The carbon balance of a Scots pine forest following severe windthrow: Comparison of reforestation techniques

Even though windthrows are the main disturbance type in European forests, their impact on forest carbon balance is generally understudied. In order to close this knowledge gap, we took advantage of a tornado event, which occurred in July 2012 in Trzebciny Forest District in northwest Poland. The main objective of this study was to determine how two completely different reforestation techniques in windthrow areas impact net ecosystem production (NEP) of this Scots pine forest ecosystem under similar meteorological conditions. The two techniques were: conventional (“Tlen I” site): uprooted stumps pulled out and removed from the site followed by ploughing, and non-conventional (“Tlen II” site): all stumps left on the site to decompose with no ploughing. Therefore, we measured carbon dioxide (CO₂) fluxes using the eddy covariance (EC) technique for four continuous years. Our results indicate that both sites became significant carbon (C) sources after the windthrow (up to 575 ± 56 g C m⁻² y^-1 in the first year, Tlen I). However, the Tlen I (conventional technique) lost over 30% less C than Tlen II during the 2015–2016 observation period. In contrast to existing knowledge, ploughing as done at Tlen I, did not substantially increase CO₂ emission, as compared to local soil ripping (non-conventional technique). The decrease in net C emission, estimated on the basis of linear model parameters, was almost five times greater at Tlen I than at Tlen II. We thus hypothesize that the annual NEP at conventionally reforested windthrow site will reach C neutrality (NEP = 0) six years after windthrow at most, while at the non-conventionally managed area it will probably take ca. 18 years. So far then, the currently widely applied conventional reforestation technique in wind-disturbed Polish forest appeared to be more effective in decreasing C losses than a technique that leaves stumps to decompose and avoids ploughing.