EVALUATING KINETIC FRACTIONATION FACTORS USED FOR ECOLOGIC AND PALEOCLIMATIC RECONSTRUCTIONS FROM OXYGEN AND HYDROGEN ISOTOPE RATIOS IN PLANT WATER AND CELLULOSE

Oxygen and hydrogen kinetic isotope enrichments occur in plant leaf wa ter during evapotranspiration. Kinetic fractionation factors for O-18 derived empirically from a variety of isotope studies of plant water c ompare favourably with fractionation factors estimated from theoretica l consideration of leaf energy balance and boundary layer dynamics. Ki netic fractionation during evapotranspiration is sensitive to the natu re of the boundary layer, which is controlled by leaf size and morphol ogy. Generally, plants with small segmented leaves have a lower compon ent of turbidity in the leaf boundary layer, which results in higher C k values, than do plants having large simple leaves and more turbulent boundary layers. Equivalent analysis of H-2 enrichment in plant leaf water reveals less consistent kinetic relations with leaf size and mor phology and an apparent temperature-dependent isotope effect acting in opposition to evaporative enrichment. Accounting for this temperature -dependent isotope effect helps to (1) reconcile the (2)Ck inconsisten cies for different leaves and (2) explain a temperature effect in whit e pine which White et al. (1994) attributed to variable biochemical fr actionation during cellulose synthesis. Improved characterization of k inetic effects during evapotranspiration has implications for isotopic study of plant water use and for constraining interpretations of pale oclimate based on the isotopic composition of plant tissues.