Do tropical species invest more in anti-herbivore defence than temperate species? A test in Eucryphia (Cunoniaceae) in eastern Australia

Tropical plants have been suggested to have higher levels of mechanical, chemical and biotic defences than temperate plants. However, comparisons have usually included deciduous species within the temperate group, which confounds the analysis since deciduous species are predicted to have a different strategy with respect to investment, nutrition and defence than evergreen species. In this study we examined levels of defence and nutrition in Five evergreen species of Eucryphia occurring along a latitudinal gradient in eastern Australia, grown under common conditions in a glasshouse. From the resource-availability hypothesis we predicted the opposite gradient in defence investment, i.e. that lowest levels of defence will occur in tropical species with potentially high growth rates and annual productivity. However, we found an increase in cell wall content, total phenolics and tannin activity, and a decrease in protein availability, with decreasing latitude and/or increasing mean annual temperature. Hence, there was a trend of increasing defence (although not in leaf toughness) and declining nutritional quality towards the tropics. These latitudinal trends were recorded in both mature and expanding leaves. The same trends were observed in leaves of two species collected from the field, indicating that the results were not peculiar to the experimental growth regime. The latitudinal trend in defence did not support our prediction based on the resource availability hypothesis and may indicate that herbivore pressure is providing an overriding selection pressure, although there are alternative explanations.