PREDICTING REGIONAL TRANSPIRATION AT ELEVATED ATMOSPHERIC CO2 - INFLUENCE OF THE PBL-VEGETATION INTERACTION

A coupled planetary boundary layer (PBL)-vegetation model is used to s tudy the influence of the PBL-vegetation interaction and the ambient C O2 concentration on surface resistance r(a) and regional transpiration lambda E. Vegetation is described using the big-leaf model in which r (s) is modeled by means of a coupled photosynthesis-resistance model. The PBL part is a one-dimensional, first-order closure model. Nonlocal turbulent transport is accounted for by means of a countergradient co rrection. The PBL model also describes CO2 fluxes and concentrations, which are driven by photosynthesis of the canopy. A number of sensitiv ity analyses are presented in which the behavior of r(s) and lambda E at an atmospheric CO2 concentration representative for the present-day situation is compared to their behavior under an approximately double d CO2 concentration. The results reveal a positive atmospheric feedbac k on r(s), by which an initial increase of r(s), due to changes in amb ient CO2 concentration, is magnified. The stomatal humidity response a ppears to be the key factor here: if r(s) increases, the air within th e canopy dries out, which causes the stomata to close further. The PBL enlarges the effect of this positive feedback loop. The model suggest s plants with a C-4 photosynthetic pathway to be less sensitive to the humidity-mediated positive feedback than plants with a C-3 photosynth etic pathway. Another important aspect of biosphere-atmosphere interac tion is the negative feedback of the PBL on transpiration. It is concl uded that the interaction between PBL and the vegetation has to be tak en into account if transpiration and its changes, due to changing surf ace characteristics, are to be predicted at the regional scale. This c onclusion applies to modeling studies as well as to extrapolation of r esults from plant physiological research or from small-scale held plot s to the regional scale.