Australia's tropical savannas are the focus of world-first research into the effects of rising carbon dioxide levels on a natural tropical ecosystem. Scientists involved in 'OzFACE', the Australian Free Air Carbon Dioxide Enrichment Study, are piping different concentrations of carbon dioxide over plots of savanna grasses and young trees, and measuring changes in plant performance, species composition, nutrient levels and carbon flow. Preliminary studies suggest a range of impacts could be expected, such as increased grass growth and drought tolerance, a thickening woody layer, and changes in competition between trees and grasses. Scientists are also looking at the effect these changes could have on grazing and the production of plant defence compounds in the leaves of trees.