Ecological characteristics of Brachychiton populneus (Sterculiaceae) (kurrajong) in relation to the invasion of urban bushland in south-western Australia

Brachychiton populneus (Sterculiaceae) (Schott et Endl.) R. Br. (kurrajong) is a small tree that occurs naturally ranging from southern Queensland to Victoria. It has been widely planted as an ornamental tree in south-western Australia. In Kings Park, B. populneus has moved from cultivation to becom e a weed in the adjoining bushland reserve. The aim of this study was to ex amine the ecology of B. populneus and the Kings Park environment in order t o identify the particular conjunction of characteristics that have led to t he species becoming a weed. The highest density of kurrajongs (69.3 trees h a(-1)) was observed in the most disturbed area of Kings Park, and there was a strong relationship between density of B. populneus and disturbance (P = 0.058). The most striking feature of the invasion was the tendency of B. p opulneus to occur beneath other tree species, and this was attributed to bi rds feeding on transported fruit in trees and rats building seed caches at their base. Mature trees produced large amounts of viable seed, but rates o f seed predation were high. Weevils, beetle larvae and omnivorous vertebrat es such as Australian ravens, magpies and introduced black rats were observ ed eating seeds. The foraging behaviour of the vertebrates may facilitate t he dispersal of seeds for relatively long distances away from parent plants . Seeds that escape predation form a transient seed bank and germinate with the onset of the winter rains. Early in their development, seedlings alloc ate resources to form a large tap-rooted tuber that has substantial starch and water reserves, allowing seedlings to survive the long dry and hot summ ers in Perth. The study observed that B. populneus could survive at least o ne fire by resprouting from basal dormant buds. Brachychiton populneus appe ars to have become a weed in Kings Park because, first, it is dispersed wid ely into new sites through the foraging behaviour of vertebrates and once g erminated has no grazing pressure, and, second, its development of a root t uber and ability to resprout means the seedlings are resilient in this freq uently disturbed Mediterranean environment. While management of existing pl ants is relatively straightforward, continued vigilance will be required to avoid reinvasion.