PARENT SESSION
Contributed Oral Session 99: Populations and Genetics: Diversity and Population Dynamics
Wednesday, August 10, 1:30 PM - 5:00 PM, Meeting Room 519 A, Level 5, Palais des congrès de Montréal

The growth-defense tradeoff and habitat specialization by plants in Amazonian forests.

Fine, Paul^{*,1, 2}, Miller, Zachariah ¹, Mesones, Italo³, Irazuzta, Sebastian⁴, Coley, Phyllis², ¹ University of Michigan, Ann Arbor, MI, USA² University of Utah, Salt Lake City, UT, USA³ Universidad Nacional de la Amazonia Peruana, Iquitos, Peru⁴ McMaster University, Hamilton, ONT, Canada

ABSTRACT- In the Peruvian Amazon, nutrient-poor white-sand forests are found immediately adjacent to nutrient-rich clay forests, each harboring a unique composition of habitat specialist trees. How does habitat specialization occur? Our hypothesis is that the combination of impoverished soils and herbivory creates strong selective pressure for plant defenses in white-sand forest. Species that have not evolved high levels of antiherbivore defense are therefore at a disadvantage in white-sand forests and are excluded by herbivores. However, the high costs of defense in white-sand specialists puts them at a competitive disadvantage in richer soils. While potential of herbivore attack did not differ between the two habitats, artificial defoliation showed that the impact of herbivory on plant mortality was significantly greater in white-sand forests. A reciprocal transplant experiment involving 20 species from 6 genera that include phylogenetically-independent pairs of closely-related white-sand and clay specialists was used to manipulate the presence of herbivores and quantify their effect on growth rates as well as to evaluate defense investment in the species. When protected from herbivores, clay specialists exhibited significantly faster growth rates than white-sand specialists in both habitats. Moreover, white-sand specialists demonstrated significantly higher resistance to herbivores relative to clay specialists. We quantified terpenes, phenolics, leaf toughness and available foliar nitrogen for the plants in the experiment. Although genera invested in different defensive strategies, we found significantly higher total defense investment overall for white-sand specialists relative to their clay-specialist congener. Furthermore, defense investment consistently exhibited a significant tradeoff against growth rate in each of the six phylogenetically-independent species-pairs. These results confirmed theoretical predictions that a tradeoff exists between growth rate and defense investment, causing white-sand and clay specialists to evolve divergent strategies. In this way, herbivores magnify differences in habitat quality and thereby increase the potential for edaphic heterogeneity to produce habitat specialization in plants.

Key words: tropical rainforest, white-sand forests, phylogenetically-controlled, reciprocal transplant experiment