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Stanford grassland project seeks answers to the riddle of global climate change

At first glance, the small, grassy field above Stanford's main campus seems an unlikely place to make predictions about the global environment.

But this unassuming meadow in the Jasper Ridge Biological Preserve is now the site of a dynamic experiment designed to forecast the effects of rising temperatures and other environmental changes on grassland ecosystems throughout the planet.

Experimental Plot 13 receives twice the normal amount of atmospheric carbon dioxide gas 24 hours a day, but no additional heat. It is divided into four quadrants, each receiving different treatments that produced dramatically different results. The upper-left quadrant (1) put forth a showy display of blue wildflowers after being given extra nitrogen, while the upper-right quadrant (2), which is exposed to extra CO2 and water, produced a mix of flowers and grass. The lower-right quadrant (3) – treated with both nitrogen and 50 percent more water – grew tall grass but no wildflowers. Meanwhile, the lower-left section (4) – fed only elevated CO2– ended up with a sparse collection of short plants.

The Jasper Ridge Global Change Project is a multi-year study focusing on what scientists believe will be four of the most significant changes affecting Earth in the next century: higher levels of carbon dioxide (CO2) in the atmosphere, warmer temperatures, increased use of nitrogen fertilizers and changes in precipitation.

The goal of the project is to document the effects of all four factors -- alone and in combination -- on the plants, microbes, animals and soil that make up the grassland ecosystem.

"Global change is going to be caused by more than just increased carbon dioxide and higher temperatures," says Harold A. Mooney, the Paul S. Achilles Professor of Environmental Biology, who helped design the project.

Mooney, a leading authority on global change, points out that farmers around the world are using ever-increasing amounts of water and nitrogen-based fertilizers to enhance crop production. These factors, he says, should be included in any long-range projection of changes in the biosphere -- the fragile envelope of air, water and soil that allows life to exist on Earth.

Mooney and biologist Christopher B. Field of the Carnegie Institution of Washington are leading the study, which is located in a fenced-off section of grassland inside Stanford's 1,189-acre Jasper Ridge Biological Preserve.

So far, the results have been dramatic.

Some plots of experimental soil have produced an explosion of wildflowers, while others are filled with tall, thick grass. And still others are virtually bare. It all depends on how much carbon dioxide, heat, nitrogen and water are added to the environment -- and in what combination.

"This is the only experiment of its kind in the world," says Philippe S. Cohen, administrative director of the Jasper Ridge Biological Preserve.

"The project promises to make important contributions to our understanding of how ecosystems respond to human-induced global changes," Cohen adds.

Experimental circles

In 1997, project scientists plotted 36 circles in the ground, each about 6 feet in diameter. A large rectangular fence was placed around the perimeter of the circles to prevent deer from prematurely consuming the results of the experiment.

All the circles were equipped with watering systems, carbon dioxide gas emitters and infrared heaters located about 3 feet off the ground.

"The idea is to simulate the kinds of changes predicted over the next 100 years," says Cohen -- a doubling of current levels of atmospheric carbon dioxide, accompanied by an average temperature increase of 2 to 6 degrees Fahrenheit.

A computer makes sure that carbon dioxide levels remain consistent by adjusting the release of CO2 gas to changing wind speeds and direction.

To measure the effect of the experimental equipment itself, four of the circular plots receive no additional water, nitrogen, carbon dioxide or heat.

Each of the remaining 32 circles is divided into four equal quadrants separated by underground fiberglass partitions to prevent roots in one section from invading a neighboring tract.

The result is 32 plots, each containing four different experiments using every possible combination of carbon dioxide, heat, water and nitrogen.

Plot 13, for example, receives twice the normal amount of atmospheric carbon dioxide gas 24 hours a day, but no additional heat.

By mid-spring, Plot 13's upper-left quadrant, which receives extra nitrogen, put forth a showy display of blue wildflowers.

But just inches away, in the lower-right quadrant of the same circle, the vegetation looks totally different. This section is treated with both nitrogen and 50 percent more water. The result: a thick cover of tall grass, but no wildflowers.

Meanwhile, the upper-right quadrant, which is exposed to extra carbon dioxide and water, contains a mix of flowers and grass, while the lower-left section -- fed only elevated CO2 -- contains a sparse collection of short plants.

What could cause such extreme differences in plant growth in such a tiny patch of earth?

"The researchers can't answer that question yet," says Cohen, pointing out that this is only the second year of a three-year experiment.

"We do know that, originally, all four quadrants looked alike," he adds.

Miniature ecosystems

According to Field, each test plot can be viewed as a true ecosystem with several thousand individual plants, mostly grasses and wildflowers.

"Overall," Field says, "these ecosystems appear much more sensitive to elevated levels of water and nitrogen than to excess carbon dioxide and warming alone. Therefore, we expect the biggest effects of global warming to be indirect."

For example, he points out that warmth indirectly affects the availability of water and nitrogen, which in turn affects a plant's growth rate.

Graduate student Erika Zavaleta also found that shrub seedlings actually did better in some experimental plots with elevated carbon dioxide levels, suggesting that CO2 could play a role in transforming grassland to shrubland.

Plants use carbon dioxide to produce carbohydrates, but Field says preliminary results show that increasing CO2 in the atmosphere does not always mean a big increase in the amount of carbon in the plant itself.

"The big unknown in the global carbon cycle is below ground," notes Rebecca Shaw, a postdoctoral researcher whose work on the project is supported by the Department of Energy.

"How are the roots responding to environmental changes?" asks Shaw.

Specifically, what is happening to the rich mix of roots, soil, microbes and animals that we rarely see?

To find out, Brazilian scientists Aristotelino Ferreira and Maria Macedo have been taking thousands of photographs of the root systems in every experimental plot using a video camera lowered into tubes buried in the soil.

The pictures will help scientists measure root growth and determine if extra carbon from the air is in fact being stored underground.

Researchers are also using chemical analysis to trace the movement of carbon and nitrogen through the plants, microbes and soil.

Unlike similar studies involving forests and other large ecosystems, the Jasper Ridge Global Change Project has the advantage of monitoring several generations of plants in a short time period.

"The annual grassland goes from seed to seed every six to nine months," says Mooney, "so you can observe an entire cycle of vegetation."

The project is underwritten by the National Science Foundation, and researchers have asked the NSF to continue funding the experiment an additional four years.


By Mark Shwartz
