From source to sink – recovery of the carbon balance in young forests.

• Young forests may return to carbon sinks already eight years after reforestation. • Increased forest productivity shortens the time needed to return to carbon sink. • Harvest of logging residues/stumps for bioenergy may improve the carbon balance. We analyzed ecosystem carbon fluxes from eddy-covariance measurements in five young forests in southern Sweden where the previous stand had been harvested by clear-cutting or wind-felled: three stands with Norway spruce (Picea abies (L.) Karst.), one with Scots pine (Pinus sylvestris) and one with Larch (Larix x eurolepis A. Henry). One of the spruce stands had the stumps harvested, one was fertilized and one without any special treatments. These stands returned from positive (sources) to negative (sinks) annual carbon fluxes 8–13 years after disturbance, depending on site productivity and management. This corresponds to approximately 15% of the rotation periods at these sites. Extrapolation in combination with chronosequence data suggests that conventionally regenerated stands reach a neutral carbon balance after approximately 30% of the rotation period. The lowest carbon emissions and shortest recovery time was observed in a stand where the stumps of the trees, in addition to the stems and logging residues, were removed after harvest. This stand not only returned to a carbon sink within this time period but the total carbon gains since disturbance also equaled the total losses after only 11 years. These results stress that production stands in southern Sweden are carbon sources during a relatively small part of the rotation period, and that this part can be considerably shortened by measures that increase productivity or reduce the amount of woody debris left after disturbance. [ABSTRACT FROM AUTHOR]