Nouabale-Ndoki
National Park in Central Africa, site of the scientists' research.
Nouabale-Ndoki
National Park is a tree-dotted enclave in Central Africa's Republic
of Congo. Heavy logging surrounds the park, but it still has one of
the largest intact forests in Africa. In recognition, it recently
became a UNESCO World Heritage Site.
Trees--thousands
of them--make up a forest. How did Nouabale-Ndoki's trees become so
numerous, and how do they stay that way?
The
answer, say biologists, lies far below the tree canopy, in the soil
where seedlings sprout.
Today
in the journal PLOS
ONE,
scientists report results of an extensive seedling experiment in
Nouabale-Ndoki National Park.
The
research, which involved sowing 40,000 seeds of five tree species, is
a new look at "seeing the forest for the trees."
The
findings, which show what limits seedling growth, are important to
reforestation efforts in areas that have been logged.
Every
tree can produce hundreds of thousands of seeds in its lifetime, but
on average, only one seed survives to adulthood, says John Poulsen of
Duke University, a co-author of the journal paper.
Other
paper co-authors are Connie Clark, also of Duke, and Doug Levey,
formerly of the University of Florida and now a program director in
the National Science Foundation's (NSF) Division of Environmental
Biology.
Which
seeds have the best chance of making it to old age?
"There
are basically two ways to look at successful seedling recruitment
[survival]," says Levey. "Species may be seed-limited or
establishment-limited."
A
tree species is seed-limited if its ability to grow is determined by
whether its seeds reach a particular location on the ground. The
seeds may arrive on the wind or simply by falling from trees.
Establishment-limited
trees are those that depend on the environment around them, rather
than on seeds landing in just the right spot. If the soil is too wet
or there is too much shade, a species is establishment-limited.
To
test the importance of these two limitations on seedling recruitment,
the scientists sowed tens of thousands of seeds.
They
chose the species randomly, which allowed the results to be
generalized to all tree species, not just the most common ones, says
Poulsen.
The
seeds were planted in different amounts in plots that stretched
across an area the size of the state of Rhode Island.
Latter-day
Johnny Appleseeds, the researchers couldn't do it alone, however.
"We
hired a small army of indigenous, Mbendzélé hunter-gatherers,"
says Clark. "These families could easily locate seeds, and we
were the beneficiaries of their intimate knowledge of the forest's
natural history."
After
the seeds were planted, the ecologists watched them grow into
seedlings over two years.
They
found that only a small fraction of seeds, some 16 percent, became
seedlings. An even smaller amount, about six percent, survived to
reach their second birthdays.
When
numbers of seeds were at one end of a spectrum--rare or abundant--the
trees' recruitment was seed-limited.
"When
seeds were at intermediate densities," says Levey, "the
chance of recruitment was influenced by environmental factors such as
soil type and sunlight."
The
importance of seed- and establishment-limitation changes over time,
Levey says. "As individual trees get older, they need the
correct soil and light exposure [become more establishment-limited]."
Not
that different from our changing needs for the right nutrients and
enough light as we reach our sunset years.
where is my link ?????
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