Ecological studies of coastal forest and its regeneration after mining

Abstract

In recent years the heavy mineral sand mining industry on the east coast of Australia has, with the aid of modem technology, found it possible to extract heavy minerals from orebodies which are often located in sand masses some distance from the present coastline. These sand masses usually support ecosystems e.g. forests which are more complex than the frontal dunes which were previously mined. Whilst such forests have little economic value in present day terms, they are of considerable value in other ways; for example, they are aesthetically pleasing to many people and they represent plant and animal gene pools. Several years ago a forested sand mass situated in the Myall Lakes National Park north of Newcastle, New South Wales, was mined for heavy minerals. Detailed pre-mining studies were made in the natural forest, a dry sclerophyll open forest with blackbutt (Eucalyptus pilularis) the dominant tree species. In addition to measurements of density and cover of each plant species, the biomass of each component of the forest, above and below ground, was estimated. The total forest biomass including dead tissues was 370 t/ha. The trees accounted for 80% of the total biomass; a little over half of the tree biomass was in the trunks. Only 3.8% of the total biomass was in the understorey tops and the litter accounted for 10% of the biomass. Each component of the biomass was analysed for nitrogen, phosphorus, potassium, calcium, magnesium and sodium. The total contents of these elements was in the order of 3,000 kg/ha. The sodium content was higher than other reported values for forests, probably due to the proximity of the forest to the sea, resulting in accessions from aerosols and rainfall. Processes whereby a coastal forest growing on a siliceous sand substrate could accumulate this mass of elements are discussed. Atmospheric accessions of most nutrient elements, with the notable exception of phosphorus, were probably in excess of annual requirements. It is hypothesised that phosphorus was accumulated very slowly by the developing ecosystem and is the key nutrient element in determining the stage of successional development and the climax condition. An experiment was carried out in the laboratory to assess the rate of movement of phosphorus down the soil profile after mining. Moderate leaching occurred when the equivalent of less than half of the mean annual rainfall received at the field research site was applied. Three field experiments were carried out to see whether management factors applied to the reconstituted site after mining would have any influence on the composition of plant species which returned to the site naturally, and on the direction of plant succession in the early years following mining. The three factors studied were duration of topsoil storage, density of cover crop and application of plant nutrients. It was found that storing the soil during the winter months and spreading it in the early summer resulted in a greater density of native plants than replacing it immediately in the winter months or storing it for short periods and replacing it in the winter months. A sorghum cover crop had little effect on native species although there were indications that the absence of a cover crop increased the density of native species. Annual applications of nitrogen based on the current practice at the time and above and below that rate, increased the percentage cover of some native species. Annual applications of phosphorus at the current rate also increased percentage cover, but the high rate depressed plant cover, indicating likely toxic effects. Potassium, calcium, sulphur, copper, zinc and molybdenum had no effect on plant cover. An absence of fertilizer resulted in the highest biomass (dry matter) of tops of native species. The current usage of nitrogen and phosphorus resulted in the highest mass on a fresh weight basis. Two of the native species and the cover crop species were grown in flowing nutrient culture in the glasshouse. All species attained their maximum growth at extremely low phosphate concentrations, indicating their ability to grow well in poor soils. The implications of the results obtained in this study on the future development of the post mining vegetation, and the use of phosphorus in slowly available forms such as rock phosphate, are discussed.