Distributional ecology of Gymnostoma australianum (Casuarinaceae), a putative palaeoendemic of Australian wet tropic forests

Abstract

The small, tropical Australian rainforest tree Gymnostoma australianum (L. Johnson) is presently only known from five sites in the environs of Thornton Peak, in north-eastern Queensland. Aggregated populations of these trees occur in open sites along rocky stream banks at various altitudes from sea level to 1200 m, and amongst granite outcrops and fern fields at high altitudes. Trees are tolerant of shallow, acidic soils but do not occur where light is limiting. The population of trees occurring on a mid-stream island in Noah Creek form monospecific stands on the central rocky portion of the island. This population is subject to moderate levels of disturbance during flooding. Lost individuals are likely to be replaced by conspecifics as seedling growth under adults is plentiful. Although disturbance enables current populations to persist, the species does not appear to be a coloniser of disturbed habitats. Gymnostoma australianum is more accurately described as a habitat specialist, occurring in marginal, low nutrient sites within various types of rain forest. These sites are narrow in range both geographically and ecologically. This restricts the expansion of current populations and has probably influenced population expansion in the past. Therefore, the hypothesis that the species, or its ancestor, was a recent immigrant from northern tropical Malesian forests was rejected. The current distribution of the species is more likely to represent the persistence and expansion of trees from local refugia.