Factors that control monsoon-rainforest seedling establishment and growth in north Australian Eucalyptus savanna

The seasonally deciduous tree species Bombax ceiba L. and Sterculia quadrifida R. Br. formed 9.1% and 2-3%, respectively, of the total basal area in a coastal dry monsoon rainforest in the Australian seasonally dry tropics. Neither species occurred in a nearby Eucalyptus savanna. 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 air temperatures than the monsoon rainforest when averaged over 1 year. Minimum air temperatures were similar. Annual mean surface soil moisture content was similar in savanna and rainforest, but savanna soils were drier in the dry season. During this time both communities had soil matric pressure potentials below 1-3 MPa. S. quadrifida established more successfully in the rainforest than in the savanna but B. ceiba establishment was < 2% in both communities. Nursery experiments also showed that B. ceiba had a consistently lower rate of establishment than S. quadrifida, and that seed establishment of both species was greatest on unsterilized rainforest and savanna soil. Adding 40% gravel to savanna soil also resulted in improved establishment. Eucalyptus litter, light, and rainforest or savanna soil were found to have no significant effect on seed establishment of either species. After 12 months 70% of B. ceiba and 40% of S. quadrifida seedlings survived following transplantation to the rainforest. No transplants survived in the savanna, but transplants of both species survived in an irrigated savanna. Grass fires were not absolutely lethal to both species on an irrigated savanna plot. Soil sterilization inhibited the growth of both species on rainforest soils in pots, but adding fertilizer eliminated the growth inhibition caused by sterilization except when B. ceiba was grown on rainforest soil. Eucalyptus litter, light and different proportions of gravel in the soil had no effect on seedling growth rates of either species. It is concluded that the long-term development of a monsoon-rainforest-savanna ecotone is critical for creating microclimatic and soil conditions favourable for monsoon-rainforest seedling establishment and growth. Experiments point toward the importance of mycorrhizas, soil fertility and soil moisture in controlling monsoon-rainforest seedling establishment. This study helps explain why so few monsoon-rainforest seedlings have been observed to establish in fire-protected Eucalyptus savannas.