Dark Island heath (Ninety-mile Plain, South Australia). III. The root systems

Abstract

This paper describes the nature of the root systems of the most important members of the heath community. Several variations of tap-root and fibrous root systems were observed. Tap-rooted species were either shallow rooting (1–2 ft) or deep rooting (6 or more feet into the clay subsoil). Two variations of deep tap-rooted species were observed. The tap-root of one decays with age; the laterals of the other produce frequent sucker shoots. In all forms of the deep tap-rooted species an extensive lateral root system was developed within the surface 12 in. of soil — the organic A₁ horizon; the tap-root and occasional secondary vertical descended, often unbranched, to the subsoil. The fibrous root system may arise from stem bases, rhizomes, tubers, or underground stocks (caudices). With the exception of underground stocks, which had extensive roots in the A₂ and A₃ to B horizons, the other forms of the fibrous root systems were confined to the A₁ horizon.

The marked concentration of roots in the organic A₁ horizon was illustrated in dry weight–depth curves. Most of the roots in the A₂, and A₁ horizons arose from the caudex of Xanthorrhoea australis R.Br.; the remainder were vertical roots which passed directly into the subsoil from the deep-rooted species.

About 70 per cent of the species recorded in the heath had morphological characteristics which enabled them to survive a fire and sprout from perennating buds buried under the surface of the ground. Thus, although the aerial organs of the heath were destroyed by fire, the root systems provided a reserve of food and nutrients for the regenerating heath. The dry weight of the root systems was therefore scarcely influenced by fire and thereafter steadily increased in the organic A₁ horizon as the stand aged.

The presence of root nodules on species of Leguminosae and Casuarinaoeae as well as of haustoria on Exocarpos sparteus R.Br. and Euphrasia collina R.Br. Is recorded.