Grassland plants are responding in unexpected ways to increased carbon dioxide in the air, a finding that could influence whether land ecosystems will continue to absorb as much carbon dioxide from the atmosphere as they do currently, says Dr. Tali Lee, a UW-Eau Claire biology professor who is among the scientists collaborating on the long-term study.
Findings from the 20-year study suggest that in the future, a majority of the Earth’s plants might not soak up as much of the greenhouse gas as previously expected, while some grasslands might take up more, says Lee, who is part of research team that includes faculty from the University of Minnesota.
“For the first 12 years, we found what we expected regarding how different types of grasses react to carbon dioxide,” Lee says of grasses planted by researchers at a study site in Minnesota. “However, during the last eight years of the study, we found the expected pattern reversed.”
The findings were published in the April 20 issue of the prestigious journal, Science. View article online.
In the study, researchers planted 88 plots at a study site in Minnesota with two different types of grasses, warm-season C4 grasses and cool-season C3 grasses.
They exposed the grasses to current carbon dioxide levels and to the elevated levels the Earth might experience later this century given human activity.
Because carbon dioxide is needed by plants to grow, plants that use the C3 photosynthetic pathway — which means most plants, including all tree species — are expected to grow more under elevated carbon dioxide as these plants are known to increase their carbon dioxide capture as levels rise, Lee says.
Growth of plants with the C4 photosynthetic pathway, such as many prairie grasses and several crops including corn, sugar cane and sorghum, are not expected to respond to higher carbon dioxide levels because they are generally less able to capture extra carbon dioxide as levels rise, she says.
“For the first decade, our experiment followed this expectation with the prairie grasses in our study,” Lee says, explaining that the C3 grasses grew more under elevated carbon dioxide levels but C4 grasses did not. “However, during the last eight years, C4 grasses actually grew more with elevated carbon dioxide, while C3 grasses did not.”
Grasslands cover 30-40 percent of the Earth’s land and play a critical part in absorbing carbon dioxide released by burning fossil fuels, Lee says.
“The findings caution us that we should not be overly confident about our assumptions regarding where, and by how much, land ecosystems will keep absorbing extra carbon dioxide out of the air,” Lee says.
Ecosystems on land currently absorb about one-third of the carbon dioxide that we emit into the atmosphere by the burning of fossil fuels, Lee says. These systems then store much of that carbon dioxide in plant biomass or in the soil, she says.
As a result, plants are currently slowing the rate of climate change, Lee says.
Whether land ecosystems can continue providing this service is important, Lee says.
The current belief is that C3-dominated grasslands in cool temperate regions help absorb some of the fossil fuel emissions of carbon dioxide by increasing their growth, while C4 grasslands in warmer drier regions will remain neutral under higher carbon dioxide levels, Lee says.
“However, if mature grasslands behave like our experiment did, the situation may be opposite,” Lee says. “Warm climate C4 grasslands might help us more with carbon dioxide removal from the atmosphere than we thought they would and cool climate grasslands might help less.”
While Lee and her collaborators are not sure why the responses of the grasses switched after a decade, it likely has to do with how these species alter the supply of another key resource, nitrogen, Lee says, noting that plants must have sufficient N to use extra carbon dioxide.
Their study showed concurrent alterations in soil N in plots with C3 grasses, Lee says.
The supply of soil N decreased over time under elevated carbon dioxide, while in plots with C4 grasses, soil N supply increased over time under elevated carbon dioxide, she says.
“These results demonstrate how important it is to have long-term experiments under more ecologically realistic conditions to accurately inform the computer models that are used to predict how vegetation will respond to changing carbon dioxide concentrations in the atmosphere,” Lee says. “This would help inform policy and direct action.”
How ecosystems develop over time is complex, so having the opportunity to study the grasses over two decades was valuable, Lee says.
“Experiments of comparable length and complexity to our study are extremely rare and our results completely surprised us,” Lee says.
In addition to Lee’s contributions to the research, numerous UW-Eau Claire undergraduate students also have participated in the research, Lee says, noting that about 15 Blugolds have had internships connected to the study.
UW-Eau Claire’s collaboration with University of Minnesota researchers gives Lee and her students access to the Cedar Creek Ecosystem Science Reserve in Minnesota, a site that offers multiple learning and research opportunities.
The National Science Foundation supports the study through its Long Term Ecological Research Program that provides funding for projects that span many years.
“This is so desperately needed to understand the workings of ecosystems at multiple time and spatial scales,” Lee says of long-term projects. “It also has provided amazing experiences for students who are hired as interns for the summer, where they are exposed to other undergraduates from around the country as well as graduate students, postdocs and other research faculty.
“The location and infrastructure inherently immerses them in an active scientific community. For some students, this has led to opportunities beyond their college careers through the experience and networking it provides.”
Having their research published in Science is significant, she says.
“I am really thrilled with this publication ,” Lee says. “I have been a part of this experiment since its first year of operation in 1998, and it is so rewarding to have data that you have collected over so many years contribute to findings like these.”
Lee’s research has previously been published in Nature, another prestigious journal.
Photo caption: Dr. Tali Lee, a biology professor, is among the scientists collaborating on the long-term study that was published in the prestigious Science journal.