Suitability of a Combined Stomatal Conductance and Photosynthesis Model for Calculation of Leaf-Level Ozone Fluxes

Abstract

Currently, the most important source of uncertainty in stomatal ozone flux (FO₃ ) modelling is the stomatal conductance (g_st ) factor. Hence FO₃ model accuracy will strongly depend on the g_st model being implemented. In this study the recently developed semi-empirical g_st model of Dewar was coupled to the widely known biochemical photosynthesis (A_n ) model of Farquhar. The g_st performance of this model combination was evaluated with a 4-month time series of beech (Fagus sylvatica L.) measurements. The g_st model was hereto optimized in two steps to a 4-day and a 8-day period. A comparison between the modelled and measured g_st to O₃ (g_stO3 ) revealed a rather good overall performance (R² = 0.77). Errors between the model combination and the measurements are thought to be largely caused by a moderate performance of the A_n model, due to poor parameterization. Two 2-day periods with distinctly differing soil and meteorological conditions were chosen to give a picture of the daily g_st performance. Although instant relative differences between modelled and measured g_stO3 are sometimes high, the model combination is able to simulate the rough daily courses of g_stO3 and hence FO₃ reasonably well. Further improvement on full parameterization of the g_st model and a well-parameterized A_n model to be linked to are needed to draw founded conclusions about its performance. Future efforts hereto are certainly justified since the model's mechanistic nature makes it a tool able to model g_st variation in space and time, O₃ effects on g_st, and effective FO₃ .

Key words

Beech - Fagus sylvatica L. - model - stomatal conductance - ozone flux.

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Guest Editor: R. Matyssek