June 23, 2003
Plant diversity threatened by climate change
and buildup of greenhouse gas, study reveals
By Mark Shwartz
Stanford News Service
Doubling the amount of carbon dioxide in the air significantly reduces
the number of plant species that grow in the wild, according to a newly
released study on climate change in California.
"One take-home message of our study is that certain kinds
of species are much more sensitive to climate and atmospheric
changes than others."
--Erika S. Zavaleta, joining
the faculty this summer as an assistant professor of environmental
The results, published in the Proceedings of the National Academy
of Sciences (PNAS), are the latest findings from the Jasper Ridge
Global Change Project at Stanford University--a multiyear experiment
designed to demonstrate how grassland ecosystems will respond to predicted
increases in temperature and precipitation caused by the continual buildup
of CO2 and other greenhouse gases in the atmosphere.
Writing in the June 16 edition of PNAS Online, researchers found
that exposing open grasslands to large doses of CO2 gas for three years
caused a nearly 20 percent reduction in wildflower species and an 8
percent decline in plant diversity overall. The addition of excess nitrogen
and other predicted climate changes caused diversity to plunge even
further, the study found.
"I was surprised how quickly we lost species over such a short
time," said the study's lead author, Erika S. Zavaleta, a UC Berkeley
postdoctoral researcher and a former Stanford doctoral student who is
joining the UC Santa Cruz faculty this summer as an assistant professor
of environmental studies. "It only took three years in our experiment.
What does that say about the impact global change will have on plant
diversity in the longer term?"
Located in the grassy foothills of Stanford's Jasper Ridge Biological
Preserve, the Global Change Project relies on a system of infrared heat
lamps, sprinklers, and emitters to simulate four conditions that climate
experts predict could exist a century from now as a result of continued
fossil fuel consumption and deforestation:
A temperature increase of 2 degrees F;
A 50-percent rise in precipitation;
Double the amount of CO2 in the air;
Higher concentrations of nitrogen pollutants in the soil.
To study the environmental impact of such future global changes, researchers
monitored 36 circular plots of land, each about six feet in diameter,
between 1998 and 2001. Four circles were left undisturbed as experimental
Each of the remaining 32 circles was divided into four quadrants--like
a birthday cake cut into equal pieces--for a total of 128 experimental
Different treatments were applied to different plots. Some were given
a single application, such as excess carbon dioxide gas, while others
received various combinations of elevated CO2, heat, water, and/or nitrogen
Initially, each plot contained between five and 20 varieties of grasses
and wildflowers. The goal of the experiment was to see how different
combinations of treatments would affect species diversity over a three-year
The results were dramatic. Plots that received all four treatments
lost more than one-fourth of their wildflower species, while those given
elevated nitrogen or CO2 suffered a 10 to 20 percent decline. However,
plots treated with excess water experienced a 10 percent increase in
wildflower diversity and a 3 percent gain in the number of annual grass
"We found that elevated CO2 caused a loss in species, while added
precipitation caused an increase. We were surprised they had such opposite
effects," said study coauthor Christopher B. Field, a professor
by courtesy of biological sciences at Stanford and director of the Carnegie
Institution's Stanford-based Department of Global Ecology. "One
hypothesis is that elevated CO2 added moisture to the soil, which tended
to extend the growing season of the dominant plants, leaving less room
for other species to grow."
On the other hand, Field noted, increasing precipitation by 50 percent
may have encouraged growth in late-season plants that normally stop
growing during the dry California summer: "We think the effects
of elevated CO2 and increased precipitation were more or less the same,
but because they were separated in time by a couple of weeks, they actually
produced opposite results. In our ecosystem here, things that happen
at different times in the season are really important."
The study also revealed that heat, in the absence of other treatments,
had no significant impact on diversity. However, when experimental plots
were exposed to higher temperatures along with excess nitrogen, carbon
dioxide, and water, the number of wildflower species plummeted.
"One take-home message of our study is that certain kinds of species
are much more sensitive to climate and atmospheric changes than others,"
"It turned out that wildflowers were much more sensitive to the
treatments than grasses were, no matter what combination of treatments
we tried," she added, noting that a large-scale change in diversity
could diminish the ability of grasslands to support birds, deer, butterflies,
and other wildlife--as well as commercial grazing.
The researchers discovered that they could make remarkably accurate
predictions of species diversity in plots where multiple treatments
had been applied simply by adding up losses and gains observed under
For example, in quadrants receiving excess nitrogen, heat, and CO2,
wildflower diversity decreased by about 27 percent--almost exactly what
would be expected if you added up the percentages of loss in quadrants
given single treatments of CO2 (18 percent), nitrogen (8 percent), and
heat (2 percent).
"One possible reason we see this overall additive response is
that the mechanisms that are driving the changes are not interacting,"
Field said. This finding could prove beneficial in forecasting how global
environmental changes will affect plant diversity in other ecosystems.
"We hope to move into the domain where we can predict responses
rather than just record them and report them," he added.
Other coauthors of the PNAS study are Harold A. Mooney, the Paul S.
Achilles Professor of Environmental Biology at Stanford; Nona R. Chiariello,
research coordinator of the Jasper Ridge Biological Preserve (JRBP);
and M. Rebecca Shaw of the Nature Conservancy.
The study was supported by the National Science Foundation; the David
and Lucile Packard Foundation; the Morgan Family Foundation; JRBP; the
Carnegie Institution of Washington; the U.S. Department of Energy; the
U.S. Environmental Protection Agency; the Switzer Foundation; the A.
W. Mellon Foundation; and the Nature Conservancy.
Return to Front Page