FACTORS THAT CONTROL MONSOON-RAIN-FOREST SEEDLING ESTABLISHMENT AND GROWTH IN NORTH AUSTRALIAN EUCALYPTUS SAVANNA

1. The seasonally deciduous tree species Bombax ceiba L. and Sterculia quadrifida R. Br. formed 9.1% and 2.3%, respectively, of the total ba sal area in a coastal dry monsoon rainforest in the Australian seasona lly dry tropics. Neither species occurred in a nearby Eucalyptus savan na. 2. Both communities occurred on deep, iron-rich, sandy loam soils. The savanna had lower concentrations of nutrients in the surface soil than the rainforest but experienced greater maximum ground surface ai r temperatures than the monsoon rainforest when averaged over 1 year. Minimum air temperatures were similar. Annual mean surface soil moistu re content was similar in savanna and rainforest, but savanna soils we re drier in the dry season. During this time both communities had soil matric pressure potentials below 1-3 MPa. 3. S. quadrifida establishe d more successfully in the rainforest than in the savanna but B. ceiba establishment was <2% in both communities. Nursery experiments also s howed that B. ceiba had a consistently lower rate of establishment tha n S. quadrifida, and that seed establishment of both species was great est on unsterilized rainforest and savanna soil. Adding 40% gravel to savanna soil also resulted in improved establishment. Eucalyptus litte r, light, and rainforest or savanna soil were found to have no signifi cant effect on seed establishment of either species. 4. After 12 month s 70% of B. ceiba and 40% of S. quadrifida seedlings survived followin g transplantation to the rainforest. No transplants survived in the sa vanna, but transplants of both species survived in an irrigated savann a. Grass fires were not absolutely lethal to both species on an irriga ted savanna plot. Soil sterilization inhibited the growth of both spec ies on rainforest soils in pots, but adding fertilizer eliminated the growth inhibition caused by sterilization except when B. ceiba was gro wn on rainforest soil. Eucalyptus litter, light and different proporti ons of gravel in the soil had no effect on seedling growth rates of ei ther species. 5. It is concluded that the long-term development of a m onsoon-rainforest-savanna ecotone is critical for creating microclimat ic and soil conditions favourable for monsoon-rainforest seedling esta blishment and growth. Experiments point toward the importance of mycor rhizas, soil fertility and soil moisture in controlling monsoon-rainfo rest seedling establishment. This study helps explain why so few monso on-rainforest seedlings have been observed to establish in fire-protec ted Eucalyptus savannas.