Invasion by Pittosporum undulatum of the Forests of Central Victoria. I. Invasion Patterns and Plant Morphology

Abstract

Pittosporum undulatum is invading eucalypt open forests in south-central Victoria, 200 km west of its native habitat in eastern Australia. This is due to horticultural plantings of P. unduiatum and to the suppression of wildfires: it has thin bark and is killed by fires which most eucalypts can survive. Near Melbourne, P. undulatum preferentially establishes around the butts of eucalypts and other established trees although in denser forests this clumping is not so obvious. Seedlings of other weed species such as Ilex aquifolium and Cotoneaster pannosa similarly aggregate around established shrubs and trees. Classification of quadrats by monothetic division showed P. undulatum to be the most important species in the community. The weedy nature of P. undulatum is clear from its rapid dispersal, early seed production and fast growth. The dense canopy dramatically reduces the light intensity and completely suppresses the original sclerophyllous understorey: only Gahnia radula remains. Soil beneath the canopy is fertile and the litter is high in nutrients but bioassays indicate that inhibitors may be present. The root system is variable, depending on soil texture and compaction. The invasion of forest remnants by P. undulatum and other weeds is threatening the survival of the eucalypt forests in urban areas.