Invasive Plants in Australian Forests, with an Emphasis on Subtropics and Tropics

Australia is both a continent and a country, with a total land area of 768 million ha. Centered over the southern hemisphere subtropical high-pressure system, the continent is mostly quite dry. The distribution of effective rainfall to most of Australia imposes limitations on where forests, and hence forest weeds, can grow. Using a very liberal definition of forest as a vegetation type that has a height of woody plants of at least 2m and overstory crown cover of greater than 20%, one may say that 21% or 164 million ha of Australia are forests (Bureau of Rural Science 2003). Of this area, 127 million ha, or 78% of all forest, is eucalypt forests of various types. Of this, only about 93,000 ha is in closed-canopy eucalypt forest (>70% cover), and the remainder is classified as either open (canopy) forest (30%–70% cover) or woodland. About 10% of the total forest area is in acacia stands, 4% in Melaleuca, and 3% in rainforest. There is a total of 1.6 million ha of plantations, including nearly 1 million ha in exotic pines (Pinus radiata, P. elliottii, and P. elliotiiP. caribaea hybrids). The remaining 600,000 ha are planted with hardwoods (National Forest Inventory 2005), mainly eucalypts, with hardwood plantations expanding at around 80-100,000 ha per year.