Processes controlling transpiration in the rainforests of north Queensland, Australia

Transpiration is a major component of the water balance of tropical rainforests and it is important to know how it might change in response to climate and/or land use change. Substantial amounts of transpiration data are now available for Australian rainforests in northern Queensland and this paper presents an analysis of these data for four different rainforest types. Measured transpiration rates are much lower than potential transpiration rates, largely because the rainforest canopy conductances are much lower than that associated with the potential rate. Canopy conductance was found to be strongly related to atmospheric vapour pressure deficit and only weakly to solar radiation; because the forests are highly coupled to the atmosphere, due to their very high aerodynamic conductance. The best predictive model of rainforest transpiration is one based directly on daily average vapour pressure deficit and solar radiation. Models based on canopy conductance can give daytime transpiration rates, but they do not account for the significant amounts (20–50%) of nocturnal transpiration that are observed. We also found little/no evidence for any direct soil moisture influence on canopy conductance or transpiration.