removed and the logging residues are either removed or left on site. Clearcutting changes the microclimate and vegetation structure at the site, both of which impact the site’s carbon balance. Peat soils with poor aeration and high carbon (C) densities are especially prone to such changes, and significant changes in C stocks and greenhouse gas exchange can be expected. We measured carbon dioxide (CO2) and energy fluxes with the eddy covariance method for two years (April 2016–March 2018) on a peatland drained for forestry. After the clearcutting, we observed a significant rise (23cm) in the water table level. The site was also a large CO2 source (first year: 3086±120gCO2m−2yr−1, second year: 2072±141gCO2m−2yr−1) after the clearcutting. These large CO2 emissions resulted from the collapse of gross primary production (GPP) following the removal of photosynthesizing trees and the decline of ground vegetation. During the second summer (June–August) after the clearcutting, GPP had already increased by 96% and total ecosystem respiration decreased by 14% from the previous summer. As a result, net CO2 emissions decreased during the second summer after clearcutting compared to the first one. The Bowen ratios were different in 2016 and 2017, starting at 2.6 in May 2016 and decreasing to less than 1.0 in August 2016, while in 2017 it varied mostly within 0.6–1.0. This was due to a 33% decrease in the sensible heat flux and a 40% increase in the latent heat flux from the 2016 values, probably due to the recovery of ground vegetation that increased evapotranspiration and albedo of the site. In addition to CO2 and energy fluxes, we measured methane (CH4) and nitrous oxide (N2O) fluxes with manual chambers. After the clearcutting, the site turned from a small CH4 sink into a small source and from N2O-neutral to a significant N2O source. Compared to the large CO2 emissions, the global warming potential (GWP100) of the CH4 emissions was negligible. Also, the GWP100 due to increased N2O emissions was less than 10% of that of the CO2 emission change.
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