A "living fossil" tree species is helping a University of Michigan
researcher understand how tropical forests responded to past climate change
and how they may react to global warming in the future.
The research appears in the November issue of the journal Evolution.

Symphonia globulifera is a widespread tropical tree with a history that goes
back some 45 million years in Africa, said Christopher Dick, an assistant
professor of ecology and evolutionary biology who is lead author on the
paper. It is unusual among tropical trees in having a well-studied fossil
record, partly because the oil industry uses its distinctive pollen fossils
as a stratigraphic tool.

About 15 to 18 million years ago, deposits of fossil pollen suggest,
Symphonia suddenly appeared in South America and then in Central America.
Unlike kapok, a tropical tree with a similar distribution that Dick also has
studied, Symphonia isn't well-suited for traveling across the ocean - its
seeds dry out easily and can't tolerate saltwater. So how did Symphonia
reach the neotropics? Most likely the seeds hitched rides from Africa on
rafts of vegetation, as monkeys did, Dick said. Even whole trunks, which can
send out shoots when they reach a suitable resting place, may have made the
journey. Because Central and South American had no land connection at the
time, Symphonia must have colonized each location separately.

Once Symphonia reached its new home, it spread throughout the neotropical
rain forests. By measuring genetic diversity between existing populations,
Dick and coworker Myriam Heuertz of the Université Libre de Bruxelles were
able to reconstruct environmental histories of the areas Symphonia
colonized.

"For Central America, we see a pattern in Symphonia that also has been found
in a number of other species, with highly genetically differentiated
populations across the landscape," Dick said. "We think the pattern is the
result of the distinctive forest history of Mesoamerica, which was
relatively dry during the glacial period 10,000 years ago. In many places
the forests were confined to hilltops or the wettest lowland regions. What
we're seeing in the patterns of genetic diversity is a signature of that
forest history."

In the core Amazon Basin, which was moist throughout the glacial period,
allowing for more or less continuous forest, less genetic diversity is found
among populations, Dick said. "There's less differentiation across the whole
Amazon Basin than there is among sites in lower Central America."

The study is the first to make such comparisons of genetic diversity
patterns in Central and South America. "We think similar patterns will be
found in other widespread species," Dick said.

Learning how Symphonia responded to past climate conditions may be helpful
for predicting how forests will react to future environmental change, Dick
said.

"Under scenarios of increased warmth and drying, we can see that populations
are likely to be constricted, particularly in Central America, but also that
they're likely to persist, because Symphonia has persisted throughout
Central America and the Amazon basin. That tells us that some things can
endure in spite of a lot of forest change. However, past climate changes
were not combined with deforestation, as is the case today. That combination
of factors could be detrimental to many species - especially those with
narrow ranges - in the next century."

The researchers received funding from the National Science Foundation and
the National Fund for Scientific Research of Belgium.
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