Seeing the forest for the trees: long‐term exposure to elevated CO2increases some herbivore densities

Abstract

AbstractThe effects of elevated CO2on plant growth and insect herbivory have been frequently investigated over the past 20 years. Most studies have shown an increase in plant growth, a decrease in plant nitrogen concentration, an increase in plant secondary metabolites and a decrease in herbivory. However, such studies have generally overlooked the fact that increases in plant production could cause increases of herbivores per unit area of habitat. Our study investigated leaf production, herbivory levels and herbivore abundance per unit area of leaf litter in a scrub‐oak system at Kennedy Space Center, Florida, under conditions of ambient and elevated CO2, over an 11‐year period, from 1996 to 2007. In every year, herbivory, that is leafminer and leaftier abundance per 200 leaves, was lower under elevated CO2than ambient CO2for each of three species of oaks,Quercus myrtifolia, Quercus chapmaniiandQuercus geminata. However, leaf litter production per 0.1143 m2was greater under elevated CO2than ambient CO2forQ. myrtifoliaandQ. chapmanii, and this difference increased over the 11 years of the study. Leaf production ofQ. geminataunder elevated CO2did not increase. Leafminer densities per 0.1143 m2of litterfall forQ. myrtifoliaandQ. chapmaniiwere initially lower under elevated CO2. However, shortly after canopy closure in 2001, leafminer densities per 0.1143 m2of litter fall became higher under elevated CO2and remained higher for the remainder of the experiment. Leaftier densities per 0.1143 m2were also higher under elevated CO2forQ. myrtifoliaandQ. chapmaniiover the last 6 years of the experiment. There were no differences in leafminer or leaftier densities per 0.1143 m2of litter forQ. geminata. These results show three phenomena. First, they show that elevated CO2decreases herbivory on all oak species in the Florida scrub‐oak system. Second, despite lower numbers of herbivores per 200 leaves in elevated CO2, increased leaf production resulted in higher herbivore densities per unit area of leaf litter for two oak species. Third, they corroborate other studies which suggest that the effects of elevated CO2on herbivores are species specific, meaning they depend on the particular plant species involved. Two oak species showed increases in leaf production and herbivore densities per 0.1143 m2in elevated CO2over time while another oak species did not. Our results point to a future world of elevated CO2where, despite lower plant herbivory, some insect herbivores may become more common.