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Deserts Threatened by Climate Change

2 November 2000 7:00 pm
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Something in the air. Excess CO2 gives exotic species a boost in wet years.

Desert plants go wild during wet years when treated to excess carbon dioxide, researchers say. The finding backs up climate change models, which predict that rising levels of atmospheric CO2 will disrupt the ecology of sensitive desert ecosystems. Experts fear that the change will favor invasive plants given to triggering wildfires.

Plentiful CO2 helps plants use water more efficiently, and atmospheric levels of CO2 are expected to double, relative to preindustrial times, by 2050. Global-warming researchers predict that such massive increases will eventually transform desert ecosystems. The boom in plant growth is expected to upset delicately balanced desert ecosystems--changing the nutrient cycle, fire cycle, and distribution of water.

A research team led by ecologist Stanley Smith of the University of Nevada, Las Vegas, decided to check. They enriched levels of CO2 above three 23-meter-diameter plots by about 50%. Six similar plots served as controls. During the lush 1998 El Niño growing season and the subsequent drought of 1999, the researchers measured plant size, density, and seed production on the plots. Carbon dioxide didn't change plant growth much during the drought, but it did during the wet year. All plants in the CO2 plots grew bigger and produced more seeds than those in control plots. However, some plants took more advantage of the situation than others; invasive species such as cheatgrass doubled their density in wet, high-CO2 conditions compared to native species such as fescue and peppergrass, the researchers report in the 2 November issue of Nature.

Long-term elevated CO2 levels could give exotic species a boost, Smith says, and extra plant matter means more fuel for fires. The effect could magnify over time, he warns, as exotic species recover faster after a blaze, building up fuel quickly for the next fire.

"What's neat about this study," says ecologist James Reynolds of Duke University, is that it tests models of how CO2 acts on arid lands with real data. The minimal effect of CO2 during dry years, he says, was a surprise, and more data will help refine models of what climate change will do to different ecosystems.

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The research was conducted at the Nevada Desert FACE Facility

Similar studies are being conducted in Minnesota

and North Carolina among others.