Nancy Marie Brown
June 01, 1987
white car driving up eroded hill, dark storm clouds

"When I consider the excessively small amount of labor required in this country [Amazonia] to convert the virgin forest into green meadows and fertile plantations, I almost long to come over with half a dozen friends disposed to work and enjoy the country; and show the inhabitants how soon an earthly paradise might be created." —Alfred Wallace, 19th-century British naturalist

In the last quarter-century, roads have been fingering into the rainforest. Settlers have followed the roads. "They come with their lives on their shoulders," says Christopher Uhl, an assistant professor in Penn State's College of Science, "full of hope. There's this tremendous energy all around."

With axe and machete, they chop a field out of the forest, burn the slashings, leave the charred logs to rot, and plant manioc cuttings. After 12 months, and two to three weedings, the roots are mature. Dug up, cleaned, peeled, grated—the bitter varieties soaked, so natural enzymes will break down the poisonous glucosides, and squeezed to remove the resulting hydrocyanic acid—dried, sieved, and baked, manioc produces a coarse, granular flour. A field will bear two or three crops, each smaller than the last, before the yield becomes discouragingly low.

Settlers tied to the land will carve a small field from the forest every two years until their holdings are patchworks of just-burned, farmed, and abandoned plots. Occasionally they might plant a field fallowed for 10 years; it will produce only one crop. A 25-year fallow will last through two crops. A 50-year fallow will grow nearly as much as an original cutting.

Settlers not tied to the land will follow the road. "Around one bend . . . we overtook a barefoot man carrying a large scythe," writes journalist Catherine Caufield about a trip through a nature preserve in Colombia. "His sunburned, wrinkled face and bony frame made him look old and frail, but his voice was robust. He was on his way to weed his plot, a hilly piece of land above the landslide that bordered the road. He had only recently cut and burned the forest to plant his crops. Did he intend to stay and cultivate the land he had cleared? ‘No, this land is not so good. When the road goes further, I will follow it to better land.' "

Near San Carlos de Rio Negro in Venezuela and near Paragominas, in the state of Para, Brazil, are small plots of farmed and abandoned land that Christopher Uhl and his colleagues are watching. Watching—and measuring and testing and calculating—has told them how the rainforest reclaims the field a farmer has turned away from in disappointment and resignation.

It takes well over a century for a farmed site to revert to true rainforest, Uhl estimates. Cutting destroys the trees and saplings. Burning kills seedlings and sprouts and cuts the germination ability of buried seeds. (From a square foot of forest soil, removed, watered, and placed in the sun, Uhl coaxed 70 seeds to germinate; 15 seeds sprouted in the test square-foot of cut-and-burned soil.) Weeding shifts the species balance from forest trees to prairie herbs, exhausting the root-sprouting reserves of forest trees, reducing the numbers of woody pioneers, and increasing the number of herbs—to 95 percent after three weedings—since only they can germinate, flower, and set seed before the farmer again grooms the field. The three years of declining productivity is synchronous with the slow decomposition of the soil-protecting, nutrient-rich root mat: An abandoned farm is largely bare soil littered with a few burned and decomposing logs and graced with a few isolated fruit trees. After one fallow year, grasses and other meadow plants dominate; the shaded areas support a few saplings. After two years, an acre of the farm holds seven tons of biomass—the combined dry mass of all its vegetation. The islands of woody plants under the fruit trees and along the logs start to expand until, by the third year, trees of the pioneer species Vismia form a partial canopy 25 feet tall, and shrubs have edged out the grasses. In five to 10 years (16 tons of biomass), other tree species take hold; the Vismia canopy reaches 36 feet and begins to die off, to be replaced by deep-forest species. After 50 fallow years, the farmed plot will contain 60 tons of biomass per acre. An acre of virgin rainforest holds 150 tons.

"Often an abandoned farm is not allowed to revert. When settlers leave their fields and head down the road, they usually sell to cattle ranchers. In the mid-1960s, the Brazilian government began offering financial incentives to ranchers; land was never more than a few dollars an acre, and in some cases, says Uhl, "the incentives were so generous it was impossible to lose money." Ranchers began buying failed farm plots and cutting virgin forest to connect them into 250-, 2,500-, even 25,000-acre pastures. In 1980, over two million acres of rainforest were cut for cattle grazing.

