Smithsonian Environmental Research Center

A new study published in this week's issue of the journal, Science, suggests that increased levels of carbon dioxide (CO₂) in the atmosphere can actually depress growth in certain plants-which means different systems may not cope as well as previously thought when atmospheric CO₂ rises.

Usually, plants grow more when given extra CO₂, and the white milkpea in Florida's oak woodland where the study took place was no exception, at least initially. "Early on, carbon dioxide enrichment doubled the growth of white milkpea" said Dr. Bruce Hungate , Associate Professor of Biological Sciences at Northern Arizona University and the report's lead author. "But for the past four years, high CO₂ has actually depressed the growth of this plant and its rate of nitrogen fixation."

The white milkpea is a wild legume, a member of the bean family, and important for its ability to fix nitrogen. Like all nitrogen fixers, it uses the energy captured in photosynthesis to convert nitrogen gas in the atmosphere into a form that it and other plants can use. "This is important because in the long run, if there is a break put on the nitrogen fixing abilities of certain plants, it could mean these plants will stop responding to high CO₂," said co-author Bert Drake a plant physiologist at the Smithsonian Environmental Research Center (SERC).

Drake and his colleagues have been monitoring how increasing carbon dioxide in the atmosphere affects the natural vegetation in a salt marsh in Maryland for 17 years and at SERC's study site in Cape Canaveral, Fla., since May 1996.

"By and large our studies show that over the long term, plants seem to respond to high CO₂ by continuing to grow," said Drake. This could mean that as atmospheric CO₂ continues to rise, the increased response of plants could help buffer the impacts on ecosystems. But the new study indicates that growth of at least some plants is actually inhibited under high CO₂ conditions. More important, if the nitrogen fixers such as the white milkpea fail to fix nitrogen, it could result in less available nitrogen for other plants, and their growth would suffer as well.

Why the flip-flop in response from the milkpea? The element molybdenum is the likely culprit. In order to fix nitrogen, plants need other nutrients. "Molybdenum is an essential part of the nitrogen fixing enzyme, nitrogenase, and it looks like the high CO₂ treatment is causing the plants to run out of this key element" said Hungate. Leaves of the white milkpea contained far less molybdenum in the high CO₂ treatment compared to the control, and plants that had less molybdenum fixed less nitrogen.

However, Drake cautions that this is merely one new piece of information in a very complex issue. "There's a great distance between the fact that one plant has stopped fixing nitrogen in this experiment and the inference that you may draw from it that this whole ecosystem will stop responding to CO₂." It is a very clear indicator, however, that different plants, and perhaps different ecosystems could respond differently as human activity continues to increase the concentration of CO2 in the atmosphere. Huntgate hopes to secure more funding to continue studying these processes.

Read more about Bert Drake's CO₂ studies at SERC.