Climate change surprise:
High carbon dioxide levels can retard plant growth
The prevailing view among scientists is that global climate change may prove beneficial to many farmers and foresters -- at least in the short term. The logic is straightforward: Plants need atmospheric carbon dioxide to produce food, and by emitting more CO2 into the air, our cars and factories create new sources of plant nutrition that will cause some crops and trees to grow bigger and faster.
But an unprecedented three-year experiment conducted at Stanford University is raising questions about that long-held assumption. Writing in the journal Science, researchers concluded that elevated atmospheric CO2 actually reduces plant growth when combined with other likely consequences of climate change -- namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil.
The results of the study may prompt researchers and policymakers to re-think one of the standard arguments against taking action to prevent global warming: that natural ecosystems will minimize the problem of fossil fuel emissions by transferring large amounts of carbon in the atmosphere to plants and soils.
"Perhaps we won't get as much help with the carbon problem as we thought we could, and we will need to put more emphasis on both managing vegetation and reducing emissions," said Harold A. Mooney, the Paul S. Achilles Professor of Environmental Biology at Stanford and co-author of the Dec. 6 Science study.
He noted that the Stanford study is the first ecosystem-scale experiment to apply four climate change factors across several generations of plants.
"To understand complex ecological systems, the traditional approach of isolating one factor and looking at that response, then extrapolating to the whole system, is often not correct," Mooney said. "On an ecosystem scale, many interacting factors may be involved."
The findings published in Science are among the first results of the Jasper Ridge Global Change Project -- a multi-year experiment designed to demonstrate how a typical California grassland ecosystem will respond to future global environmental changes.
Located in a fenced off section of Stanford's 1,189-acre Jasper Ridge Biological Preserve, the novel experiment was designed to simulate environmental conditions that climate experts predict may exist 100 years from now: a doubling of atmospheric CO2; a temperature rise of 2 degrees F; a 50 percent increase in precipitation; and increased nitrogen deposition -- largely a byproduct of fossil fuel burning.
Launched in 1997, the Jasper Ridge experiment was conceived by Mooney and Christopher B. Field, a professor by courtesy in Stanford's Department of Biological Sciences and director of the Carnegie Institution's Department of Global Ecology, also located on the Stanford campus.
"Most studies have looked at the effects of CO2 on plants in pots or on very simple ecosystems and concluded that plants are going to grow faster in the future," said Field, co-author of the Science study. "We got exactly the same results when we applied CO2 alone, but when we factored in realistic treatments -- warming, changes in nitrogen deposition, changes in precipitation -- growth was actually suppressed."
To mimic future climate conditions, Field, Mooney and their colleagues mapped out 36 circular plots of land, each about six feet in diameter. Four plots are virtually untouched, receiving no additional water, nitrogen, carbon dioxide or heat. Each of the remaining 32 circles is divided into four equal quadrants separated by underground partitions to prevent roots in one section from invading neighboring tracts. In these smaller quadrants, researchers study all 16 possible combinations of elevated and normal CO2, heat, water and nitrogen.
The plots thicken
The biggest surprise from the study was the discovery that elevated carbon dioxide only stimulated plant growth when nitrogen, water and temperature were kept at normal levels.
"Based on earlier single-treatment studies with elevated CO2, we initially hypothesized that, with the combination of all four treatments together, the response would be additional growth," said W. Rebecca Shaw, a researcher with the Nature Conservancy of California and lead author of the Science study.
But results from the third year of the experiment revealed a more complex scenario. While treatments involving increased temperature, nitrogen deposition or precipitation -- alone or in combination -- promoted plant growth, the addition of elevated CO2 consistently dampened those increases.
"The three-factor combination of increased temperature, precipitation and nitrogen deposition produced the largest stimulation [an 84 percent increase], but adding CO2 reduced this to 40 percent," Shaw and her colleagues wrote.
The mean net plant growth for all treatment combinations with elevated CO2 was about 4.9 tons per acre -- compared to roughly 5.5 tons per acre for all treatment combinations in which CO2 levels were kept normal.
However, when higher amounts of CO2 gas were added to plots with normal temperature, moisture and nitrogen levels, aboveground plant growth increased by nearly a third.
Why would elevated CO2 in combination with other factors have a suppressive effect on plant growth? The researchers aren't sure, but one possibility is that excess carbon in the soil is allowing microbes to outcompete plants for one or more limiting nutrients.
"By applying all four treatments, we may be repositioning the ecosystem so that another environmental factor becomes limiting to growth," Field observed. "For example, by increasing plant growth as a result of adding water or nitrogen, the ecosystem may become more sensitive to limitation by another mineral nutrient such as phosphorous, potassium or something else we hadn't been measuring."
A new five-year experiment is underway at the Jasper Ridge site to analyze potential limiting nutrients in the soil along with microbial-plant interactions and the molecular biology of the vegetation.
Field and his colleagues say that their ultimate goal is to use the results of the Jasper Ridge study to forecast what will happen to other ecosystems -- from alpine tundra to tropical rainforests.
"In the past, people have argued that perhaps we don't really need to worry about fossil fuel emissions, because increased plant growth will effectively pull elevated CO2 concentrations out of the atmosphere and keep the world at the appropriate equilibrium," he added. "But our experiment shows that we can't count on the natural world, the unmanaged world, to save us by pulling down all the atmospheric CO2."
Added Mooney: "Our study demonstrates that there is still a lot
to learn about the factors that regulate global climate change. But we also know
a lot already, more than enough to engage in a serious discussion about action
to reduce CO2 emissions from burning fossil fuels and clearing
Other coauthors of the Science study are former Stanford doctoral student Erika S. Zavaleta, now a Nature Conservancy post-doctoral fellow at the University of California-Berkeley; Nona R. Chiariello, research coordinator of Stanford's Jasper Ridge Biological Preserve; and Elsa E. Cleland, a graduate student in the Stanford Department of Biological Sciences.
The study was supported by the National Science Foundation, the Morgan Family Foundation, the David and Lucile Packard Foundation, the Jasper Ridge Biological Preserve, the Carnegie Institution of Washington, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the Switzer Foundation and the A.W. Mellon Foundation.