A pasture is productive for only five to 10 years. After as few as three or four, the grasses begin to lose vigor because of soil infertility (large pastures are not usually fertilized), insect attack (the year-round growing season in the Amazon guarantees year-round pests), and competition with weeds. After about six years, more than 50 percent of the pasture is covered with inedible weeds, its soil phosphorus is too low to grow grass, and the soil itself is so compacted (rain and cattle hooves combine to make it 30 percent denser than the surrounding rainforest) that grass seedlings, even if given phosphorus, couldn't become established. "Pastures in this condition," Uhl writes, "are of little use for cattle grazing: A cow gains less than 20 pounds per acre per year, compared to 50 pounds per acre in a first-year pasture." Often, a pasture such as this is simply abandoned, in hopes that it will revert to forest.

"But modern disturbances like cattle pastures," says Uhl, "are larger than anything that has come before, and quite prolonged. The forest does not appear able to come back."

The limiting factors, he explains, do not include soil infertility (the mineral soil underneath a rainforest's root mat is just as infertile as that of a six-year-old pasture; rainforest plants are adapted to it). Uhl and Dan Nepstad, a doctoral student from Yale University, found that water stress was much more of a problem. "For the three to four months when rainfall is low in this part of the Amazon," says Uhl, "the plants in pastures are kept at permanent wilting conditions."

Another serious barrier noticed by Uhl and Charles Yohn, a Penn State graduate student, concerns the logistics of seed dispersal. "We didn't find any forest tree seeds in the abandoned pastures," explains Uhl. "Why? Ninety percent of rainforest trees have seeds that require a bat or a monkey or a bird to transport them. Of the 150 bird species in one of our study areas, we found only five that ate fruits and defecated seeds and would also cross into large clearings. These were birds that ordinarily lived on the edges of the forest or in second-growth areas. None of the birds we caught in the deep forest were caught in the pasture. So the birds don't reseed the open areas. When we tried to place seeds in the pastures ourselves, most were quickly removed by ants and rodents—you can be walking down the transect putting out seeds, and when you look back, they're all gone. Finally, the seedlings that did sprout were attacked by leaf-cutter ants; those that survived the ants wilted in the next dry season."

Ranchers bent on keeping their land will often burn a degraded pasture and reseed it. The burned weeds fertilize the grass; fire cuts the insects' reproductive chain. Since the late 1970s, when a national monetary crisis forced the Brazilian banks and government to reduce their support for ranching, burning has become much more common for weed control, being much less expensive than hand-cutting with machetes or spraying with herbicides. The chance of a fire going out of control has, until recently, been small: Undisturbed rainforest is too moist to burn.

The recent drop in government support, however, has also impelled the Brazilian ranchers to begin selling timber rights—under the original land distribution laws, half of each holding (of whatever size) was to be left forested. Selective logging in a rainforest is ecologically sound when done carefully. It usually creates one or two canopy gaps in an acre; these gaps—since only the trunks of the trees are removed, leaving the nutrient-rich twigs, leaves, and roots—behave like natural tree falls, quickly growing back.

"Unfortunately, as Uhl and Robert Buschbacher of the University of Georgia note, careful logging is the exception in tropical rainforests. "Because the cutters are not the landowners," they write, "they have no vested interest in doing the work carefully. Cutters with chain saws fell all potentially harvestable trees, cutting many more than are actually harvested. Bulldozers follow and sloppily drag undamaged boles to spur roads, killing many saplings. The end result is thousands of square kilometers of cut-up forest scarred with bulldozer tracks and laden with dead slash."

The moisture-holding canopy is gone in a carelessly logged forest. If the landowner then decides to burn off the weeds in his pastures, the fire will quickly spread to the logging slash. "After a fire," says Uhl, ""you have a strange, parklike atmosphere. What happens to the trees that are left when the services—pollination, seed dispersal, etc.—provided by animals cease? It seems reasonable to assume that fires would disrupt the vital linkages between the canopy and the forest floor—linkages related to nutrient retention, regeneration, pollination, seed dispersal, cover from predators.

"I have studied disturbance ecology in Amazonia for the past 10 years, and this is the first time I have seen, within a grouping of development strategies, the clear potential for the degradation of the entire Basin."

Last Updated June 01, 1987