Combining global environmental changes yields surprising ecosystem response

Scientists have discovered that elevated atmospheric CO2 (carbon dioxide) can suppress plant growth when increases of this important greenhouse gas are combined with a broad suite of already-occurring environmental changes. According to Christopher Field, project leader and director of the new Department of Global Ecology of the Carnegie Institution of Washington, “We are now getting a much richer picture of ecosystem responses to global environmental changes, and the traditional view that elevated CO2 always stimulates plant growth simply isn’t correct.” The research is published in the December 6, 2002, issue of Science.

Many past studies of global-change impacts on plants and ecosystems have focused on responses to increases in atmospheric CO2. But realistically, global changes are much more than just elevated CO2. They include global warming, altered rainfall, and increases in biologically available nitrogen compounds produced during fossil-fuel combustion. These other global changes can have major impacts on plants and ecosystems. A new study by scientists at the Carnegie Institution of Washington, the Nature Conservancy, and Stanford University shows, for the first time, how these other global changes alter the response of a natural ecosystem to increased atmospheric CO2. According to lead author Rebecca Shaw, “In the third year of the experiment, plant growth increased in the plots treated with CO2 alone, as in many other experiments. It also increased in plots exposed to the other global changes–warming, increased precipitation, and fertilizing with nitrogen –alone or in combination. But, when we added carbon dioxide, the effect of the other treatments was suppressed. The elevated CO2 in this situation pushed the response back toward the initial conditions.”

Over the last hundred years, the concentration of CO2 in the atmosphere has increased by more than 30%. The planet has warmed by about 1 ºF. Rainfall has increased in some regions and decreased in others. And human actions have more than doubled inputs of biologically available nitrogen. Elevated atmospheric CO2 increases plant growth in many experiments, but most past experiments studied impacts of CO2 alone or in combination with one other factor. The results of the Carnegie-led experiment reveal new dimensions of ecosystem responses to global change. In the California grassland studied by this team, elevated CO2 suppresses plant growth in many treatments, especially treatments where growth at normal CO2 is fastest. Field noted, “When we look at impacts of realistic global changes on whole ecosystems, we see a broad range of responses. We do not yet know whether responses will be similar in other ecosystems, but our wide range of treatments helps open the door to understanding global-change impacts on ecosystems not yet studied.”

This research was conducted over a three-year period at Stanford University’s Jasper Ridge Biological Preserve. The small stature and short life span of the plants in this California grassland ecosystem make it a model system –one that is relatively simple to study, but with results that can be used to help interpret global-change responses of all the world’s land ecosystems.

Carnegie’s new Department of Global Ecology –launched July 1, 2002, on the campus of Stanford University — grew from a century of ecological research at Carnegie’s Department of Plant Biology, also at Stanford. Using the latest technology–from satellite imagery to the tools of molecular biology–Carnegie scientists have been analyzing the complicated interactions of Earth’s land, atmosphere, and oceans. Building from biological details at the level of biochemistry and physiology, they link data and concepts from the microscopic to the global scales. The interdisciplinary Carnegie team views the planet through a biological lens to probe the function, assess the fragility, and explore the integration of the world’s ecosystems. They tackle issues such as the global carbon cycle, the role of land and oceanic ecosystems in regulating climate, the interaction of biological diversity with ecosystem function, and much more. According to Field, “We know too much about the influence of mechanisms that span biology, geology, and atmospheric sciences to stick with traditional disciplinary approaches for global studies. We need to establish a new, interdisciplinary scientific field–global ecology.”

The Carnegie Institution of Washington (http://www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments in the U.S.: Embryology, Geophysical Laboratory, Terrestrial Magnetism, The Observatories, Plant Biology, and Global Ecology.