DURHAM, N.C. -- In findings with implications
for the future of a commercially-important tropical wood, Duke University
ecologist Laura Snook has discovered that seedlings of American mahogany
trees seem to become successfully established only on open land.
Snook's research in Mexico's Yucatán Peninsula suggests that replacing the many stately mahoganies felled by loggers in tropical forests will not be a simple matter.
While mature mahogany trees normally grow in thick woods, she has found no evidence that new mahogany seedlings will successfully grow in the shade of other trees -- mahogany trees included.
Snook is deeply concerned about the "mahogany deserts" she said are being created by South and Central American logging practices. Remaining mahogany reserves are currently being "mined," not "managed," she said in an interview.
" 'Mining' means you take it all out until it is gone, and then you walk away," she said. " 'Management' says we want to harvest mahogany today and tomorrow and the next day. At the moment, mahogany is not being treated like a renewable resource for the most part. In 99 percent of cases, it's being mined."
Snook, an assistant professor of the practice of conservation biology at Duke's Nicholas School of the Environment, details her findings in an article published in this month's Botanical Journal of the Linnean Society. She will also discuss her research at the International Conference on Big Leaved Mahogany: Ecology, Genetic Resources and Management, to be held Oct 22-24 in San Juan, P.R.
To successfully regenerate mahogany trees in natural settings, Snooks's studies have shown that that mahogany seeds must land upon temporary clearings created by high winds and subsequent fires. They must then compete for survival with other plant species also taking root there.
"They apparently co-evolved with disturbance, which is very common in forests," Snook said in an interview. "The longleaf pine in the Southeastern United States is similar. It depends on fire. It has co-evolved with fire."
While such a stark survival strategy dooms most mahogany seeds to fail, not many successes are needed to carry on a species that may live and reproduce for 500 years or more, she added. But logging practices at odds with nature could threaten the American mahogany's future, she warns.
Snook, a diplomat's daughter who received a bachelor's degree in history from Iowa's Grinnell College before becoming a scientist, has become a specialist on American mahogany ecology since first exploring the subject during 1987-92 for her doctoral research at the Yale School of Forestry and Environmental Studies.
Searching the scant available publications, most dating from several decades earlier, she learned that the only true mahogany species (Swietenia macrophylla) now providing commercial timber grows only in tropical forests of South and Central America.
Another American species native to a few Caribbean islands (Swietenia mahogani ) was logged from the 16th to 18th centuries but now lingers in a few isolated habitats, including South Florida's Everglades. Meanwhile, African and Philippine "mahoganies" are foreign imposters, she said.
Swietenia macrophylla towers up to 180 feet tall, with a wide leafy crown that dominates other trees. In the dry season, its leaves turn a deep mahogany red and then shed, mimicking the autumn ritual of northern deciduous trees. Before their leaves fall, mahogany crowns' distinctive color allows timber companies to spot marketable mahogany trees from overflying aircraft, Snook said.
Because the growth rings of mahoganies she has examined don't form at annual intervals, Snook can only guess at the species' maximum age. Based on extrapolation and evidence, including old photographs of huge harvested logs and four-foot-wide boards, she speculates that mahoganies may live up to 600 or more years.
High in the forest canopy -- the only layer of the jungle to regularly experience wind -- mahogany seed pods resembling wood-covered avocados extend upright from the trees' branches. After ripening, the pods' outer shells crack open and fall off to reveal seed clusters that look like parasols.
As treetop winds blow, individual wing-shaped seeds can then detach and flutter to the ground "like little helicopters," Snook said. She estimated that airborne dispersal can sow mahogany seeds at least 180 feet away from the parent tree.
Early in her research, Snook realized that almost nothing was known about how mahogany trees propagate in the wild, even though their reddish brown wood has been commercially prized for 500 years.
For the first 400 years, woodsmen simply cut the trees down with little thought to replenishing the supply, she says. Then, when the virgin timber began running out starting around 1900, foresters assumed they could grow replacement mahogany in same-species plantations.
