Do the antiherbivore traits of expanding leaves in the Neotropical tree Inga paraensis (Fabaceae) vary with light availability?
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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.
KeywordsYoung leaves Phenolics Saponins Canalization Amazonia
GS appreciates help with field and lab work from P.H.S. Soares, C. Brancolina, E.R. Araújo, L.S. Chalco, E.Z. de Albuquerque, M.C. Vaz; M.T.B. da Silva, J.B. da Silva, J.F.T. Junior, S. Khachaturyan, R. Bixenmann, D. Denney, E. Murakami. We acknowledge valuable logistical support from BDFFP/INPA in Manaus, AM, Brazil; we thank J.M. da S Vilhena for identifying the ants; J. Seger and F. Adler for enlightenment regarding statistics; C. Dambros and E. Spear for R programming assistance, B. Lenz, C. Emer, K. Kitajima and anonymous reviewers for thoughtful comments on the manuscript. Field collections were conducted in Manaus, Brazil and complied with current state and federal laws. Plants were exported through permit number 107895 from the Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis. G.S. received a Master scholarship and M.R.L. a research fellowship from the Brazilian’s Conselho Nacional de Desenvolvimento Científico e Tecnológico. T.A.K., P.D.C., and J.L. were supported by National Science Foundation Grants DEB-0234936, DEB-0640630, DEB-1135733 and Research Experience for Undergraduate supplements. This is publication number 592 in the Biological Dynamics Forest Fragments Project Technical Series.
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