Evaluating climatic and soil water controls on evapotranspiration at two Amazonian rainforest sites

Interactions between the biosphere and the atmosphere have profound impacts on the functioning of the Earth system. One of the most important areas of biosphere–atmosphere interaction is the Amazon basin, which plays a key role in the global cycles of carbon, water and energy. The Amazon is vulnerable to climatic change, with increasingly hot and dry conditions expected over the next 50–100 years in some models. The resulting loss of carbon from the Amazon basin has been suggested as a potentially large positive feedback in the climate system. We investigated the differences in atmospheric demand and soil water availability between two sites; Manaus, in central Amazonia, where evapotranspiration was limited in the dry season, and Caxiuanã in eastern Amazonia, where it was not. New soil hydraulic data including water release and unsaturated hydraulic conductivity curves were collected at Caxiaunã using the instantaneous profile method (IPM), pressure plate analysis and tension infiltrometry. These data were compared to existing data from Manaus. The plant available soil water at the Caxiuanã site was 2.1–3.4 times larger than the Manaus site. The hydraulic conductivity curves indicated the existence of a secondary macropore structure at very low tensions (−0.05 kPa to −1 kPa), potentially caused by biogenic macropores, but did not vary with respect to soil water potential between sites. In addition, differences in the climatic severity of the dry season were estimated. The maximum soil water deficit, projected using a simple model of forest water use, was similar between the sites. No difference in climatic severity between sites was found and we conclude that below-ground supply of water, rather than climatic differences, were likely to have caused the contrasting dry season behaviour at the two sites. These findings indicate that, in combination with other factors, heterogeneity in soil water retention capacity may exert strong controls on the spatial variation in forest responses to climatic change.