Plant Biodiversity Boosts Ecosystems' Capacity To Absorb Carbon Dioxide

Embargoed by Nature until 2 p.m. EDT Wednesday, April 11

Contacts:

Peter Reich, University of Minnesota department of forest resources, (612) 624-4270, [email protected]

Deane Morrison, University News Service, (612) 624-2346, [email protected]

PROTECTING PLANT BIODIVERSITY WILL HELP SAFEGUARD ECOSYSTEMS' CAPACITY TO RESPOND TO ELEVATED CARBON DIOXIDE

MINNEAPOLIS / ST. PAUL--Human activity is shaping ecosystems to contain fewer species of plants at a time when levels of atmospheric carbon dioxide and nitrogen pollution are on the rise. It has been debated whether plants will slow the accumulation of atmospheric carbon dioxide by absorbing more carbon dioxide and storing it as plant tissue. A study led by the University of Minnesota has found that prairie plots with greater plant biodiversity respond to augmented carbon dioxide and nitrogen more vigorously than plots with fewer plant species. If the findings hold for ecosystems worldwide, human simplification of ecosystems will hamper ecosystems' ability to remove carbon dioxide and nitrogen from circulation and perform other services. The work will be published in the April 12 issue of Nature.

While rises in atmospheric carbon dioxide are well known, nitrogen is being deposited in increasing amounts from fertilizers and nitrogen-containing gases emitted by industrial burning. In addition to the effects of agriculture and industry, humans are simplifying ecosystems by replacing native forests and grasslands with monocultures of crops and trees, lawn grass and other landscapes in which the number of species is lower than in the natural ecosystems they replace.

"This is the first study to examine how three elements of global change--plant biodiversity and increases in carbon dioxide and nitrogen--interact to affect biomass, carbon and nitrogen cycling and species composition," said lead investigator Peter Reich, a plant physiologist and ecologist in the department of forest resources at the University of Minnesota. "When levels of carbon dioxide and nitrogen were elevated to those we will see later this century, plots with greater numbers of plant species made better use of these nutrients than plots with few species."

The experiment, called BioCON (biodiversity, carbon dioxide and nitrogen) was conducted at the university's Cedar Creek Natural History Area in Minnesota. The experiment uses a unique technology to grow plants under elevated carbon dioxide concentrations in a natural field environment, without chambers or greenhouses. Carbon dioxide levels are raised by controlled releases of carbon dioxide-enriched air over the open-air plots. In 1997, 296 plots of land, each four square meters in area, were planted with one, four, nine or 16 species of prairie plants chosen at random from among 16 species.

Beginning in 1998, half the plots were grown under a 50 percent augmentation of carbon dioxide, the other half in ambient air. Half the plots in each carbon dioxide level also received additional nitrogen, representing the rates of nitrogen deposition from atmospheric emission in industrialized regions.

Plant biomass in each plot was measured twice in 1998 and twice in 1999. Plant biomass is almost half carbon, and thus is a direct indication of carbon accumulated via photosynthesis. When their biomass was compared to controls, plots with one species increased biomass seven percent in response to carbon dioxide elevation while plots with 16 species increased their biomass by 22 percent. When both carbon dioxide and nitrogen were elevated, the increases were 17 percent for one-species plots and 36 percent for plots with 16 species.

The relative inability of species-poor plots to convert carbon dioxide and nitrogen to biomass is of concern, said Reich. For one thing, about one-third of the extra carbon dioxide pumped into the atmosphere by fossil fuel use is absorbed by oceans, terrestrial plants and soil. If a decline in plant diversity lessens the ability of the terrestrial biosphere to act as a "sponge" for excess carbon dioxide, atmospheric carbon dioxide levels could rise faster and climate could change faster. Also, ecosystems with lower diversity would be able to perform less of the services humans depend on them to do.

"We need plants for fiber, food and services such as water purification and soil stabilization," said Reich. "To the extent biodiversity is decreased, we may negatively affect these functions."

Several reasons may explain why species-rich plots have more capacity to respond to increasing carbon dioxide and nitrogen, Reich said. First, diverse plots are more likely to contain species that respond well to an increased supply of these nutrients. Second, diverse mixes are likely to contain species with a range of "lifestyles," or ways to utilize the nutrients. For example, if plant species vary in root depth or in the time of year they grow most readily, nutrients could be shared and used more efficiently. Third, a diverse array of plants is likely to contain species that can facilitate each other's functioning, such as plants that can convert atmospheric nitrogen into soil nitrogen in forms useful for other plants, and plants that require large amounts of soil nitrogen.

"We need to consider the disadvantages of decreasing diversity as we manage our landscapes," said Reich. "We need to better understand how diversity affects responses to global changes, such as rising carbon dioxide levels, so that we can manage ecosystems to lessen potential negative impacts."

Working with Reich were colleagues from the University of California, Berkeley, Brookhaven National Laboratory and the University of Nebraska, Lincoln, as well as the University of Minnesota. The study was funded primarily by the U.S. Department of Energy, with additional support from the National Science Foundation.