Water use efficiency as a means of up scaling carbon fluxes from leaf to stand
Evalutaion of the suitability for using Water Use Efficiency (WUE, the relation between carbon uptake and water use) as a means of up-scaling carbon fluxes from leaf to stand for deciduous forests . The suitability of WUE as assessing stand carbon fluxes relies on the hypothesis that the leaf WUE is a constant function of vapor pressure deficit (a measure of air humidity) and thus not reliant on e.g. light exposure. Here, WUE is constant for the full tree and sap flow (measurements of the amount of water passing trough the stem) can be used to assess tree and stand carbon uptake.
To perform the analyses, a large number of leaf carbon and water flux data from a Danish beech forest was used to assess the environmental influence on leaf WUE. It was found that WUEN - the leaf WUE normalized to the Vapor Pressure Deficit (VPD) - is dependent on incoming PAR (the part of sunlight useful for plants) below 500 micromol m-2 s-1 and, for the full range, dependent on VPD. Nevertheless, WUEN is independent of both different levels of the canopy and for the variation in environmental parameters.
Because of this light dependency, the up scaling was performed using the light distribution within the canopy assessed with a forest carbon and water flux model called MAESTRA. The up scaled daily canopy WUE for daytime fluxes (6-18 hours) was in agreement with observed eddy flux based canopy WUE estimations and it was, as for the leaf WUE, dependent only on VDP and light, thus independent on other environmental factors. This means that canopy WUE can be successfully estimated using the up-scaled WUE relationships if incident radiation and VPD within the canopy is known.