Grasslands to Replace Most U.S. Forests?

National Geographic News: NATIONALGEOGRAPHIC.COM/NEWS
August 6, 2008

Will Grasslands Overtake U.S. Forests Due to Warming?
William Cocke for National Geographic News

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.

Wildfires

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.

© 1996-2008 National Geographic Society. All rights reserved.

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