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The theory of resource availability suggests that slower growing species should be better defended against herbivores. We tested this theory by examining differences in growth, defense and herbivory between sexes and among individuals of a dioecious tree species, Acer negundo. Male trees had significantly faster growth rates than females. Leaf defense levels were estimated by measuring leaf characters, such as nitrogen and water content, toughness, tannins and total phenolic compounds. Female and male trees could be distinguished from one another using a discriminant function analysis based solely on leaf characters. Leaves from female trees were tougher than those from males, although no consistent differences were found between the sexes in other leaf characters. When all leaf characters were combined in a multiple regression, females were significantly better defended than males. Further-more, data from the field showed that males consistently had higher damage rates than females. Data from the literature on other dioecious species suggest that males commonly suffer greater herbivory than females. Similar negative correlations between growth rate and defense, and between defense and herbivory were found among individuals of a single sex. These data support the hypothesis that slow plant growth rates select for high defense and also suggest that growth rate differences may explain the sexual dimorphism in defenses and herbivory of dioecious plants.
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