Prevalence of monodominant vigorous tree populations in the tropics: Herbivory pressure on Tabebuia species in very different habitats

Abstract

1 We have studied the reproductive performance, early establishment and insect herbivory in two Tabebuia species found in contrasting distant habitats. Tabebuia ochracea and T. aurea are sympatric in the cerrado (Brazilian savanna), whereas T. aurea also colonizes floodplains in the wetlands of the 'Pantanal Matogrossense' as large monodominant stands, but is sparsely distributed in the cerrado. 2 We test the hypothesis that T. aurea has a more vigorous genotype in the Pantanal than in the cerrado, which may be due to a lower negative selection caused by insect herbivory, positive selection for better soil conditions and an increased growth in the face of stochastic flooding. In addition, we predict that T. ochracea is better adapted to withstand higher herbivory rates than T. aurea, and is thus more successful in the cerrado. 3 Kruskal-Wallis analysis and Wilcoxon paired-sample tests were used to compare adult tree reproductive traits. A reciprocal transplant experiment, combined with a glasshouse experiment, was used to investigate herbivory on half-sib seed families found in each habitat. 4 Over a 2-year period, T. ochracea had the higher investment in flower and seed production, which compensated for lower germination rates. However, in the Pantanal, T. aurea had the greatest conversion of flowers to fruit. 5 Fewer seeds of T. aurea survived and germinated in the cerrado than in the Pantanal, but seedlings from the Pantanal grew faster than those from the cerrado. The growth rate of T. ochracea was approximately four times lower than that of T. aurea. 6 Herbivory on seedlings was lower in the Pantanal than in the cerrado. Although seedlings of T. aurea from the Pantanal suffered the highest rates of herbivory in the cerrado, they survived equally to seedling genotypes from the cerrado. Tabebuia aurea had a higher mortality in both the cerrado and under controlled environmental conditions. 7 Faster growth rates may evolve when favourable abiotic conditions prevail. Although herbivores may promote habitat specialization by restricting colonization of poor soil environments by fast-growing species, herbivory pressure may select for tree genotypes with conservative growth rates in newly colonized habitats with poor soil conditions and intense herbivory. © 2006 The Authors.