Temporal variations of the 18O/16O signal of the whole-canopy transpiration in a temperate forest

Biosphere- atmosphere exchange of water vapor isotopes plays an important role in the global atmospheric O-18- CO2 and O-18- O-2 budgets. In this paper, we report the results of the first continuous measurements of isotope ratios of water vapor and the evapotranspiration flux in a temperate forest over one full growing season. We found that the O-18/O-16 isotopic signal of the whole-canopy transpiration (delta(T)) was not in steady state with respect to plant source water. The departure from steady state was greatest at night and on days of low transpiration rates. Relative humidity was an important driver on timescales shorter than a few hours; on the diurnal timescale, the nonsteady state behavior was driven by relative humidity and the covarying transpiration rate. On average, delta(T) was lowest in midmorning and highest at midnight, with an average peak-to-peak variation on the order of 15 parts per thousand over the growing season. A diurnal variation of 60 parts per thousand or more was observed on some days. On the seasonal timescale, delta(T) was tightly coupled with the precipitation isotope ratio in the early growing season and fluctuated around the isotope ratio of the stem water of overstory trees in the late growing season. The temporal shift suggests that the forest switched its water source from the shallow to the deep soil pool and that the overstory trees dominated the whole stand transpiration in the late growing season. Using isotopic partitioning, we estimated that the overstory trees contributed roughly 70% to the whole-stand transpiration water loss during the growing season.