Abstract
Juvenile trees are expected to be more heavily defended against browsing mammals than mature plants. Juvenile and mature trees of Acacia tortilis and A. nilotica occurring at Nylsvley, Northern Transvaal, were quantitatively compared in terms of some potential chemical and physical defences. Neither species showed any significant difference between juvenile and mature trees in terms o f total polyphenol content, condensed tannin content, protein precipitating ability or protein content in leaves. Both species showed age-class differences in spinescence. In A. nilotica, thorns on branch tips were longer and more closely spaced and leaves were smaller in juveniles than in adults. Hence juveniles of this species appear to be physically more heavily defended than mature plants. In A. tortilis, curved thorns were longer, but straight thorns were shorter than in mature trees. There was no difference between age classes in overall thorn density, but juveniles had a higher curved to straight thorn ratio. It is not obvious what the effects of these differences might be on mammalian browsers.
Highlights
Woody plants that are browsed by mammals when still juvenile may show retarded growth and increased time to reach reproductive maturity (Bryant el al. 1983: Bryant el al. 1991b)
The length of all thorns within 250 mm of the branch tip on the same branches from which leaves were col lected was measured to the nearest millimetre
The density of thorns was measured by counting the number of thorns or thorn pairs within 250 mm of the branch tip
Summary
Woody plants that are browsed by mammals when still juvenile may show retarded growth and increased time to reach reproductive maturity (Bryant el al. 1983: Bryant el al. 1991b). Woody plants that are browsed by mammals when still juvenile may show retarded growth and increased time to reach reproductive maturity Woody plant defences against mam malian herbivores could be expected to be expressed more strongly in juveniles than in mature trees The objective of this study was to assess whether some potentially defensive traits were expressed more strongly in juvenile Acacia trees than in mature trees. No studies comparing anti-herbivore defences of juvenile and mature Acacia trees have yet been reported. Chemical defences include secondary metabolites (Freeland & Janzen 1974; Bryani & Kuropat 1980; Cooper & Owen-Smith 1985; Cooper el al. 1988; Bryant el al. 1991c), high fibre contents (Bell 1983) or low nutrient levels (Lundberg & Astrom 1990)
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