Age-related changes in defensive traits of Acacia tortilis Hayne

The theory of plant defences proposes that investments in physical and chemical defences are driven by the risk of herbivore damage, and limited by the cost of producing the particular defensive trait in terms of resources that could be directed to other sinks, such as growth and reproduction. We sampled twigs of 18 mature Acacia tortilis trees and their cohort of juveniles to test some predictions of this hypothesis. We expected a higher allocation of defensive traits to leaves and twigs in the young plants than in the mature ones as a result of a higher risk of damage by ungulates at the juvenile stage. Our results show that the juvenile plants produce more spines along their twigs, but have lower concentrations of phenolic compounds in their leaves than in the mature ones. We also expected a negative relation between the concentration of foliar nutrients and phenolic compounds, as predicted by the carbon/nutrient hypothesis. Only mature plants showed this pattern, Reproduction (in mature plants) and water stress (in juvenile plants) did not relate to allocation to secondary compounds as predicted by current hypotheses of plant defence.