The objectives of this paper are threefold: 1) to present a performance evaluation of the gradient-diffusion method for flux isotope ratio measurement in the surface layer over tall vegetation, 2) to examine the oxygen isotopic composition of water vapor flux at a range of time scales including diurnal variations, rain shower time scales, and seasonal changes, 3) to identify emergent properties of the water vapor isotopic exchange at the whole-ecosystem scale.

The gradient diffusion method consists of a tunable diode laser analyzer, a dripper calibration device, and a gradient interface. The measurement is conducted in a mixed temperate forest in southern New England from May to October 2005. Isotope compositions of rain, stream, sap and soil water are analyzed to support the interpretation of the flux isotope ratio data. Eddy fluxes of sensible heat, water vapor, carbon dioxide, and momentum are also measured at the site.