“We were expecting to find rapid extinction, a total
change in the forest.” — CArloS JArAmillo
Warm spell spurred biodiversity in
South American tropical rain forest
By Alexandra Witze
Some like it hot, including the plants living in
South America’s tropical
rain forests 56 million
years ago.
As average global
temperatures spiked
by 5 degrees Celsius
over a period of 10,000
years — a geologic blink
of an eye — plant diversity in northern South
America also soared,
researchers report in the
Nov. 12 Science.
“We were expecting to find rapid
extinction, a total change in the for-
est,” says study leader Carlos Jaramillo,
a paleobotanist at the Smithsonian
Tropical Research Institute in Balboa,
Panama. “What we found was just the
opposite — a very fast addition of many
new species, and a huge spike in the
diversity of tropical plants.”
The fossil study raises questions
about how tropical forests might
respond as atmospheric carbon diox-
ide levels rise because of fossil fuel
burning and other industrial activities.
Though today’s forests may not respond
to warming the same way ancient for-
ests did, researchers say the findings do
suggest that at least some plants are sur-
prisingly adaptable.
“This kind of work is critically impor-
tant,” says Scott Wing, a paleobotanist at
the Smithsonian Institution in Washing-
ton, D.C., who was not part of the study.
“We’re beginning to map out what hap-
pened in different places during this
huge perturbation of the carbon cycle
and climate system.”
Researchers call the warming the
The diversity of pollen found in deposits from northern
South America suggests that tropical rain forest plants
thrived there during a warm spell 56 million years ago.
Paleocene-Eocene Thermal Maximum,
because it took place at the boundary
between the Paleocene and Eocene
epochs of geologic time. It’s the closest analog scientists have to the global
warming they expect in the future,
though on a much slower scale; today,
instead of a 5 degree Celsius increase
over 10,000 years, researchers expect a
2 degree C increase over just the next
century, with more warming after that.
Only a few places on land preserve
evidence of how plants and animals
responded to the Paleocene-Eocene
heat, most in temperate or northern
latitudes. In Wyoming, Wing and other
researchers have found fossils suggesting that as things heated up, species from
more southern regions moved into the
area temporarily. But some tropical forests are already in the hottest places on
the globe, so there is no still-warmer
place from which other species might
have moved to populate these spots.
Many think these forests are already
close to the maximum temperature at
which they can survive.
To probe this question, Jaramillo
and colleagues spent seven years scouring South America for sedimentary
rock outcrops with ages spanning the
Paleocene-Eocene boundary. Eventually
the researchers narrowed their list to
three sites in Colombia and Venezuela.
By taking samples of pollen and other
plant fossils from rock layers below and
above the boundary, the team could gauge
the diversity of plant types in those places
before, during and after the hot spell.
Before the warming, the landscape was
covered by a tall, damp rain forest with
even more species than the Amazon has
today, Jaramillo says. As temperatures
rose into the Eocene, more plant groups
appeared in the rock record —mainly
angiosperms, the flowering plants that
are the largest and most diverse plant
group on Earth. Once the warming
abated about 200,000 years later, those
new plants stuck around for good.
Unlike in Wyoming, where native
plants moved off the scene during hot
spells and then returned, the South
American plants apparently dealt with
the heat by diversifying in a great evolutionary burst. “This shows that plants
have the genetic variability already built
in to cope with high temperatures and
high CO2,” says Jaramillo.
But that doesn’t mean tropical forests
will necessarily thrive under future climate change. Oliver Phillips, a tropical ecologist at the University of Leeds
in England, says that it’s a stretch to
suggest that plants at the end of the
Paleocene would have responded to
warming the same way modern plants
would. “Very few modern genera, let
alone species, were extant 56 million
years ago,” he says, “and our modern
plants therefore evolved with different
climate tolerances.” Tropical plants are
far more likely to go extinct in the near
future than to diversify into new species, he says.
So even though global warming is
expected to raise temperatures the most
at polar latitudes, it may have the greatest biodiversity impact in the tropics. s
Francy carvajal/SmithSonian tropical reSearch inStitute
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