Carbon concentration variations in the roots, stem and crown of mature Pinus pinaster (Ait.)

Stands of maritime pine (Pinus pinaster Ait.) cover about one million hectares of land in south-western France and produce 19% of all French timber, thanks to the intensive management methods employed. Evaluations of carbon fixation and storage in this forest are facilitated by its general homogeneity with respect to soil, climate and tree genetics. However, initial assessments were based on basic values for expansion factors and carbon concentration in the biomass, and more accurate results could be obtained. The aim of the present study was to estimate the carbon concentration in the 13 main compartments of mature P. pinaster shoots and roots, describing sources of variation within these compartments and quantifying precisely the corresponding carbon contents. The biomass distribution per compartment in the shoots and roots of 12 trees with a range of social status is given. It was obtained by joint architecture and dry weight measurements. The root systems were uprooted with a mechanical shovel and measured by 3D digitizing. Biomass allometric prediction equations per compartment according to girth at breast height were developed. The carbon concentration was analysed in 300 samples from four trees, taking into account their architecture. The carbon concentration varied largely between compartments and showed a quadratic relationship with relative height in the four stem compartments and in branches and buds. It showed a negative exponential relation with root diameter. The carbon concentration of needles was not related to their age or their relative height in the crown. Carbon concentration variations were in accordance with the tissue chemical composition found in literature. The biochemical concentration of softwoods organs is extensively reviewed in the paper. The weighted mean carbon concentration reached 53.6% in the shoots and 51.7% in the roots. This resulted to 53.2% at tree level. The carbon content in the pine stand was 74 t C per hectare. Between and within compartment variations in carbon concentration should be considered in carbon content evaluations and in structural–functional models. The underestimation of carbon storage in mature P. pinaster stands and sawnwood products reaches 6% when the usual 50% conversion factor is used.