Sap flow measurements as a basis for assessing trace-gas exchange of trees

The assessment of trace-gas uptake by plants is of basic interest in plant ecophysiology and atmospheric chemistry. For tall vegetation and extensive canopies micrometeorological methods and modelling of deposition or combinations of both are usually the methods of choice. However, distinguishing between the aerodynamically driven components of deposition and stomatal uptake is difficult and estimates of plant uptake remain uncertain. Canopy conductance derived from sapflow measurements of trees represents an important and highly variable component to determine the uptake of trace gases by trees under free atmospheric conditions. The theory of the assessment of trace-gas uptake by the sapflow-based approach is reviewed and exemplified for the uptake of ozone, nitrogen oxides, and ammonia into coniferous and deciduous tree species. First results on the stomatal ammonia compensation point (χ_s=0.11–0.27 nmol mol^?1) of the coniferous tree species Picea abies determined by a bio-assay are reported and compared with published values on herbaceous plants and gas-exchange approaches for P. abies. For a summer period in 2003, the ground-area scaled uptake rate of gaseous NH₃ by P. abies was more than twice as high (1.38 nmol m^?2 s^?1) than the uptake of NO_x (0.53 nmol m^?2 s^?1). Estimates of ground-area scaled O₃ uptake and phytomedically relevant O₃ doses of Fagus sylvatica were found to be significantly less under dry conditions in August 2003 (cumulative uptake 2.3 mmol m^?2) than in years with sufficient soil water supply despite higher atmospheric O₃ concentrations in 2003. Cumulative ground-area scaled O₃ uptake of Pinus cembra reached 150 mmol m^?2 during the growing season at an alpine site. Preliminary results and future perspectives are discussed for the transfer of the approach to the uptake of carbon dioxide and hence to determine total net primary production of trees. This novel approach has the potential to reduce uncertainties of C fluxes measured by the eddy-covariance technique and biogeochemical plot studies. It also allows to determine flux components like heterotrophic and autotrophic respiration separately as residuals from budget equations. Overall, it is concluded that the sapflow-based methodology contributes a new quality of flux data significantly improving our current understanding of biospheric aspects of trace-gas fluxes into tall vegetation.