Deborah Lawrence has made her findings during five years of research at a remote Dayak village whose inhabitants periodically clear 2? acre patches of land for rice farming or rubber and fruit tree cultivation. In between cultivating these patches, the Dayaks allow them to revert to native shrubs and trees during 20 year long "fallow" periods.
These fallow and active farming sites, in turn, are nested within a matrix of "primary" forest land that has never been cleared for farming -- forming a mosaic of differing land uses.
In a report prepared for presentation Monday at the Ecological Society of America's annual meeting, Lawrence described her recent analyses of soils and trees within seven grown-over fallow sites that had not been cultivated for 10 years, and three other never-disturbed primary forest sites. Her research was supported by the National Science Foundation, the World Wildlife Fund, the National Security Education Program and the Garden Club of America.
Lawrence's previous studies in the same area have shown that the Dayak method of shifting cultivation actually makes soils more fertile than those in undisturbed primary forest. Those results themselves are "surprising," she said in a recent interview at Duke, because they contradict negative perceptions of "slash and burn" methods that the Dayaks themselves employ to clear land.
In her newest study, Lawrence found that -- especially when she combined all her data from all her sites -- tree biodiversity was "negatively correlated" with soil fertility. In other words, the less carbon, calcium, magnesium, nitrogen, phosphorus and potassium she found in the soil, the more varieties of trees she found growing in a given study plot.
Each of those elements is an essential nutrient, Lawrence said. Carbon is the building block for all organic life, while nitrogen and magnesium both contribute to the photosynthetic process that uses sunlight to create plant sugars. Phosphorus is essential for energy production. Calcium is used to make plant cell walls. And potassium contributes to plant water regulation.
When she separated the fallow site data from that for the primary forest sites, Lawrence found some interesting differences. Not only was there more tree diversity in the never-disturbed woodlands, she said, but to some extent, the relationship between biodiversity and soil fertility was actually reversed when primary forest plots were analyzed alone.
With increasing fertility, diversity increased in primary forest, whereas it decreased in periodically-cultivated plots. "It's as if cultivation takes the soil over some critical threshold where nutrients increase to the point that they then actually limit diversity in a negative way," said Lawrence, who is specializing in biogeochemistry and community ecology.
Her latest findings offer mixed signals for the prospects of reclaiming former tropical rain forests that have been cleared of their trees. On the one hand, she said, they corroborate a paradoxical finding of other scientists. Tropical rain forests may grow the world's most diverse array of plant life. But conditions are so delicately balanced there that the soils actually contain relatively few nutrients.
That means that subsistence farmers who clear tracts of the rich forests in the hopes of a quick return in bountiful row crops are quickly disappointed. They then must move on to destroy additional rain forest, leaving behind soil too depleted to grow anything but scrub grass.
But Lawrence's findings also suggest that Borneo's Dayaks have found a way to manage rain forest land much more productively by alternating various parcels between rice production, tree gardening, and decades-long resting periods.
"It's a broad mosaic of uses, much richer in species than any other land use system around," she said. "It's certainly richer than tree plantations or oil palm plantations or clove plantations or pulpwood plantations. But, within this land use system itself, there's no doubt that there's a decline in diversity and that it will probably continue."
By improving the soil's fertility, though, Dayak methods may actually create the conditions for the long process of generating the next rain forest, Lawrence added. All that would be needed would be seed from nearby remaining patches of primary forest.
"It means that we don't have to bring fertilizers in to get the forests to come back," she said. "We just have to leave them alone and make sure there are seed sources."
Fluent in Indonesian, Lawrence collected an oral history of the changing uses of each patch of land in her study area by gaining the Dayak's trust over a five-year period. She would ask farmers how many times they had "opened" a given patch for cultivation, knowing that there were intervening fallow periods of approximately 20 years.
She also asked the farmers what they remembered about their parents and grandparents use of various tracts. "I think that by going back three generations you can get a rough but pretty reliable estimate about how many times a patch was used," she said.
Land in her study area around the Dayak village of Kembera has been in continuous though shifting cultivation for at least 200 years.