An Assessment of the Allelopathic Potential of Eucalyptus

Abstract

Previous studies have shown that various Eucalyptus species can yield allelopathic chemicals which may be effective in suppressing understorey vegetation. However, the techniques employed in many studies do not resemble natural ecological processes. This study used germination of Lolium and growth of Lolium, Lemna, Eucalyptus and Acacia to test for allelopathy. Extraction techniques mimicked typical daily rainfall rates upon quantities of foliage, leaf litter and bark litter that are typically encountered in forests; root leachates were obtained hydroponically; stemflow was obtained following rainfall; soils were leached with water; and volatiles from leaves were studied in an enclosed chamber. Fresh intact leaves caused little growth suppression, in contrast to coarsely chopped leaves and extracted leaf essential oils which were both highly suppressive. Whole leaf litter, shed bark and, especially, stemflow yielded suppressive leachates. Evaporative concentration of leachates in soils was demonstrated, which increased their inhibitory effect. It is apparent that allelopathy must be considered in relation to rainfall and the soil water balance. Decay was shown to reduce the allelopathic effects of leaf and bark litter leachates but some inhibitory chemicals remained after 5 months. It was concluded that allelopathy is likely to be a cause of understorey suppression by Eucalyptus species especially in drier climates.