How does ontogeny in a Eucalyptus species affect patterns of herbivory by Brushtail Possums?

Abstract

Summary 1. Comparisons of adult and juvenile plant stages are often confounded by other factors such as differences in height between plants. However, these factors can be teased apart using common garden experiments and appropriate sampling designs. 2. Using paired sampling of upper and lower canopies of Eucalyptus nitens trees,which were growing in a common environment trial and only one of which had undergone the transition to adult foliage, this study assessed the effects of ontogeny and canopy position on the feeding preferences of Common Brushtail Possums (Trichosurus vulpecula) in captive feeding trials under choice conditions. 3. Possums preferred juvenile foliage over adult foliage. In contrast, canopy position did not influence preferences. 4. Adult foliage contained significantly less sideroxylonals (a compound known to deter herbivory by Brushtail Possums), but was significantly tougher (with thicker cuticle, more fibre, more lignin and a higher percentage dry mass) than juvenile foliage, suggesting that leaf toughness was more significant than defensive chemistry in influencing intake by possums. Adult foliage did not differ significantly from juvenile foliage in nitrogen, total phenolics or essential oil concentration. 5. Greater resistance to herbivory of adult foliage compared with juvenile foliage by a significant browser of eucalypt tree foliage could contribute to selection for an earlier change from juvenile to adult foliage in areas that are heavily browsed by possums.