Restoration of plant biodiversity beneath tropical tree plantations in Northern Australia

Understorey colonization by native species was assessed in timber plantations on the Atherton Tablelands in North Queensland, Australia (latitude 17 degrees S). We surveyed 151 plots (each 78.5 m(2) in area) in plantation monocultures of the exotic Pinus caribaea and the natives Araucaria cunninghamii, Flindersia brayleyana and Toona ciliata ranging in age from 5 to 63 years. A total of 350 species, including 176 tree species, were found beneath the plantations. On similar sites, F. brayleyana had a significantly greater number of colonizing species than A. cunninghamii which, in turn, had significantly greater numbers than P. caribaea. Tree species richness and the number of species regarded as 'late successional' increased significantly with age for the two species where a range of ages were measured. Age explained a considerably greater proportion of the variation in richness in P. caribaea plantations (r(2) = 0.79) than in A. cunninghamii plantations (r(2) = 0.22), and there tended to be more late successional species in younger A. cunninghamii plantations. The distance from the rainforest to the plantation edge generally had little effect on the number of tree species, although there was a slight trend of richness decreasing away from the plantation boundary for younger plantations and in F. brayleyana plantations at one site. Between 80 and 90% of the tree species found in the plantations were primarily bird-dispersed, with similar proportions of primarily bird-and wind-dispersed species among the plantations of different tree species. The implications of these results for conservation of biodiversity and ways of managing plantations to conserve this diversity are discussed. (C) 1997 Elsevier Science B.V.