Will Grasslands Overtake U.S. Forests Due to Warming?

William Cocke
for National Geographic News
August 6, 2008

Climate change may cause grasslands to spread to parts of the United States that are currently covered in forest, a new study says.

If local climates become more extreme due to global warming, then entire ecotones—boundaries between ecosystems—could shift, the study says, highlighting the central United States, where prairie gives way to forests of the east.

"People generally expect that the climate is becoming more variable with climate change," said study author Michael Notaro of the University of Wisconsin's Center for Climatic Research.

"If the climate becomes more variable year-to-year, then potentially, you may have less vegetation, more fire, then shifts in these different boundaries," he said.


An unstable climate would prove fatal to certain types of trees and advantageous for short-lived plants, such as grasses.

The buildup of combustible plant material caused by long, wet periods followed by extended droughts may increase the size and frequency of wildfires, Notaro said.

The combination of fire and drought, coupled with extreme temperature swings, favors certain types of trees, as well as grass.

"The evergreen tree tends not to do as well with larger variability," Notaro said. "Part of the reason is, if you kill an evergreen tree, it takes a long time to grow back compared to a grass or even a deciduous tree."

Deciduous trees limit water loss by shedding their leaves, whereas evergreens, which need to keep their needles year-round, are sensitive to water loss, particularly during the winter months.

By favoring the expansion of grasses over woody plants, less consistent climate patterns over time could reduce total global vegetation cover, Notaro said.

Shifting Boundaries

In the central U.S. an ecotone marks the transition from grasslands and prairies to the west and forests to the east. The current boundary exists largely because the western climate is more extreme—varying throughout the year between hot and cold, and wet and dry.

"If there was no variability, then the whole forest in the eastern United States would shift into the central United States," Notaro said.

Over the course of decades, if global warming causes extreme weather, as expected, the opposite will occur: Grasses, which go dormant during drought and thrive after fire, would move east to exploit the habitat of trees that are unable to compete for scarce resources and ravaged by wildfires.

The ecotone transition from closed forest to open canopy is, by nature, highly variable, said Ronald P. Neilson, a scientist with the U.S. Forest Service in Corvallis, in Oregon.

An increasingly extreme climate "would tend to push the ecosystem to a lower density of overstory [forest canopy] and a more open type of a system," said Neilson, who is not involved with the new study.

Notaro used a dynamic global-vegetation model with climate data from the 20th century for his study, which appears in the journal Climate Dynamics.

By contrasting model results driven by mean, or average, climate data against an experiment driven by climate data with year-to-year fluctuations, Notaro was able to identify the impact of climate fluctuations on global vegetation patterns.

He presented his findings Tuesday at a meeting of the Ecological Society of America in Milwaukee.




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