The annual carbon budget of a French pine forest (Pinus pinaster) following harvest

Eddy covariance measurements of net ecosystem exchange (NEE) of carbon dioxide and sensible and latent heat have operated since clear felling of a 50-year old maritime pine stand in Les Landes, in Southwestern France. Turbulent fluxes from the closed-path system are computed via different methodologies, including those recommended from EUROFLUX (Adv. Ecol. Res. 30 (2000) 113; Agric. Forest Meteorol. 107 (2001a, b) 43 and 71), and sensitivity analysis demonstrates the merit of post-processing for accurate flux calculation. Footprint modeling, energy balance closure, and empirical modeling corroborate the eddy flux measurements, indicating best reliability in the daytime. The ecosystem, a net source of atmospheric CO2, is capable of fixing carbon during fair weather during any season due to the abundance of re-growing species (mostly grass), formerly from the understorey. Annual carbon loss of 200-340 g m(-2) depends on the period chosen, with inter-annual variability evident during the 18-month measurement period and apparently related to available light. Empirical models, with weekly photosynthetic parameters corresponding to seasonal vegetation and respiration depending on soil temperature, fit the data well and allow partitioning of annual NEE into GPP and TER components. Comparison with a similar nearby mature forest (Agric. Forest Meteorol. 108 (2001) 183) indicates that clear-cutting reduces GPP by two thirds but TER by only one third, transforming a strong forest sink into a source of CO2. Likewise, the loss of 50% of evapotranspiration (by the trees) leads to increased temperatures and thus reduced net radiation (by one third), and a 50% increase in sensible heat loss by the clear cut.