Tropical tree seedlings grown in elevated CO₂ experience lower rates of herbivory

Human activities, especially the burning of fossil fuels over the past ~160 years, have increased atmospheric CO₂concentrations by more than 50% compared to pre-Industrial levels. As CO₂ levels are projected to continue rising significantly for the foreseeable future, little is known about the influence of elevated CO­₂ (eCO₂) in crucial terrestrial biotic interactions, especially in the tropics. Plants and insects comprise the vast majority of terrestrial biomass, and their ubiquitous interaction – herbivory – is essential to understand forest functioning. Under climate change scenarios, understanding how forests and their arthropod food webs will respond to eCO₂ will decrease the great uncertainties around these predictions. We conducted an eCO₂ experiment on 6 species of tropical tree seedlings in an intact tropical lowland forest. Using open-top chambers at La Selva Biological Station in Costa Rica, we elevated daytime CO₂ to 654±0.17 ppm, compared to an average of 351±0.06 ppm in ambient controls. We conducted weekly surveys for two months to estimate leaf herbivory and arthropod abundance on each leaf, across 600 seedlings. We found that after 8 weeks, the plant-level herbivory on leaves grown in eCO₂ decreased more than two-fold compared to ambient controls. The proportion of leaves with any amount of herbivory was also significantly lower in eCO₂. Even though we consistently observed more spiders in the eCO₂ chambers, the abundance of herbivores on the plants was not significantly different over the course of the experiment, suggesting that plant traits may be less favorable to herbivores in eCO₂ in this experiment.