Reconstructing the δ18O of atmospheric water vapour via the CAM epiphyte Tillandsia usneoides: seasonal controls on δ18O in the field and large-scale reconstruction of δ18Oa

Using both oxygen isotope ratios of leaf water (O-18(L)) and cellulose (O-18(C)) of Tillandsiausneoides in situ, this paper examined how short- and long-term responses to environmental variation and model parameterization affected the reconstruction of the atmospheric water vapour (O-18(a)). During sample-intensive field campaigns, predictions of O-18(L) matched observations well using a non-steady-state model, but the model required data-rich parameterization. Predictions from the more easily parameterized maximum enrichment model (O-18(L-M)) matched observed O-18(L) and observed O-18(a) when leaf water turnover was less than 3.5d. Using the O-18(L-M) model and weekly samples of O-18(L) across two growing seasons in Florida, USA, reconstructed O-18(a) was -12.60.3 parts per thousand. This is compared with O-18(a) of -12.4 +/- 0.2 parts per thousand resolved from the growing-season-weighted O-18(C). Both of these values were similar to O-18(a) in equilibrium with precipitation, -12.9 parts per thousand. O-18(a) was also reconstructed through a large-scale transect with O-18(L) and the growing-season-integrated O-18(C) across the southeastern United States. There was considerable large-scale variation, but there was regional, weather-induced coherence in O-18(a) when using O-18(L). The reconstruction of O-18(a) with O-18(C) generally supported the assumption of O-18(a) being in equilibrium with precipitation O-18 (O-18(ppt)), but the pool of O-18(ppt) with which O-18(a) was in equilibrium - growing season versus annual O-18(ppt) - changed with latitude.