Biodiversity Gives Carbon Sinks a Boost

By Cat Lazaroff

UPTON, New York, April 13, 2001 (ENS) - The more diverse an ecosystem, the better it can serve to absorb carbon dioxide - a potent greenhouse gas - from the atmosphere, a new study suggests. The research has important implications for ongoing international negotiations over the best way to address global climate change, and the role that so called carbon sinks should play.

Biodiversity is an important factor regulating how ecosystems will respond to increasing atmospheric carbon dioxide, say researchers from the U.S. Department of Energy's Brookhaven National Laboratory and their collaborators. The team of investigators, led by Peter Reich of the University of Minnesota, just released results from a major field study that appears in the April 12 issue of the journal "Nature."

FACE ring

One of the test plots planted for the biodiversity study, known as a free air CO2 (FACE) ring (Two photos courtesy Brookhaven National Laboratory)
All plants absorb carbon dioxide as they grow, but different species absorb carbon at different rates - and different environmental conditions can also affect how well plants absorb carbon.

The scientists found that more diverse plant ecosystems were better able to absorb carbon dioxide (CO2) and nitrogen, both of which are on the rise due to human activities and industrial processes.

"The key implication of this research is that, in response to elevated levels of CO2 and nitrogen, ecosystems with high biodiversity will take up and sequester more carbon and nitrogen than do ecosystems with reduced biodiversity," said Brookhaven plant physiologist David Ellsworth, one of the study authors.

The findings could influence international efforts to control emissions of greenhouse gases like carbon dioxide. Industrialized countries, particularly the United States, support the use of carbon sinks - vegetation that absorbs carbon dioxide - as a mechanism for reducing the amount of CO2 added to the atmosphere.


Plant physiologist David Ellsworth takes measurements in a FACE ring
Critics of carbon sinks argue that placing limits on industrial sources of CO2 is the best way to reduce carbon emissions, and emphasizing carbon sinks could take attention away from the need to cut emissions.

In addition, some fear that nations will advocate replacing old growth forests - which have largely completed the part of their life span in which they absorb the most CO2 - with tree plantations or other fast growing crops. Incentives to create carbon sinks could prove devastating to efforts to preserve the planet's remaining pristine ecosystems.

The study released this week lends credence to arguments that intact ecosystems do a better job of regulating environmental problems than do manmade landscapes.

The experiment, called BioCON - for Biodiversity, CO2 and nitrogen - is the first field study to test the idea that plant species diversity influences the responses of natural ecosystems to elevated CO2 and nitrogen levels. The project was performed in a scientifically controlled grassland environment at the Cedar Creek Natural History area of the University of Minnesota, using free air CO2 enrichment technology.

This experimental free air CO2 (FACE) technology was developed by Brookhaven National Laboratory to study the effects of extra CO2 on plants in their natural environment, rather than in greenhouses or other enclosures.


Tree plantations, like these papaya trees, replace diverse natural ecosystems with a single tree species (Photo courtesy Dennis Gonsalves, Department of Plant Pathology, Cornell University)
Each FACE facility consists of six 20 meter diameter experimental plots, each encircled by a ring of five foot tall vertical pipes capable of releasing different amounts of CO2. Computers monitor the wind speed, wind direction and CO2 level within each ring, and adjust the release of CO2 to achieve the atmospheric concentration that is expected to occur 50 years from now.

In the BioCON study, the six rings were each subdivided into experimental plots measuring two by two meters. In 1997, these subplots were planted with different numbers and varieties of grassland plant species, creating a range of species diversities.

The experimental plots within three of the rings received no additional CO2, while the other three rings were bathed in CO2 that was about 50 percent above current concentrations. Beginning in 1998, half the plots received additional nitrogen, comparable to the high rates of nitrogen that are deposited in industrialized regions.

At the end of both the 1998 and 1999 growing seasons, the scientists measured the total amount of plant matter, known as biomass, per square meter in each plot. Biomass is an indicator of the amount of carbon the plants accumulate through photosynthesis, the process by which green plants use CO2, water and sunlight to grow.

Nitrogen, an important plant nutrient, is absorbed from the soil to become part of the biomass.

The researchers learned that elevated levels of CO2 and nitrogen resulted in increased biomass when compared with plots exposed to ambient levels of CO2 and nitrogen. The effect, however, was greatest in plots with high biodiversity as compared to those with fewer species.


When old growth forests, like this one in Mt. Hood National Forest, Oregon, are clearcut, timber companies often replant the areas with a single tree species (Photo courtesy American Lands)
"These findings suggest that protecting biodiversity worldwide will contribute to safeguarding the capacity of ecosystems to capture a larger fraction of additional carbon and nitrogen entering our environment due to industrial processes," said Brookhaven ecologist George Hendrey, who led development of the FACE system and is another coauthor on the current study.

One reason for the greater uptake of CO2 and nitrogen in biodiverse plots may be that the different plant species bloom and go to seed at different times. The plot is therefore absorbing CO2 and nitrogen over the entire growing season, rather than just part of it.

Other interactions among the plant species may also be contributing to the effectiveness of the biodiverse plots as carbon sinks, the scientists said.