An investigation of the effect of the dehydroangustione present in the leaf litter of Backhousia angustifolia on the germination of Araucaria cunninghamii - An experimental approach to a problem in rain-forest ecology

Abstract

Failure of Araucaria cunninghamii to regenerate in secondary rain forest containing a high proportion of Backhousia angustifolia has been observed at Good Night Scrub, near Maryborough, Qld. The possibility that this failure is due primarily to the presence of toxic compounds in the leaf litter of Backhousia has been investigated. In laboratory tests, substrates containing between 1 % and 5% by weight of free dehydroangustione (in this case the major constituent of the leaf oil of Backhousia) have been found to hinder germination of Arazccaria. However, the hypothesis is not supported by experiments carried out in the field; germination of Araucaria was significantly higher on a litter enriched with freshly gathered Backhousia leaves (and found to contain 1.3-1.6% by weight of dehydroangustione) than on the natural litter. Moreover, a watering treatment resulted in a further significant increase in germination of Araucaria on an enriched litter containing 1.9-2.2% by weight of dehydroangustione. Evidence has been obtained that substrates prepared from milled leaves or from extracts of plants (which are frequently used to demonstrate "antibiotic effects" in this type of investigation) are not comparable with substrates of intact leaves. Presumably all of the toxic compound present in the substrate has access to the test species in the former cases, but this does not appear to be so when substrates of intact leaves are used. It is concluded that further experimentation in the field is essential before the "antibiotic effects" revealed in laboratory experiments can be considered to be of probable ecological significance in a natural plant community. It is suggested that the failure of Araucaria to regenerate in the environment studied is more likely to be due to the early death of seedlings than to inhibition of germination.