Dark Island heath (Ninety-mile Plain, South Australia). VI. Pyric succession: changes in composition, coverage, dry weight, and mineral nutrient status

Abstract

The dynamic changes in the composition, dry weight, and mineral nutrient status of heath following fire have been investigated. The overall growth (dry weightltime) curve for the aerial organs of the heath is essentially exponential. Soil moisture is conserved by burning and, provided climatic conditions are favourable, regeneration of all species is rapid. Annual species are rare and are found only in the first year after a fire. Many species are fire-resistant and regenerate rapidly from buriedperennating buds; the others reproduce in great numbers from seeds. The number of propagules varies with the age and composition of the parent stand. The initial regrowth, dominated by Xanthorrhoea australis, produces annually over 500 kg dry weight per acre. Two or three years after a fire the regrowth of Casuarina pusilla and a wealth of undershrubs form alarge part of the stand. Growth is much slower with only 240 kg dry weight produced annually per acre. During this period many species or the understorey reach their peak and die; the major species are reduced in number. This decrease in numbers may be through natural senescence of the species, but is certainly hastened by competition for water and, to a lesser extent, light. It continues throughout the development of the heath. After about 10 years, the numerous seedlings of Banksia ornnta dominate the stand, probably owing to reduced competition from the understorey plants. A dry weight of 180 kg per acre is produced annually over the next 5 years. After this time (15 years) there is a continuous fall in the annual growth rate to 160 kg per acre towards the 50-year period. Of 36 species recorded after a fire only 20 persisted after 25 years, five of these 20 contributing less than 1 kg dry weight per acre. Only ten of these species persist after 50 years and most of these are greatly depleted in numbers. Almost 15,000 kg dry weight per acre were found in the 50-year stand dominated by massive plants of B. ornata and X. australis. Apart from the first 10 years when the underground organs contribute considerable food reserves to the regenerating aerial organs, the evidence suggests that these organs increase in dry weight per acre almost as much as those above ground. Nutrients from this very infertile soil, the Makin sand, steadily aocumulate in the underground organs, often at the expense of the aerial organs. Translocation of many nutrients (P, N, K, Ca, Cu, Zn, and Mn) to aerial organs may be greatly reduced, that of some elements almost to zero. This must contribute greatly to the decreasing growth rate of the aerial organs. As Casuarina and Phyllota spp. contain greater concentrations of nutrients than the other species their requirements are presumably greater. They are eliminated early under nutritional stress. Gradually only those species survive in which the concentration of nutrient elements is low, namely Banksia spp. and Xanthorrhoea. However, as over 50 per cent. of the nutrients in the aerial organs of these species are bound in fruits and dead leaves, even these species must suffer nutrient stress and degradation of the stand must inevitably occur, to be followed by regeneration on the release of the nutrients. The frequency of fire is such that the heath does not mature. Regular firing is essential to maintain many elements of the flora. Changes in the nutrient levels of soil and litter are also indicated.