Regeneration of Three Tree Species in Arid South-Eastern Australia

Abstract

Grazing effects by introduced mammals on tree seedlings in the arid zone have caused concern for the long-term future of some tree species. This was investigated by measuring stem girths of Casuarina cristata Miq., Heterodendrum oleifoliurn Desf. and Myoporum platycarpum R.Br. in arid-zone wood- lands. Episodic seedling regeneration of Myoporum is inferred following series of wet years in the 1950s and 1970s. Although young Myoporum plants appear unpalatable to stock, regeneration may be sup- pressed by rabbit-grazing in some areas. Almost no Heterodendrurn seedlings were found but suckering was common after burning and clearing. This species is highly palatable to grazing mammals and suckers may grow to safety above the browse line only where stock are absent. Evidence for recent Casuarina seedling regeneration was limited to one cohort, on a site that had been flooded. While suckers can be found locally where surface roots become exposed, suckering appears insufficient to perpetuate many Casuarina stands.

Mammal-grazing seems important in strongly limiting regeneration. While Casuarina and Heterodendrum stand densities decline due to natural senescence and occasional fire, the only species regenerating into gaps is Myoporum. There is concern for the future of Casuarina and Heterodendrum, given their widespread failure to regenerate despite the wettest 3 years ever recorded, in 1973-75. On pastoral land, at least 10 years without stock-grazing might be needed for successful regeneration of Heterodendrurn. In reserves, some nature conservation programs for woodlands might require virtual eradication of all exotic mammalian herbivores, including goats.