Abstract, Global Change Open Science Conference, Amsterdam, the Netherlands, July, 2001.

The GCTE Elevated CO2 Network

R. J. Norby, Körner, Ch. and Pataki, D. E.

The Elevated CO2 Network is a primary activity of the GCTE Focus on Ecosystem Physiology. Whole-ecosystem manipulative experiments are encouraged that will simultaneously vary CO2 concentration and other critical controllers of ecosystem processes, such as nutrients and water. The primary goal of these experiments is to identify and quantify the mechanisms underlying ecosystem responses. The experiments make use of various techniques for elevating CO2 concentration, such as controlled environment chambers, open-top field chambers, FACE (Free-Air CO2 Enrichment technology), and natural CO2 springs, and they are located in a broad range of ecosystems around the world, including agricultural systems, grasslands, bogs, deserts, and forests.

GCTE has sponsored various synthesis activities through the Elevated CO2 Network. Following a major 1996 synthesis, which highlighted many of the uncertainties and difficulties in relating the results of small-scale experiments to larger-scale ecological questions, subsequent research projects within the network have focused on ecosystem-relevant experiments and measurements. Research programs have increasingly been attempting to describe how the primary responses to [CO2] will be manifested in future ecosystems, understand the feedbacks between those primary responses and the atmospheric and climatic systems, and develop plant and ecosystem models to make the predictions of plant responses to a future atmosphere. GCTE synthesis activities have considered issues that are important in biogeochemical models of ecosystem response, such as root dynamics, the effects of [CO2] on litter chemistry and decomposition, and interactions between [CO2] and environmental stresses.

Although the GCTE CO2 network encompasses a wide variety of experimental approaches, the employment of FACE technology was especially encouraged for exposing intact ecosystems. FACE experiments in agricultural systems are providing data for validating crop production models. FACE experiments in forests are resolving uncertainties about carbon cycling responses. In two projects supported by the U.S. Department of Energy, net primary productivity of coniferous and deciduous forest stands has increased about 25% in response to elevated [CO2] in the atmosphere. Increased NPP in the pine stand is recovered in woody biomass, but in the deciduous forest it is allocated to fast-turnover pools (leaves and fine roots). In unmanaged systems such as the desert and prairie, effects of [CO2] on biodiversity may be the predominant issue. The FACE experiment in the Nevada desert, for example, documented a surprising interaction between [CO2], rainfall variability, and an invasive species that has the potential to accelerate the fire cycle, reduce biodiversity, and alter ecosystem function in the deserts of western North America.

A synthesis of the effects of rising [CO2] that apply across a wide range of ecosystems cannot be undertaken lightly. Although we can safely conclude that in most systems photosynthesis is increased by CO2 enrichment, predictions about the whole-system behavior of future ecosystems require an understanding of how the primary responses to [CO2] interact with the attributes of the different systems. Nevertheless, the scientists, research projects, and funding agencies that contribute to the GCTE Elevated CO2 Network have made tremendous progress in providing the data and understanding needed for making—and having confidence in—predictions about the future.