U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Project   Effects of elevated CO2 and O3 on insect-mediated ecosystem processes in a northern deciduous forest

Principal investigator:   Richard L. Lindroth

Herbivory

Project goal

Assess the independent and interactive effects of elevated concentrations of CO2 and O3, in constructed hardwood forest communities, on: (a) abundance and diversity of herbivorous insect communities, (b) rates of herbivory, and (c) quantity and quality of frass and greenfall (i.e., green litterfall resulting from insect feeding) produced in the experimental plots.

Ecosystem being studied

Constructed stands of (a) a mixture of genotypes of the pioneer species trembling aspen (Populus tremuloides) and (b) a mixture of aspen and another pioneer species, paper birch (Betula papyrifera). These species are important components of many northern temperate forests, and contribute significantly to pulpwood harvests in the Great Lake States. In addition, trembling aspen is the dominant angiosperm tree species of the boreal forest and is the most widely distributed tree species in North America.

The experiment is part of the program-sponsored DOE Rhinelander, Wisconsin, FACE [free-air CO2 enrichment] experiment.

Results

Insect abundance and biodiversity were assessed via trapping and visual censusing in 2006 and 2007. Data are being compiled and analyzed; preliminary results indicate that insect abundance and community composition differ among CO2 and O3 treatments, and that the treatments differentially affect insect functional groups (e.g., herbivores vs. predators).

Cumulative herbivory rate (total proportion of canopy leaf area removed through late August of the growing season) was measured for 2006 and 2007 in aspen-birch and mixed aspen genotype plots. Overall herbivory rates (averaged across years and CO2 and O3 treatments) were 10.3% for aspen and 11.0% for birch. Herbivory was markedly higher (about 40-110% increase) in elevated CO2 plots than in ambient CO2 plots. The effect of elevated O3 compared to ambient O3 on herbivory was less pronounced (about 10-20% decrease). The magnitude of elevated CO2 and O3 effects varied across years, aspen genotypes, and tree species. These data indicate that the effects of elevated CO2 and O3 on feeding damage by insects in deciduous forests of the future are likely to be significant, but also temporally variable and species-specific.

Average herbivore input (frass and greenfall) from the canopy to the forest floor, averaged across years and CO2 and O3 treatments, was 6.58 g m-2 yr-1 for aspen plots and 8.23 g m-2 yr-1 for aspen-birch plots. In both 2006 and 2007, the quantity of herbivore inputs was higher (about 40% increase) in stands with elevated CO2 than those with ambient CO2. On the other hand, elevated O3 had smaller and intermittent negative effects (about 20% reduction) on herbivore input quantity.

Why this is important

Plant-insect interactions are qualitatively and quantitatively important to the structure and functioning of many terrestrial ecosystems, including northern hardwood forests in the United States. In spite of this importance, little is known of the potential effects of rising concentrations of CO2 and/or O3 on the interactions between herbivorous insects and the trees they feed on. This project is expected to provide a wealth of unique information that will be important to forecasting potential ecological effects, mediated through insect-tree interactions, of the ongoing changes in atmospheric composition brought about by energy production from fossil fuels. The project will be the first to document how elevated CO2 and O3 concentrations, singly and in combination, might affect primary production and nutrient cycling in hardwood forests by altering relationships between herbivorous insects and trees.

To date, project results indicate that increasing CO2 and O3 will affect the abundance and species composition of forest insect communities. The net result will depend on interactive effects of elevated CO2 and O3 on herbivory; for the concentration combination used in this experiment, leaf damage has generally increased.

This research is done to address potential shifts in ecosystem-scale nutrient cycling caused by elevated CO2 and/or O3 as mediated through insect herbivore--tree interactions.

Methods

The project is taking advantage of the large and successful investment that the program has made in the DOE Rhinelander FACE experiment to experimentally study effects of elevated concentrations of CO2 and O3 on herbivorous insects and their interactions with trees. The study includes censuses of herbivorous insect communities in the two forest habitats (i.e., aspen communities [five genotypes] and aspen-birch communities).

The project is quantifying feeding damage to aspen (multiple genotypes) and birch trees and links damage amounts (estimated loss of net primary production) to CO2 and O3 effects on plant chemistry (i.e., plant biochemical composition). The project is documenting effects of elevated CO2 and O3 concentrations on the quantity and quality of organic matter deposited on the forest floor through frass and green litter fall (these include indirect and direct effects of the CO2 and/or O3 treatments on leaf quality).

Results will be integrated with modeling efforts at the site to better predict the effects of changes in the concentrations of CO2 and O3 on primary production (i.e., loss of primary production) and nutrient cycling in northern hardwood forests.

Personnel

Richard L. Lindroth, University of Wisconsin

John J. Couture, University of Wisconsin

Timothy D. Meehan, University of Wisconsin

Funding period:   July 2006 to present