Net Ecosystem Productivity of a mature temperate deciduous oak forest: reconciling fluxand biometric estimates

The eddy covariance (EC) technique measures the turbulent exchanges of gasses between atarget ecosystem and the atmosphere. Provided conditions for the applicability of the ECtechnique are met (i.e. turbulent enough atmosphere, flat terrain etc.), the time integration ofturbulent exchanges of CO2 estimates the net ecosystem productivity (NEP, in terms of carbon,neglecting small losses of CH4, VOC and DOC). Since its inception, questions have emergedregarding the ability of EC to measure NEP, and independent measurements of NEP throughbiometric methods have frequently shown discrepancies between estimates. Here we compareNEP estimates of a mature, 150-yr old temperate oak forest (Fontainebleau-Barbeau, ICOS FRFon),established over the past 19 years. The NEP_EC of this 150-yr old forest is of 504 +/- 72 gCm-2 yr-1, which places it in the high range of data for mature temperate deciduous forests.Measurements of soil respiration and below-canopy EC fluxes suggest that respiration fluxes areunderestimated at FR-Fon, probably in relation to its location at the edge of a plateauoverlooking a river 50-m below. However the integration of NEP_EC over time compares well tothe one obtained from biometric estimates (combination of wood increment, litterfall, rootproductivity and variations of the soil carbon stocks), yielding 562 gC m-2 yr-1. Interestingly,we estimate that 80% of NEP was stored as an increment of the standing wood biomass, while20% ended in the accretion of the soil organic carbon stock, yielding a 10 per mil increase of theSOC stock per year, coherent with trends reported for forest soils in France, Germany andFinland.