Dry season conditions determine wet season water use in the wet-dry tropical savannas of northern Australia

Daily and seasonal patterns of transpiration were measured in evergreen euc alypt trees growing at a wet (Darwin), intermediate (Katherine) and dry sit e (Newcastle Waters) along a steep rainfall gradient in a north Australian savanna. Relationships between tree size and tree water use were also deter mined. Diameter at breast height (DBH) was an excellent predictor of sapwood area in the five eucalypt species sampled along the rainfall gradient. A single relationship existed for all species at all sites. Mean daily water use was also correlated to DBH in both wet and dry seasons. There were no signific ant differences in the relationship between DBH and tree water use at Darwi n or Katherine. Among the sites, tree water use was lowest at Newcastle Wat ers at all DBHs. The relationship between DBH and tree leaf area was similar between species and locations, but the slope of the relationship was less at the end of th e dry season than at the end of the wet season at all locations. There was a strong relationship between sapwood area and leaf area that was similar a t all sites along the gradient. Transpiration rates were significantly lowe r in trees at the driest site than at the other sites, but there were no si gnificant differences in transpiration rates between trees growing at Darwi n and Katherine. Transpiration rates did not vary significantly between seasons at any site. At all sites, there was only a 10% decline in water use per tree between t he wet and dry seasons. A monthly aridity index (pan evaporation/rainfall) and predawn leaf water potential showed strong seasonal patterns. It is pro posed that dry season conditions exert control on tree water use during the wet season, possibly through an effect on xylem structure.