THE HERBIVORY COST OF N2 FIXATION AND ITS EFFECT ON THE ABUNDANCE OF NITROGEN FIXING TREES IN A TROPICAL FOREST

Abstract

Nitrogen-fixing trees provide an essential nutrient input to tropical ecosystems that enhances the ability of these ecosystems to sequester atmospheric CO2. However, these trees face unique environmental constraints that may limit this nutrient input and therefore the size of the terrestrial carbon sink. In particular, the growth and survival of nitrogen fixing trees may be constrained by leaf tissue loss due to herbivores that preferentially target their nutrient-rich leaves. I collected measurements of herbivory rates in a Panamanian tropical forest to determine whether herbivory is a major factor limiting nitrogen fixation. To expand upon previous studies, I focused on seedlings and tested for differences in herbivory due to leaf age, stand age, and leaf nutrient content. I also constructed herbivore exclosures to separate the effects of insect and large mammal herbivory. I then incorporated this data into the Ecosystem Demography Model version 2 to simulate the herbivory cost on nitrogen fixers and examine the trade-off between herbivory costs and fixation benefits in tropical forests under current and future climate conditions. The field data indicate that nitrogen fixers experience higher rates of herbivory than non-fixers, especially in young leaves, and that this cost remains significant throughout forest succession. This preference may be explained in part by differences in nitrogen content among leaves. There was no evidence of mammalian herbivory, so herbivory came primarily from insects. Simulations demonstrated that herbivory can act as a major constraint on nitrogen fixers in tropical forests and that this constraint may increase with elevated CO2, limiting the amount of carbon tropical forests may sequester in the future.