Sweet potato: key to Hawaiian dryland agriculture systems?

New archaeological studies from the famed Kona Field System appear to confirm the dramatic expansion in organized Hawaiian agriculture starting as early as the year 1300, and certainly by the late 1400s.

In terms of calories produced, one of the key crops of the drylands was the sweet potato. Did the late arrival in the Islands of this rich source of carbohydrates prompt the development of extensive, highly organized dryland agriculture? It's possible, researchers said. The crop was popular enough that large sections of land were dedicated exclusively to its production.

(Image: Sweet potato in a modern dryland agricultural application.)

The new study, published in the May 2009 issue of the Journal of Archaeological Science, includes a dozen radiocarbon dates from the Kona agricultural fields, but the authors also studied pollen, starch grains and cellular material from plant materials found in the soil at a dozen locations.

The paper is “Sweet potato (Ipomoea batatas) and banana (Musa sp.) microfossils in deposits from the Kona Field System, Island of Hawaii,” by Mark Horrocks, of The University of Auckland School of Geography, Geology and Environmental Science in New Zealand, and Robert Rechtman, of Rechtman Consulting in Keaau, Hawaii.

“The oldest radiocarbon ages of the sampled deposits are 1300–1625 AD and 1310–1470 AD,” the authors write.

The Kona Field System is perhaps the most famous dryland agricultural system in the archipelago. It is a vast area on the slopes between Kailua and Honaunau on the Big Island, where plantings of sweet potato, paper mulberry, breadfruit, dryland taro, bananas and other crops were grown in elevation zones linked to the species' moisture requirements.

Researchers have suggested that this kind of large-scale, organized agriculture could not have occurred without a centralized governing authority, and the establishment of the Kona Field System has been linked by some to the first island-wide governance, perhaps by the famed chief 'Umi-a-Liloa.

Horrocks and Rechtman said they were able to identify various kinds of pollen, and the remains of banana leaves and starch from the roots of plants, including of sweet potatoes.

They found that pollen evidence for trees and shrubs appeared to suddenly decline as the cultivated plants appeared—suggesting land clearing of forested areas for agriculture. There was also evidence that sweet potatoes were being grown exclusively in some specific areas.

“The apparent absence of starch and xylem remains of other tuberous crops archaeologically identified elsewhere in Polynesia suggests that tuberous cropping within the study area was mono-specific,” the authors wrote.

Other places in Hawai'i also had agricultural field systems. There is another well-known field system at Kohala. And on Moloka'i, there is recent evidence that the Kalaupapa peninsula, whose agricultural fields were once believed to be post-European, extended well back into the prehistoric period.

Researcher Mark McCoy, now an anthropologist with San Jose State University, conducted extensive research on the peninsula that juts northward from the base of Molokai's cliffs. In a paper published in 2005 in the Journal of the Polynesian Society, “The Development of the Kalaupapa Field System, Moloka'i Island, Hawai'i,” he cites dates similar to those of the Kona Field System.

“This field system was largely ignored in previous discussions of Hawaiian agriculture because it was initially assessed to be a 19th century construction,” he wrote.

Radiocarbon dates of botanical material found on the peninsula and in adjacent valleys suggest that there may have been humans there perhaps as early as 800 AD and almost certainly by 1200 AD.

There are questions about the validity of the earliest dates, but certainly by 1200, Hawaiians were in the area and beginning to establish settlements. And while there is some evidence of agricultural activity, it appears this was not particularly intensive for another 250 years.

That's when the serious land clearing started.

“Widespread burning across the Kalaupapa Peninsula, which signals of the beginning of the Kalaupapa Field System, does not commence until 1450-1550,” McCoy wrote.

The field system appears to have been abandoned at Kalaupapa in the late 1700s, and then reopened about 1850, as Hawaiian farmers began providing potatoes and other crops to the participants in the California gold rush.

This revival of agriculture at Kalaupapa was halted in 1866, when the crown relocated native residents of the Kalaupapa region to establish the Hansen's disease colony there.

In general, McCoy said that the field system studies, along with other archaeological work, tends to suggest that residents settled and farmed the wetter regions of the Islands first, and then moved into the drier areas.

Why did the move from wet to dry occur about the same time in many areas? Perhaps it was population pressure. Perhaps it was the arrival well into the last millenium of the sweet potato, a crop from South America. Sweet potatoes, 'uala in Hawaiian, do not appear in the earliest archaeological sites, suggesting they had not yet arrived.

“There are a number of equally attractive, alternative hypotheses to explain concurrent construction of field systems in the Hawaiian Islands, including the late introduction of the sweet potato, population pressure, an increased demand for social production to fuel the political economy, or a combination of some or all of these,” McCoy wrote.

© Jan TenBruggencate 2009