Do the antiherbivore traits of expanding leaves in the Neotropical tree Inga paraensis (Fabaceae) vary with light availability?

Abstract

Treefall gaps in tropical forests have a profound effect on plants growing in the understory, primarily due to increased light availability. In higher light, mature leaves typically have increased anti-herbivore defenses. However, since the majority of herbivory occurs while leaves are expanding, it is important to determine whether defense expression during the short period of leaf expansion is canalized (invariant) or plastic in response to variation in light. Therefore, we examined young leaves of Inga paraensis (Fabaceae) saplings growing along a light gradient in a terra-firme forest in Central Amazonia. We quantified leaf production and expansion time, dry mass of phenolics, saponins, and nitrogen, ants attracted to extrafloral nectaries, and leaf consumption. Over the entire light gradient, the number of leaves produced per flush increased by 50 % and the mass of phenolic compounds by 20 %, but no other traits changed. On average, 39 % of leaf area was consumed with no difference across the light gradient. Alone, none of the leaf traits was a significant predictor of leaf consumption, except for phenolics, which showed a positive relationship. Multiple regressions showed that leaf consumption was positively related to more leaves per flush and a higher concentration of phenolics in leaves. Unlike studies of mature leaves, young leaves of I. paraensis show low plasticity in defense traits across a light gradient, suggesting that leaf development is canalized.