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threshold, trees work to maintain a thick
canopy that acts as a barrier to stop fire
from spreading.
“These two papers tell us that these
feedbacks really do operate at all scales,”
says Audrey Mayer, an ecologist at
Michigan Technological University in
Houghton. “They’ll make us have to
redo some of our assumptions about how
things are going to change in the future.”
Many global climate models, for
instance, assume a smooth transition
between savanna and forest as temper-
ature and rainfall change. But the new
work suggests that forests could appear
or disappear quickly, Mayer says, espe-
cially if people complicate the picture.
“You can’t just plant a couple of trees and
they’ll grow up and the forest will come
back,” she says. “You have to fight those
internal feedbacks.”
Staver and her colleagues are search-
ing for savanna-forest transitions that
are occurring right now. “These things
are definitely happening,” she says, “and
the new work tells us it could be even
more widespread than we’d thought.”
Studying where landscapes are changing
could help the scientists better under-
stand what causes ecosystems to tip
from one category to the other.
Where the wild trees are
Savanna
bistable, savanna
bistable, forest
Forest
Easy come, easy go Certain areas (labeled here as bistable) throughout the tropics and
subtropics are poised to switch from forest to savanna, or vice versa, if conditions such as
rainfall change just a little. the livelihood of farmers or ranchers in these areas could be at risk.
For their part, the Dutch scientists
have developed “resilience maps” that
show which places are most likely to
tip from savanna to forest or vice versa.
Farmers scratching out a living in western Africa or ranchers running cattle
on the fringes of the Amazon might
use such maps to learn how viable their
livelihoods are likely to be in coming
decades.
Locals could thus spend more time and
energy working to keep the ecosystem the
way it is, perhaps by building extra capac-
ity for storing water or by cutting back
on logging. Or residents could cut down
more trees to tip a forest into a grassy
rangeland for their animals. “These maps
can be a tremendous tool for all kinds of
organizations,” Holmgren says.
A lot of what scientists know about changes on earth’s
surface comes thanks to two remote-sensing instruments called moDiS, for moderate resolution imaging
Spectroradiometer. one moDiS launched in 1999 on
nASA’s terra satellite, and a second soared into space
three years later aboard the sister Aqua satellite. by
regularly measuring a wide range of light and other
radiation reflecting off the earth’s surface, moDiS can
build up a high-resolution picture of changes in planetary phenomena such as cloud cover, deforestation
and even the smoke from forest fires in bolivia seen at
left. terra passes over the equator every morning, and
Aqua does so every afternoon. together they see the
entire earth’s surface every day or two —making them
an all-seeing, never-sleeping eye in the sky for environmental dynamics. —Alexandra Witze
From top: A.C. StAver ET AL/SCIENCE 2011; nASA viSible eArth
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