The effect of stomatal conductance formulation on the study of ozone fluxes to vegetation.
Altimir, Nuria*,1, Kolari, Pasi1, Bäck, Jaana 1, Hari, Pertti1, 1 Department of Forest Ecology, Helsinki, Finland
ABSTRACT- Tropospheric ozone can be toxic to vegetation, particularly the portion that reaches mesophyll via stomata. This uptake is commonly predicted via estimation of stomatal conductance, for which there are various formulations. These have been developed to aid the description of transpiration or photosynthesis and its correspondence to the prediction of ozone uptake -albeit convenient- is not free of problems. Ozone can also be scavenged before entering the foliage by mechanisms incompletely understood. These could involve interactions with surface elements (epicuticular waxes, water film, organic volatiles) and are referred as non-stomatal sinks. These sinks seem to correlate with temperature, solar radiation, and the presence of moisture. Analyses of the non-stomatal sinks are based on nocturnal ozone flux data on the assumption that the stomatal conductance is negligible. Or they are based on diurnal data from which the stomatal uptake has been subtracted. Thus, assumptions made for the stomata behaviour affect the determination of toxicologically significant uptake as well as conclusions on the stomatal sink mechanisms. To analyse the relevance of such assumptions, we compare the surrogation from measured water vapour stomatal conductance, the multiplicative model of Jarvis, and the photosynthesis-stomatal models of Ball-Berry, and Makela-Berninger-Hari. We make use of measurement on Scots pine foliage at the boreal field station SMEAR in Finland. We analyse different approaches to stomatal conductance, discuss the uncertainty associated to their use and how related inaccuracies are transported to the study of ozone flux whether into stomata or surfaces. We have observed a modulation of the non-stomatal sink by what has been measured to be epicuticular water films. Preliminary results suggest the treatment of water vapour pressure deficit is determinant in the context of ozone flux estimations, both in how its relation to stomatal conductance is formulated as well as how it is measured.
Key words: ozone uptake, stomatal conductance, non-stomtal sink, water films
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