Insect herbivory
in a forest under
elevated CO2

The objectives of the proposed research are to determine how elevated atmospheric CO2 affects plant-herbivore interactions in complex and simple communities, and to quantify the photosynthetic competence of leaves following herbivory. The projected increase in CO2 caused by human activities alters the chemical and structural characteristics of leaves. These changes, such as increases in leaf starch content or decreases in nitrogen, reduce the quality of foliage as a food source. Folivores may respond by increasing consumption to compensate for poor food quality; however, this response is highly variable and depends in part on the availability alternative food sources. Using Free-Air CO2 Enrichment (FACE) technology, we will investigate how projected changes in atmospheric CO2 affect leaf quality and the feeding behavior of folivorous arthropods in natural settings.

FACE employs vertical vent pipes extending through the plant canopy to precisely control atmospheric CO2 levels in plots within an otherwise intact ecosystem. The control and treatment plots are contiguous with the surrounding ecosystem, and open to the full suite of folivores and their predators.  In North Carolina, the amount of herbivory and the population dynamics of the major folivorous arthropods will be compared for 14 hardwood species growing in the naturally occurring understory of a pine plantation under ambient (~360 ml/l) and elevated (~560 ml/ l) CO2, the level expected in 2050.  We hypothesize that the total amount of herbivory by generalists will not change in this complex community as they choose nearby species whose foliage quality has not responded to elevated CO2. Parallel measurements will be made in Illinois where a soybean monoculture will be grown under elevated CO2 with FACE.  In this system we expect per capita herbivory to increase as leaf quality decreases for all plants. In North Carolina and Illinois, measurements of leaf structure and chemistry will be paired with measurements of herbivory and arthropod population dynamics to determine if CO2 induces compensatory feeding by specific herbivores or if the population size or community structure of herbivores is altered. 

With the exception of severe outbreaks, it is generally accepted that herbivory has a relatively minor effect on ecosystem productivity. This view rests on measurements of the amount of leaf tissue removed by herbivores but does not consider the photosynthetic competence of the remaining leaf tissue.  We recently built a novel fluorescence imaging system that provides detailed maps of the component reactions of photosynthesis across the surfaces of intact leaves. Preliminary results indicate that photosynthesis is inhibited at substantial distances from the area of leaf tissue removed, suggesting that simply measuring the loss of leaf tissue greatly underestimates the damage by herbivores. Using this instrument we will test the hypothesis that different feeding guilds of arthropods (chewers, suckers, gall formers) will differentially affect remaining leaf tissue in forest trees and soybean. These new measurements of the effect of herbivory on photosynthesis together with the experiments on the effects of elevated CO2 on herbivory will provide new insights into how plant-herbivore interactions will alter ecosystem productivity in a future high-CO2 world.  

See also http//sciencenow.sciencemag.org/cgi/content/full/2001/810/4.