But mahogany plantations in the Americas have not lived up to expectations, she said, mainly because of the shootborer caterpillars that thrive all year in the tropics.
Shootborers can quickly decimate young mahogany saplings grown close together in rows without the protection of a surrounding cover of other tree species, she said. In nature, the surrounding trees would divert many of the caterpillars from attacking the mahoganies.
"There are people who are still working with mahogany plantations," Snook added. "But what we're trying to do is imitate nature. My interest is trying to regenerate mahogany in natural forests to sustain the value of those forests as production systems so they will not be converted to pasture or agriculture."
To learn how to regenerate mahogany, Snook began studying the trees growing within a model forestry project called Plan Piloto Forestal in Quintana Roo on Mexico's Yucatán peninsula.
She found that mahogany trees there were sparsely dispersed among other species at an average density of one tree per hectare (about 2.5 acres), though occasionally in clumps of up to 20 mahoganies per acre. Mahogany trees grow so big and wide that they must be spaced far apart to "keep from crowding each other out," she said.
What Snook couldn't find was equally important. There were no mahogany seedlings successfully growing under the forest canopy. Any seedlings she found were instead "on roadsides and abandoned agricultural fields," she said.
Back in the forest, Snook also found young mahogany trees growing where she detected signs of old hurricane or fire damage.
Snook then began a systematic search for more sites of past disturbances by conducting "oral history" interviews with local residents.
"I had to be careful not to lead people into answers," recalled Snook, who speaks fluent Spanish. "I asked them things like, 'Is there any part of the forest where you know something happened in the past? Can you take me there?'" She also used local people's memories to date the ages of post-disturbance trees.
Like Sherlock Holmes, the fictional detective about whom Snook read avidly while at Grinnell, she collected multiple clues to unravel the mystery.
Hurricanes periodically blow through Yucatán forests, she said. "They knock down some trees and branches, but most of the trees survive." However, the fallen debris provides ample fuel "that ignites in the next dry season," she added. "And only 15 percent of the trees survive the subsequent fire."
But the odds are especially good that mature mahoganies will be left standing. "They're very good survivors," with large buttresses, aerodynamically shaped crowns and stout branches that resist wind damage, plus thick bark to protect against flames, she said.
Those older trees can then provide sources of seed to grow new mahoganies in the clearings opened up by wind and fire.
The regeneration process may be even more complicated. In a recent revisit to Quintana Roo, Snook found that mahogany seeds can actually successfully germinate and begin growing in the shade. It's only after they run out of their own stored food -- perhaps a year after germination begins -- that the tiny shoots need open sunlight to make food through the photosynthetic process.
At a study site in Belize as well as in Mexico, Snook is dropping mahogany seeds onto cornfields to see if the species can also be regenerated and managed in conjunction with shifting agriculture.
"I'm sure that's what happened in the past," she said. "Not only did mahoganies have the benefit of hurricanes and fires. They also had the benefit of Mayans who created little openings in the forest and then abandoned them after they finished a year or two of corn production."
Using a rough calculation, Snook predicted that a 500-year old mahogany tree may produce half a million seeds over its long life. "Most seeds die," she said. "Very few come to anything. But only one seed can replace its parent."
Another consideration is that mahogany seeds are only viable for a matter of months before they biodegrade and die, she noted. That means they cannot be stored and preserved indefinitely in the ground the ways seeds of many other plants can.
In her article in Botanical Journal of the Linnean Society, Snook warns that the way mahoganies reproduce makes them "vulnerable to logging, first because juvenile mahoganies are not found in the understory, and secondly because logging operations shortcircuit mahogany regeneration processes. ..."
Logging interferes "by selectively removing almost all mahogany seed sources while leaving standing competing vegetation of other species," she writes.
Snook's article also supports the American mahogany's listing under Appendix II of the Convention on International Trade in Endangered Species as "a species which although not necessarily now threatened with extinction may become so unless trade in specimens of such species is subject to strict regulation..."
Such a listing would not restrict trade, but would require that mahoganies be monitored and that producing nations define sustainable harvesting levels and issue logging permits, she said.