ON THE SPATIAL VARIABILITY OF BIOPHYSICAL FACTORS AND ITS INFLUENCE ON MEASURED NET ECOSYSTEM EXCHANGE OVER FOREST

摘要

All natural forest ecosystems exhibit spatial variability at some range of scales. Measured fluxes of CO_2 are thus only expected to be representative of a forest ecosystem to the extent that the biophysical forcings in the flux footprint reflect average forest conditions. Here, we examine the influence of spatial variability in biomass (as expressed by the normalized difference vegetation index, NDVI), and the amount of absorbed photosynthetically active radiation (APAR) on the estimate of net ecosystem exchange of CO_2 from eddy-covariance measurements at a height of 46 m above a forest canopy of h = 26 m at the Morgan-Monroe State Forest (MMSF, Indiana, USA) AmeriFlux site (Schmid et al., 2000). An IKONOS satellite scene is used to determine the distribution of NDVI in the vicinity of the MMSF site. Slope angle information is obtained from a digital terrain model. Growing season average PAR is calculated for each grid-point. First incoming solar radiation is modeled across the topographical domain using the method of Moore et al. (1993). Direct and diffuse beam solar radiation are calculated based on atmospheric optical transmission, estimated from observations on the MMSF tower (including a cloudiness index). Slope and aspect effect on exposure to direct beam radiation and the sky view factor effect on receipt of diffuse beam radiation is derived using the terrain model. APAR is assumed to be related to NDVI and PAR by APAR≈α_p PAR[1-exp(-LAI)] , (1) where α_p ≈ 0.94, and LAI ≈117.35 NDVI - 9.01 (Baret and Guyot 1991; Wulder et al. 1998).