Ba. Wilson et Dmjs. Bowman, FACTORS INFLUENCING TREE GROWTH IN TROPICAL SAVANNA - STUDIES OF AN ABRUPT EUCALYPTUS BOUNDARY AT YAPILIKA, MELVILLE-ISLAND, NORTHERN AUSTRALIA, Journal of tropical ecology, 10, 1994, pp. 103-120
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.