Pittosporum undulatum Vent. was identified as an invader of forests and woodlands in southern Australia over 30 years ago (Gleadow and Ashton 1981). At the time it was predicted that its high reproductive potential, suppression of competitors, broad tolerance of environmental challenges, and changed management practices could result in serious infestations and threaten the regeneration of native eucalypt forests, unless steps were taken to control it. That prediction is becoming a reality. Seventy years ago, at Menzies Creek, in the Dandenong Ranges, east of Melbourne, there was only one female pittosporum tree in the entire village; now there are up to 6 000 saplings and mature trees per hectare. There are no eucalyptus seedlings under the coalescing canopy and plant diversity under that canopy is reduced.
In order to determine the rate and direction of invasion into neighbouring areas, a good approximation of tree age is required. A citizen science project, co-ordinated by one of us (JW), worked with the local primary school to estimate the age of trees from their circumference. Single stemmed trees (n=39) were felled 30 cm above ground level and their age determined using tree rings at two sites; at the edge of the invasion canopy, and within it. The correlation between age and circumference was highly significant at both sites; however the slopes differed between trees growing at the edge and those growing in a closed canopy. Incorporating plant growth parameters into an invasion model will determine the time it will take for P. undulatum to invade non-managed forests in the region. It was estimated that the invasion front is progressing southward from Menzies Creek at about 80 metres year-1. The eastern and western invasion fronts now threatening the Dandenongs are about 4 km apart, so if the above rate applies to those fronts, and no treatment or control measures are put in place, all the forests and woodlands in the Dandenongs will be infested within 30 years.
The gender ratio of P. undulatum suggests that only about 30% of the population needs to be treated or controlled – the female trees carrying viable capsules – thereby reducing the cost of treatment greatly. While there are significant knowledge gaps in the area of population genetics, sex ratios and growth rates in different environments, it is important that coordinated action be taken soon to preserve the biodiversity of areas of significance to greater Melbourne and other areas of Victoria and Australia, wherever its invasions have become well established.
Cite this article as:
Gleadow, R. and Walker, J. (2014). The invasion of Pittosporum undulatum in the Dandenong Ranges, Victoria: realising predictions about rates and impact. Plant Protection Quarterly 29(3), 111-117.