Continental and local climatic influences on hydrology of eucalypt-Nothofagus ecosystems revealed by δ2H, δ13C, and δ18O of ecosystem samples

Marrying knowledge of variation in large-scale climatic parameters such as rainfall and evaporation to physiological ecology has long been argued as a powerful approach to advancing understanding of hydrology of catchments. Widely used hydrological models for assessing water yield depend on key plant attributes such as whether or not plant water use is coupled to atmospheric and/or soil water status. We analyzed δ2H and δ18O signatures of long-term rainwater collections from coastal sites near Melbourne, Australia, and in northwest Tasmania and compared them with their counterparts in rainwater, stream water, soil water, and twig water data collected over a 2 year period from south facing Eucalyptus-Nothofagus forests in the Cement Creek catchment of Mt. Donna Buang, about 70 km east of the city of Melbourne. Within the catchment, progressive evaporative enrichment among rain, soil, and twig water accounted for observed δ2H and δ18O signatures. We observed a major shift in both δ2H and δ18O signatures of rain and stream water between 2001/2002 and 2003. The shift was likely due to changes in atmospheric conditions and especially relative contributions to rainwater from oceanic and continental sources. Our plant isotope data argue strongly that radiation, not water, is a primary limitation for Nothofagus on Mt. Donna Buang. The δ18O and δ13C signatures of foliage varied with season and with topography. For the wettest and driest years in the 1978-2000 record, strong and consistent altitudinal gradients in δ13C of tree rings had a slope of between 4"° and 5"° km-1 of altitude. Our data highlight the importance of background (e.g., rainfall and atmospheric CO2) isotope data to interpretation of plant isotope data. Taken together with the literature, our isotope data lead us to conclude that (1) in periods of close to average rainfall, the observed meteoric water line at Mt. Donna Buang and at other stations in southeast Australia departs strongly in slope and intercept to that observed for rainfall collected within reasonable proximity of oceans worldwide, due mainly to contributions of rainfall that originate in airstreams from the far northwest of the Australian continent (the Indian Ocean) and (2) in drought periods, the failure of such continentally influenced rainfall ensures local meteoric water lines exactly match those recorded elsewhere for near-coastal regions. Finally, the hydrologic behavior of eucalypt forests varies strongly, from typically water-limited conditions that require more complex modeling (i.e., including a significant climatic influence on plant water use), to areas where stomatal conductance, and hence transpiration, is not constrained by water availability, and modeling may be more simply based.