text-only page produced automatically by LIFT Text Transcoder Skip all navigation and go to page contentSkip top navigation and go to directorate navigationSkip top navigation and go to page navigation
National Science Foundation Home National Science Foundation - Biological Sciences (BIO)
Biological Sciences (BIO)
design element
BIO Home
About BIO
Funding Opportunities
Awards
News
Events
Discoveries
Publications
Advisory Committee
Career Opportunities
BIO Program Director and Reviewer Opportunities
Supplements & Other Opportunities
See Additional BIO Resources
View BIO Staff
BIO Organizations
Biological Infrastructure (DBI)
Environmental Biology (DEB)
Emerging Frontiers (EF)
Integrative Organismal Systems (IOS)
Molecular and Cellular Biosciences (MCB)
Proposals and Awards
Proposal and Award Policies and Procedures Guide
  Introduction
Proposal Preparation and Submission
bullet Grant Proposal Guide
  bullet Grants.gov Application Guide
Award and Administration
bullet Award and Administration Guide
Award Conditions
Other Types of Proposals
Merit Review
NSF Outreach
Policy Office
Additional BIO Resources
The BRAIN Initiative
FY 2015 BIO Budget Excerpts
BIO's Guidance on Data Management Plans
Dear Colleague Letters: BIO and Foundation-wide
List of BIO Cyberinfrastructure Reports
National Ecological Observatory Network (NEON)
Partnership for Undergraduate Life Science Education (PULSE)
Supplements & Other Opportunities
Science Across Virtual Institutes (SAVI)
Broadening Participation Activities
NSF's Career-Life Balance Initiative
Interdisciplinary Research
BIO Reports
NSF Strategic Plan: 2011-2016
NSF Information Related to the American Recovery and Reinvestment Act of 2009
Merit Review (effective Jan. 14, 2013)
Image Credits
Other Site Features
Special Reports
Research Overviews
Multimedia Gallery
Classroom Resources
NSF-Wide Investments

Email this pagePrint this page

Discovery
Seeing the Forest for the Trees: Seed Dispersal, Environmental Conditions Matter in African Forests

Ecologists discover when, how tropical trees regenerate

Nouabale-Ndoki National Park in Central Africa, site of the scientists' research.

Nouabale-Ndoki National Park in Central Africa, site of the scientists' research.
Credit and Larger Version

May 14, 2013

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 6 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.

--  Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov

Map showing the location of Nouabale-Ndoki National Park.
Location of Nouabale-Ndoki National Park.
Credit and Larger Version

Close up image of a two-year-old seedlings of African mahogany
Two-year-old seedlings of a species known as African mahogany survive in the tropical forest.
Credit and Larger Version

A canopy tree in Nouabale-Ndoki National Park.
Reaching toward the sky: A canopy tree in Nouabale-Ndoki National Park.
Credit and Larger Version

A Mbendzele research assistant measuring tree seedlings using a ruler
Gaston Abeya, a Mbendzele research assistant, measuring tree seedlings.
Credit and Larger Version

Adult African elephant and baby in a river.
The forest elephant is one of many species that depend on tropical forests in Central Africa.
Credit and Larger Version



Email this pagePrint this page
Back to Top of page