THE EXPANSION OF LEPTOSPERMUM-LAEVIGATUM ON THE YANAKIE ISTHMUS, WILSON PROMONTORY, UNDER CHANGES IN THE BURNING AND GRAZING REGIMES

The distribution of selected vegetation types on the Yanakie Isthmus, Wilson's Promontory National Park, was mapped from aerial photographs from 1941, 1972 and 1987. The main changes in the vegetation dynamics were: (1) an expansion of Leptospermum laevigatum into grasslands and into Banksia integrifolia woodlands with herbaceous understoreys, and (2) a stabilisation of dunes by shrubs dominated by Leptospermum laevigatum. The total area of L. laevigatum shrubland and scrub increased from 2179 ha in 1941 to 3436 ha in 1972 and 4516 ha in 1987. Land-use changes in this period included the exclusion of fire in the early 1970s, after a history of regular burning, and an increase in grazing pressure primarily due to population expansions of the rabbit and the eastern grey kangaroo. Fire was not a prerequisite of the L. laevigatum expansion on the Isthmus because the spread continued after fire was excluded; nor was fire the primary cause of the expansion because the percentage yearly increase in the area of L. laevigatum was, on average, similar before and after 1972. An increase in gazing pressure was identified as the probable cause of the L. laevigatum expansion due to: (1) the exposure of bare ground, and (2) the restriction of the feeding range of cattle (known to graze both L. laevigatum and Acacia sophorae an the Isthmus).