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Within-canopy and ozone fumigation eﬀects on d 13 C and D 18 O in adult beech ( Fagus sylvatica ) trees: relation to meteorological and gas exchange parameters

Summary In this study, the eﬀects of diﬀerent light intensities either in direct sunlight or in the shade crown of adult beech ( Fagus sylvatica L.) trees on d 13 C and D 18 O were determined under ambient (1 · O 3 ) and twice-ambient (2 · O 3 ) atmospheric ozone concentrations during two consecutive years (2003 and 2004). We analysed the isotopic composition in leaf bulk, leaf cellulose, phloem and xylem material and related the results to (a) meteorological data (air temperature, T and relative humidity, RH), (b) leaf gas exchange measurements (stomatal conductance, g s ; transpiration rate, E ; and maximum photosynthetic activity, A max ) and (c) the outcome of a steady-state evaporative enrichment model. d 13 C was signiﬁcantly lower in the shade than in the sun crown in all plant materials, whilst D 18 O was increased signiﬁcantly in the shade than in the sun crown in bulk material and cellulose. Elevated ozone had no eﬀect on d 13 C, although D 18 O was inﬂuenced by ozone to varied degrees during single months. We observed signiﬁcant seasonal changes for both parameters, especially in 2004, and also signiﬁcant diﬀerences between the study years. Relating the ﬁndings to meteorological data and gas exchange parameters, we conclude that the diﬀerences in D 18 O between the sun and the shade crown were predominantly caused by the Pe´clet eﬀect. This assumption was supported by the modelled D 18 O values for leaf cellulose. It was demonstrated that independent of RH, light-dependent reduction of stomatal conductance (and thus transpiration) and of A max can drive the pattern of D 18 O increase with the concomitant decrease of d 13 C in the shade crown. The eﬀect of doubling ozone levels on time-integrated stomatal conductance and transpiration as indicated by the combined analysis of D 18 O and d 13 C was much lower than the inﬂuence caused by the light exposure.