Is pre-commercial thinning compatible with carbon sequestration? A case study in a maritime pine stand in northwestern Spain

The mid-term effects on carbon sequestration of two consecutive pre-commercial thinnings with different intensities were analysed in a heavily stocked Pinus pinaster Ait. stand in northwestern Spain. Thinning was carried out 5 and 8 years after seedling establishment. Intense thinning reduced the initial sapling density by 92 per cent after the first treatment and by 87 per cent after the second; moderate thinning reduced the density by 82 per cent after the first treatment and by 84 per cent after the second. The effect on foliar efficiency of the first thinning was transient, but was prolonged by the second thinning. Live carbon biomass stock (in saplings and understorey) decreased after thinning, especially after the most intense thinning. However, when residues were taken into account, the carbon biomass decreased by only 18 per cent in the intensely thinned plots, and by 2 per cent in the moderately thinned plots, relative to the untreated plots, by the end of the study period. Very early thinning of these heavily stocked young stands was found to have a reduced mid-term effect on carbon sequestration per land area, when carbon stock from understorey and residues were considered.