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12/06/93

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Mauna Loa volcano: the 'white rat' of ecosystems studies

STANFORD -- One of the biggest problems that an ecologist faces is teasing apart the interconnectedness of natural systems.

Ten years ago, Stanford biologist Peter Vitousek began studies in a place that could have been designed specifically for this purpose, a place where basic factors like temperature, moisture, and the number of years that organisms have grown in the soil can be varied, one factor at a time. Vitousek can study plant growth and decomposition by varying rainfall, for example, and keeping soil age and temperature constant.

Just as medical researchers use laboratory rats to supply data useful for human medicine, Vitousek and other scientists use this model ecosystem for basic studies of complex environmental questions - for example, to show how ecosystems in warm or cold climates contribute to the carbon dioxide balance of the atmosphere, and thus to the feedback systems that may drive global warming. But instead of working in a lab or a greenhouse, they have a laboratory built by nature - an entire mountain that is incidentally the most voluminous volcano on earth.

Mauna Loa, the 13,675-foot volcano on the island of Hawaii, will be intensively studied by earth scientists over the next 10 years as one of several "Decade Volcano Demonstration Projects." On Dec. 7, Vitousek reported to a session of the American Geophysical Union on a decade already spent on the slopes of Mauna Loa studying soil nutrients, climate, and the balance of plant growth and decomposition that adds or extracts carbon and nitrogen from the atmosphere.

"Mauna Loa is a wonderful study site because you can isolate on one of several simple factors at a time in a way that you just can't do anywhere else," Vitousek said. For example, the entire mountain is built up of lava flows built upon lava flows: The slopes are smooth and gentle, with rocks of nearly the same chemical composition but different ages. Thanks to careful U.S. Geological Survey mapping, Vitousek said, he can compare plant growth on a 3000-year-old lava flow on the wet windward side of the mountain, where rainfall averages 200 inches a year, with a 3000-year-old flow on the dry side, where plants get 20 inches of moisture a year.

"Then, there can be no cross section more constant for studying temperature than taking a lava flow and following it down a mountain to the sea," Vitousek said. Temperatures range from alpine to tropical, depending on elevation, but the seasonal variation at any given altitude is only about 5 degrees Fahrenheit all year round.

Hawaii's isolation in the Pacific Ocean is also beneficial for his studies. Relatively few dominant species have arrived to colonize the slopes of Mauna Loa, so similar plants can be studied at very different locations.

Vitousek studies plant growth and soil composition at different sites around the mountain to look for answers to two general questions. He is looking for the factors that control the balance between plant production and decomposition, and thus the balance between carbon gain to the atmosphere (as plants decay) and carbon loss to the atmosphere (as plants grow). And he is interested in general principles that explain how soil nutrients limit plant productivity, depending on climate and the age of the soil.

As one example, Vitousek said his research team has shown that as they travel down in altitude and up in temperature on Mauna Loa's slopes, plant production increases in a linear fashion, increasing by a constant amount with every constant change in elevation. Decomposition increases exponentially, doubling after 1,000 meters, quadrupling after 2,000 meters or so. This suggests that in a warming tropical climate, plants will decay and release nutrients to the soil and the air at a faster rate than new plants take up the nutrients and grow.

One of Vitousek's graduate students, Alan Townsend, devised a model to calculate what this balance might mean during a period of global warming. He calculated that warmed-up tropical forests would decay more rapidly than plants could grow. They would produce extra carbon dioxide, which would act in a feedback loop to contribute to more warming. But in arctic tundra, a slight global warming would speed up plant production that had been inhibited by chilly temperatures. This would remove some CO2 from the atmosphere, and partly reverse the warming trend.

"The world is much more complicated than Mauna Loa," Vitousek said. "By studying Mauna Loa, we get data about a set of major processes and how they vary. Then you have to start thinking about real- world complications on continents."

He said that understanding a simple ecosystem is like understanding how organs work in a laboratory animal like a white rat. "You can apply that understanding in more complex systems - not every detail of it, because the complex systems include important new phenomena. But your path to understanding more complex systems is smoothed by your understanding of the simple ones."

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