This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In a fast-disappearing desert oasis, scientists are trying to bring a forest back to life – and discovering the imprint of a lost civilization amidst the vanishing trees.
One of the driest places on Earth, South America’s Atacama Desert — a 1,000 kilometer stretch along the western coast of Peru and northern Chile — receives almost no rainfall. But there are isolated patches of green amidst the desert sands, thanks to water from an unusual source — fog.
In some parts of the Atacama coastline, moisture from clouds formed over the Pacific Ocean drifts inland and sweeps over the desert slopes. The fog isn’t wet enough to produce rain, but it does provide enough moisture for isolated islands of vegetation called lomas. Grasses, cacti, shrubs and small trees flourish in these fog oases, surrounded by mountains and shifting dunes.
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Scientists estimate the South American lomas support some 1,400 plant species, more than 40% of which are found nowhere else. But the lomas are important for another reason too: people depend on them to scratch out a living in an otherwise barren land.
At one time the lomas spread over 15,000 km2, or nearly 6,000 square miles. That was before local people started cutting down the trees for firewood and timber and to make room for pasture. In the past 50 years, overgrazing and deforestation have taken their toll, such that the lomas have shrunk to a fraction of their original size. By the 1980s, nearly 90% of their original expanse had returned to desert.
Fog catchers
The largest remaining lomas lie in a six square mile area in southwest Peru, in a community called Atiquipa. There, about 450 people live by herding and subsistence farming. When water shortages in the late 1990s made farming extremely difficult, the community turned to experts for help.
A Peruvian research team led by Percy Jiménez of the Universidad Nacional de San Agustín confirmed the source of the region’s precious water: fog capture by trees.
During periods of fog, the taller trees of the lomas collect tiny water droplets on their trunks and leaves, condensing the mist before it evaporates or drifts away. Droplets drip down the trees and filter into the surrounding soil.
Fog cascades over a forest dominated by tara trees in the Atiquipa lomas in southwest Peru. Filmed with funding from the Biodiversity Conservation Programme of the Banco de Bilbao Vizcaya Argentaria Foundation (FBBVA).
Average annual precipitation in Atiquipa is less than 2.5 inches (6 cm) a year, but under the tree canopy that can swell to nearly 20 inches (50 cm).
Without wells, rivers or streams to provide fresh water, the people of Atiquipa depend on the fog-harvesting properties of the vanishing trees for survival, funneling the fog water through irrigation channels and collecting it in reservoirs at lower elevations. "If the trees disappear, then the water supply won’t be enough to support human settlement," said Luís Balaguer, a restoration ecologist in Madrid who has been studying the Atiquipa lomas since 2005.
Amidst the vanishing trees, the imprint of a lost civilization
For more than two decades, researchers have studied the trees and searched for ways to bring the lomas back to life.
One tree in particular has proved critical for capturing fog — a small but sturdy tree with leafy branches known as Caesalpinia spinosa, or tara tree. By reintroducing the tara trees, researchers hope to create favorable conditions for other species as well.
But reintroducing the trees hasn’t been easy. For one thing, the remaining lomas don’t seem to support new seedlings. "We were surprised by the extremely low number of seedlings we found in the forest," said Balaguer, co-author of a study of the lomas recently published in PLoS ONE.
If the older trees aren’t replaced by younger ones, the researchers fear, this may be the last generation of tara trees.
Genetic surveys also turned up surprising results: Leaf samples collected at ten different locations in Peru, Colombia and Bolivia revealed no genetic differences among the trees.
The most likely explanation, scientists say, is the trees didn’t get there alone. "One or a few genotypes must have been widely planted and traded by people at some point in the past, until they spread throughout the region," said Rafael Rubio de Casas, a biologist at the National Evolutionary Synthesis Center who was part of the PLoS ONE study.
Early accounts by the first Spanish explorers describe how the Incas used the tara trees. The seed pods were a source of tannins for tanning leather, and their black seeds and roots were a source of dye.
The Incas may have spread tara trees intentionally for their useful properties, or even unintentionally, when livestock grazed on tara seed pods. But the puzzle of why the tara aren’t increasing in numbers remains unsolved.
Perplexing plants
Scientists from Peru, Spain, France and the United States have spent years searching for the answer. Microbiologists have tested soils for beneficial bacteria and fungi that may be key to the trees’ health, and that might be missing from the lomas — but so far nothing has turned up.
"I don’t think it’s a problem of lack of genetic diversity either, since this species grows from seeds but can also re-sprout from roots and cuttings," Balaguer said.
One thing that may be missing in addition to adequate water, Balaguer said, is a means of dispersing the seeds. Tara seeds have a hard outer coating that needs to wear away before the seeds will germinate. Passing through an animal’s gut will do the trick. The Incas raised llamas and alpaca which likely ate and dispersed the tara seeds, but the most common livestock in the region today —mostly cattle, sheep and goats —don’t touch them.
Restoration, redefined
The project comes at a time of heated debate about what ecological restoration is and isn’t, and why and how one should do it, said study co-author James Aronson, author of many books and articles on the subject.
For ecosystems with a long history of human impact, the central tension revolves around what to restore in areas where "rewilding" is impossible.
"Some landscapes have been transformed so drastically by human activity that there’s little point in trying to restore natural 'pristine' wilderness," Balaguer said. "People have been living in the Atiquipa area for more than 12,000 years."
One thing is clear —the lomas won’t grow back on their own. If the trees are to make a comeback, the scientists say, continued human intervention may be key.
In the early 2000s the community of Atiquipa started planting tara seedlings, one at a time, by hand. The seedlings were first raised in nurseries from local seeds, then transplanted into the lomas and carefully watered once a week. The community followed the planted seedlings carefully over the next few years, checking their growth along the way.
"But intervention must be more than just planting trees," Aronson said. Restoring the lomas will require continued coordinated efforts among scientists, the government, and the local community. "Hundreds of similar efforts around the world have failed because outsiders or the government sought to impose ideas and technology top down," he added.
Whether this particular experiment will work, then, remains to be seen. But so far at least, the new seedlings seem to be doing well.
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The team’s findings were published in the August 2 issue of PLoS ONE:
Balaguer, L., R. Arroyo-García, et al. (2011). Forest restoration in a fog oasis: evidence indicates need for cultural awareness in constructing the reference. PLoS ONE 6(8): e23004. Funded by the Biodiversity Conservation Programme of the Banco de Bilbao Vizcaya Argentaria Foundation (FBBVA).