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 a
target ecosystem and the atmosphere. Provided conditions for the applicability of the EC
technique are met (i.e. turbulent enough atmosphere, flat terrain etc.), the time integration of
turbulent 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 emerged
regarding the ability of EC to measure NEP, and independent measurements of NEP through
biometric methods have frequently shown discrepancies between estimates. Here we compare
NEP 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 gC
m-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 are
underestimated at FR-Fon, probably in relation to its location at the edge of a plateau
overlooking a river 50-m below. However the integration of NEP_EC over time compares well to
the one obtained from biometric estimates (combination of wood increment, litterfall, root
productivity 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, while
20% ended in the accretion of the soil organic carbon stock, yielding a 10 per mil increase of the
SOC stock per year, coherent with trends reported for forest soils in France, Germany and
Finland.