Suppression of Coastal Heath Vegetation by Eucalyptus baxteri

Abstract

Eucalyptus baxteri produces a zone of suppression beneath its canopy when growing in coastal heath. Dominant species including Casuarina pusilla and Leptospermum myrsinoides are suppressed, but species such as L. juniperinum and Xanthorrhoea australis are not. Investigations of shoot water potential, soil nutrient levels and shading failed to suggest suppression by competition. E. nitida produces a slightly deeper shade but fails to produce a similar pattern of differential suppression. Chemical inhibition (allelopathy) of Casuarina pusilla, Eucalyptus viminalis and Triticum aestivum by E. baxteri was shown to exist under laboratory conditions. Foliar leachates of E, baxteri are inhibitory in bioassays and contain gentisic and ellagic acids. Litter leachates are also inhibitory in bioassays and contain gentisic, gallic, sinapic, caffeic and ellagic acids. Both leachates also contain several unknown phenolic aglycones, numerous glycosides, and terpenoids. Topsoil extracts are also inhibitory and contain resins and, possibly, terpenoids. The suppression zone is associated with the allelopathic ability of E. baxteri, and is maintained either through the direct transfer of foliar leachates to leaves of suppressed species, through root absorption of foliar and litter leachates, or as a consequence of mycorrhizal inhibition by such leachates.