Carbon dynamics of intensively managed forest along a full rotation

Temperate and tropical forests are increasingly exploited for wood and biomass extraction and only one third of forest area was considered as primary in the recent FRA in 2010. Management practices affect the soil-forest-atmosphere continuum through various effects on soil and surface properties. They result ultimately in either positive or negative changes in the biomass and soil carbon pools but, if any, few datasets or modeling tools are available for quantifying their impacts on the net carbon balance of forest stands. To analyse these effects, the net half-hourly fluxes of CO2, water vapour and heat exchanges were monitored for 23 years in two closed stands of maritime pines in southwestern France. Carbon content of the aboveground biomass was measured annually and soil pools 10-early in the younger stand and 5-yearly in the mature stand. For analysing the data collected and disentangling the climate and management effects, we used the three components process-based model GRAECO+ (Loustau et al. this session) linking a 3D radiative transfer and photosynthesis model, MAESTRA, a soil carbon model adapted from ROTH-C and a plant growth model.Eddy flux data were processed, gapfilled and partitioned using the methodological recommendations (Aubinet et al. 2000, Adv. Eco. Res:30, 114-173, Falge et al. 2001, Agr. For. Meteo. : 107, 43-69, Reichstein et al. 2005, Glob. Change Biol., 11:1424-1439). Analysis of the sequence showed that, whether by an increased sensitivity to soil drought compared to the pines or by a rapid re-colonization of the inter-row after understorey removal and plowing, the weeded vegetation contributed to create specific intra-annual dynamics of the fluxes and therefore, controls the dynamics of carbon balance of the stand.After three growing seasons, the stand was already a carbon sink, but the impact of thinning and weeded vegetation removal at the age of 5-year brought the balance to almost neutral. We interpret this change as the combined effects of the reduction of the LAI, the enhancement of the heterotrophic respiration related to the decomposition of dead materials and the improvement of the mineralization of the large stock of soil organic matter by tillage. At the mature stage, the stand remains consistently a carbon sink and CO2 fluxes were insensitive to thinning. Conversely, the carbon balance was sensitive to climate effects as evidenced by repeated drastic reductions in NEP caused by soil drought. Our data underlines the importance of disturbances linked to forest management for the forest carbon balance during the early stage of tree growth. Since management intensification tends to shorten the forest life cycle and enhance the share of the young stages, our results confirm that the consequence of management operations on the carbon cycle in forest may revert intensified forest stands from a net sink to a source and should be accounted for carefully