In the News
“ Some object got captured into orbit, made two close passages. Survived the first, not totally damaged — then, 50 years later, it
came back in and that was the end of it. ” — Mark Showalter, page 11
genes & Cells Body clocks and disease
Longevity clues in centenarian’s genes
atom & Cosmos Saturn’s skewed ripples
Challenging faster-than-light neutrinos
earth Climate change and the New World
humans Dual migration into Asia
Body & Brain Finding the cause of tinnitus
By alexandra witze
Like Coke versus Pepsi, tropical and ecosystems come in two choices: forest or grassland. New research shows these two
options can switch abruptly, and there’s
rarely any in-between.
As such, many of these ecosystems are
particularly vulnerable to future changes
such as rising temperature, scientists
say. With just slight shifts in rainfall or
other factors, people living in what is
now tropical rainforest might suddenly
find themselves in scrubland populated
by a different mix of plants and animals — where people’s livelihoods might
have to change dramatically.
“That transition is not going to hap-
pen smoothly,” says Milena Holmgren,
an ecologist at Wageningen University in
the Netherlands. “The evidence is show-
ing there are these big jumps.”
Holmgren and her colleagues describe
the finding in the Oct. 14 Science . Another
group, from Princeton University and
South Africa’s national research council,
reports similar conclusions in a second
paper in the same journal.
In theory, the relationship between
rainfall and tree cover should be
straightforward: The more rain a place
has, the more trees that will grow there.
rain tips balance
between forest
and savanna
Amount of tree cover can
shift suddenly and abruptly
STORY ONE
Small changes in factors such as fire and rainfall can determine whether a
landscape will be covered by a dense rainforest (left) or open savanna (right).
But small studies have suggested that
changes can occur in discrete steps. Add
more rain to a grassy savanna, and it
stays a savanna with the same percentage of tree cover for quite some time.
Then, at some crucial amount of extra
rainfall, the savanna suddenly switches
to a full-fledged forest.
But no one knew whether such rapid
transformations happened on a global
scale. Separately, both research groups
decided to look at data gathered by the
MODIS instruments on board NASA’s
Terra and Aqua satellites, which sense
vegetation cover and other features
of the land surface. This information
included how much of each square
kilometer of land was covered by trees,
grasses or other vegetation. Both teams
focused on the tropics and subtropics
of Africa, South America and Australia,
because those areas are thought to be
least disturbed by human activity.
Looking at the numbers, Holmgren’s
group identified three distinct eco-
system types: forest, savanna and a
treeless state. Forests typically had
80 percent tree cover, while savannas had
20 percent trees and the “treeless” about
5 percent or less. Intermediate states —
with, say, 60 percent tree cover — were
extremely rare, Holmgren says. Which
category a particular landscape fell into
depended heavily on rainfall.
www.sciencenews.org
November 5, 2011 | SCIENCE NEWS | 5
