U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Project   Impacts of elevated CO2 and O3, alone and in combination, on the structure and functioning of a northern hardwood forest ecosystem: operating the aspen FACE experiment

Principal investigator:   David F. Karnosky

leaf response

Leaf level photosynthesis (Karnosky et al. 2003)

Project goal

Determine effects of elevated [CO2] and [O3], alone and in combination, on the structure and functioning of model northern hardwood forest ecosystems. This is accomplished by operating the DOE Rhinelander, Wisconsin, FACE (free-air CO2 enrichment) experiment, which is used by multiple users and research projects (see also the Rhinelander FACE experiment website).

Ecosystem being studied

Constructed stands of (a) the pioneer species trembling aspen (Populus tremuloides), (b) a mixture of aspen and another pioneer species paper birch (Betula papyrifera), and (c) a mixture of aspen and the late successional species sugar maple (Acer saccharum). These three species are important components of many northern hardwood forests and make up about 60% of pulpwood harvests in the Great Lake States. Trembling aspen is the dominant angiosperm tree species in the boreal forest, and is the most widely distributed tree species in North America.


A key result of this project is the long-term successful operation of the FACE experiment (see the facility performance page for operational statistics). About 70 scientists have collected data at the experimental site or are using results from the experiment in model-based studies.

Ecological effects. Elevated [O3]--at 1.5 times ambient values--completely offset the beneficial effect of elevated [CO2] (i.e., 200 ppm greater than ambient) on the growth of aspen in the aspen-only portions of the research plots. Elevated [O3] partially offset the beneficial effect of elevated [CO2] on tree growth in the aspen-birch and aspen-maple portions of the research plots.

The elevated ozone treatments have been observed to delay bud break, decrease leaf area, delay fall senescence, and exacerbate the influence of pest outbreaks. For example, it was found that elevated O3 caused a 3-5 fold increase in Melampsora leaf rust of aspen, altered aphid and forest tent caterpillar survival and growth, and decreased soil carbon deposition in plots with elevated CO2.

biomass response

Biomass responses of aspen. Treatments: (1) ambient, (2) elevated CO2, (3) elevated O3, (4) elevated CO2 + O3. From King et al. (2005).

Why this is important

Concentrations of both CO2 and O3 have increased significantly in the atmosphere enveloping forests during the past century, and both have been projected to increase in the coming decades. Because both CO2 and O3 can affect forest structure and functioning (elevated CO2 may generally be beneficial whereas elevated O3 can be detrimental), it is important to understand how these changes in atmospheric composition might affect future forests and the many goods and services that they provide to society.

Many models used to predict effects of increasing atmospheric [CO2] on tree growth and forest productivity do not include effects of simultaneous increases in [O3]. The scientific results being obtained in the DOE Rhinelander FACE user facility indicate that such predictions may be inaccurate, and overstate the growth of future northern hardwood forests. Accurate predictions of effects of atmospheric changes on forests during the next 50 to 100 years are important for forest industry and large forest landowners who try to plan for the future.

Community structure of northern forests could change as a result of future changes in atmospheric composition, as indicated by the observation of greater birch dominance in elevated [CO2] and elevated [O3] rings relative to the ambient rings. Changes in dominance might be important to production forestry, recreation, and effects of climatic change on forests.


A state-of-the-science open air exposure system is being used to examine the effects of elevated carbon dioxide (537 ppm [CO2]) and ozone (1.6 x ambient [O3]) on growth, productivity, trophic interactions, and numerous ecosystem processes in constructed northern hardwood stands. The experimental design is a full factorial including 3 replicate exposure rings for each of the following treatments: ambient, +CO2, +O3, and the combined +CO2 and +O3 treatment. Each ring is populated with three constructed populations of trees. The eastern half of each ring includes a mixture of aspen clones, the northwestern quarter includes a mixture of aspen and sugar maple, and the southwest quarter a mixture of aspen and paper birch.

FACE ring

Layout of the experimental plots.

Elevated CO2 and O3 treatments occur during daylight hours from aspen bud break to leaf drop and when wind speeds are greater than 0.5 m/sec and less than 4 m/sec. Fumigation with O3 does not occur when leaves are wet for any reason (rain, fog, or dew events) or when predicted daily maximum temperatures are less than 15 degrees Celsius.


David F. Karnosky, Michigan Technological University

Kurt S. Pregitzer, University of Nevada, Reno (subaward)

Donald R. Zak, University of Michigan (subaward)

Alistair Rogers, Brookhaven National Laboratory

Keith Lewin, Brookhaven National Laboratory

John Nagy, Brookhaven National Laboratory

Kevin E. Percy, Canadian Forest Service, Fredericton (unfunded collaborator)

Neil Nelson, USDA Forest Service, Rhinelander (unfunded collaborator)

Mark E. Kubiske, USDA Forest Service, Rhinelander (unfunded collaborator)

Richard L. Lindroth, University of Wisconsin (unfunded collaborator)


Karnosky DF, Zak DR, Pregitzer KS, Awmack CS, Bockheim JG, Dickson RE, Hendrey GR, Host GE, King JS, Kopper BJ, Kruger EL, Kubiske ME, Lindroth RL, Mattson WJ, McDonald EP, Noormets A, Oksanen E, Parsons WFJ, Percy KE, Podila GK, Riemenschneider DE, Sharma P, Thakur R, Sober A, Sober J, Jones WS, Anttonen S, Vapaavuori E, Mankovska B, Heilman W, Isebrands JG (2003) Tropospheric O3 moderates responses of temperate hardwood forests to elevated CO2: a synthesis of molecular to ecosystem results from the Aspen FACE project. Functional Ecology 17:289-304

King JS, Kubiske ME, Pregitzer KS, Hendrey GR, McDonald EP, Giardina CP, Quinn VS, Karnosky DF (2005) Tropospheric O3 compromises net primary production in young stands of trembling aspen, paper birch and sugar maple in response to elevated atmospheric CO2. New Phytologist 168:623-635

Funding period:   September 1995 to present