Studies addressing hydrological and ecophysiological responses to climate change remain relatively rare in Australian ecosystems. We examined the responses of stand-scale processes (sap flow, gross primary productivity (GPP), and water-use-efficiency (WUE)) and leaf-scale processes (stomatal conductance (gs) and photosynthesis (A)) to increasing atmospheric CO₂ concentration and temperature and decreasing vapour pressure deficit (VPD) using a Soil-Plant-Atmosphere (SPA) model. The model was applied over a one year period at a remnant open woodland in south-eastern Australia. SPA is a mechanistic model that simulates ecosystem primary production and water balance over fine spatial and temporal scales in response to soil water characteristics, plant variables, phenological data and meteorological conditions. Previous validation of the model showed it could account for up to 89% of variation in sap flow at the study site. 

Results/Conclusions

The climate scenarios used corresponded to changes in humidity, temperature, rainfall and CO₂ projected by the most recent CSIRO forcasts, which were modelled under a range of IPCC climate change scenarios. Annual sap flow of the tree canopy was most sensitive to declining VPD (14% reduction in sapflow in comparison to current conditions when VPD reduced by 25%). However,  this was off-set slightly when combined with increasing CO2 concentrations (10% reduction in sapflow when [CO₂] increased 50%). As expected, total annual GPP increased as CO2 concentration increased and decreased as VPD increased. The combined effect of CO₂, VPD and temperature changes (+50%, -25% and +2oC respectively) was an increase in GPP of 25%. Similarly, WUE (defined as GPP/sap flow) increased 56% under changing climate conditions. Mean daily A increased 34% in response to this last scenario, while mean daily gs decreased 4%. Therefore, predicted climate conditions in 2070 will lead to lower plant water use and stimulate productivity, causing improved WUE in this ecosystem.