Carbon dioxide uptake of a forested region in southwest France derived from airborne CO2 and CO measurements in a quasi-Lagrangian experiment

International audienceThis paper presents a Lagrangian budgeting approach to quantify the uptake of CO2 by vegetation at horizontal scales of several tens of kilometers. For this purpose, CO2 and meteorological parameters were measured from a small aircraft during four flights in June 2001 over a flat homogeneous and productive temperate forest in the Landes region ( southwestern France). Additional CO measurements were made in order to identify and quantify the potential influence of anthropogenic emissions on the net CO2 flux derived from the measurements. For one of four flights, Lagrangian conditions were nearly perfectly fulfilled. On average, the CO2 mixing ratio in the boundary layer decreased at a rate of 0.11 ppm km(-1), yielding an average CO2 uptake by the forest of 16 +/- 2.5 mumol m(-2) s(-1) between 1230 and 1430 UT. Our result is about 15% smaller than the local net ecosystem exchange measured by eddy covariance at a tower north of the flight domain and about 12% higher than a regional estimate based on remote sensing data for the whole experimental area. The contribution of anthropogenic emissions to the regional CO2 budget was estimated from the CO measurements to be to <0.5 mu mol m(-2) s(-1)

Elucidating physiology of plant mediated exchange processes using airborne hyperspectral reflectance measurements in synopsis with eddy covariance data

International audienceThe Carbo Europe Regional Experiment Strategy (CERES), conducted between May and June 2005, aims to determine a spatially resolved regional balance of carbon dioxide fluxes using different methodological approaches bundled within an international framework. In this study we elucidated several meteorological and physiological parameters determining plant mediated exchange processes using airborne hyperspectral reflectance measurements in synopsis with meteorological information like eddy flux covariance data. We used an airborne hyperspectral system to record spatial and temporal transects of vegetated areas surrounding flux tower sites in the Bordeaux / Landes Region (France). Alongside hyperspectral measurements a comprehensive range of meteorological and surface flux parameters were measured at a flux tower site within the same time frame. Unsupervised data analysis using cluster analysis was performed on predefined spectral wavelength windows. Additionally multiblock principal component analysis was used as explanatory data driven tool to reveal underlying data structures and to elucidate potential interdependencies between airborne hyperspectral reflectance data and meteorological ground measurements. In detail temporal, physiological changes in relevant ground information like temperature, humidity, vegetation mediated CO2 flux, latent heat flux or photosynthetic light use efficiency on the one hand and selected regions of the hyperspectral signatures, such as reflectance in the region of 531nm, which determines variations in the photochemical reflectance index (PRI) or in the spectral regions of chlorophyll fluorescence were examined. Unsupervised cluster analysis revealed coupled dependencies between changes in the derivate spectra in the range of 720 to 740 nm with gross photosynthetic uptake rate, global radiation and time of day. Multiblock principal component analysis revealed that first derivative reflectance in wavelengths from 500 to 540 nm and between 680 and 750 nm had higher loading values in respect to observed ground variables global radiation and gross photosynthetic uptake rate respectively. The significance of these findings is discussed