FACTORS INFLUENCING TREE GROWTH IN TROPICAL SAVANNA - STUDIES OF AN ABRUPT EUCALYPTUS BOUNDARY AT YAPILIKA, MELVILLE-ISLAND, NORTHERN AUSTRALIA

Most of the land surface of Metville Island, Australia's second larges t island, is covered in Eucalyptus savanna. One exception is an area a t Yapilika where a large tract of savanna is dominated by Acacia shrub s. An ordination analysis of 122 quadrats revealed that the boundary o f Eucalyptus dominance did not correspond to a major change in florist ic composition. Detailed transect studies at one site on the boundary showed that Eucalyptus trees were abruptly replaced by a band of Grevi llea trees which gradually gave way to Acacia shrub dominance. There w as a gradual change in the floristic composition of the savanna across the boundary. The distributional limit of Eucalyptus was found to be independent of any hydrological discontinuity. There was a slight decr ease (<2.5 m) in altitude from Eucalyptus to Acacia savanna. The Acaci a savanna soils were sandier and their surface soil had significantly lower concentrations of Ca and Mg and significantly greater concentrat ion of Al compared with the Eucalyptus savanna soils. Eucalyptus seedl ings planted in the three savanna communities were not found to be und er drought stress (pre-dawn leaf xylem potentials of >-0.9 MPa) during the dry season. Over a 12 month period Eucalyptus tetrodonta and E. m iniata seedling growth was not significantly different on the Acacia o r Eucalyptus savanna, although this result may be due to the counterac ting effects of greater soil fertility and tree competition in the Euc alyptus savanna and lower soil fertility in the treeless, and hence co mpetition-free, Acacia savanna. This hypothesis is supported by the si gnificantly greater growth of Eucalyptus seedlings on fertilized Acaci a savanna soils. The limited production, dispersal and establishment o f Eucalyptus seeds and the greater frequency of fires in the Acacia sa vanna probably explains the abrupt limit to Eucalyptus dominance along the edaphic gradient.