Influence of suburban edges on invasion of Pittosporum undulatum into the bushland of northern Sydney, Australia

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Abstract
Suburban edges are shown to exert a major influence on invasion of the native Pittosporum undulatum Vent, in dry sclerophyll bushland of northern Sydney. Transect data from fifteen urban bushland sites spanning approximately 90 years of development indicate significant increases of P. undulatum with time. Basal area, density and frequencies of plants in all size‐classes increased significantly with age of adjoining development. Mean basal area at edges of old sites was 5700 times greater than in comparable bushland in larger reserve interiors. The effect of age was compounded by the greater impact of edge effects in narrower reserves of older suburbs. Basal areas and proportion of reproductive plants decreased significantly with distance into the reserve. Older sites contained larger but fewer individuals at the edge, compared with high densities of smaller plants further into the reserve, suggesting an advanced successional stage. The overall relationship of density with distance was not significant. The observed clumping of seedlings beneath canopies is consistent with previous work on seed dispersal by vectors, and enhanced seedling recruitment in safe sites. Pittosporum basal area and density reached no maximum levels with age, implying ongoing invasion. The study indicated some factors which appeared to promote the establishment of P. undulatum in the study area. Signs of fire were negatively correlated across all sites with basal area and density of P. undulatum. Human disturbance was positively correlated with the same variables. On the basis of findings in this study, some broad priorities for management of Pittosporum in urban bushland are suggested.