Volcanic Soils Yield New Clues About The Emergence Of Powerful Chiefdoms In Hawaii

On the islands of Kauai, O'ahu and Molokai, the principal crop was taro – a starchy plant grown in irrigated wetlands where the supply of water was usually abundant.

But on Maui and the Big Island of Hawaii, the main staple was the sweet potato – a more labor-intensive crop planted in relatively dry fields where success depended on adequate seasonal rainfall. Some anthropologists say that, by the late 1700s, sweet potato production had reached its maximum capacity. As a result, the chiefdoms on Maui and Hawaii began aggressively coveting the taro ponds that flourished on other islands. Pressure to find new sources of food may be one reason why Kamehameha, chief of the island of Hawaii, launched an invasion in 1795 that culminated in his eventual conquest of the entire island chain.

But one question has long troubled anthropologists, ecologists and historians alike: Why was large-scale sweet potato farming confined to just a few areas of Maui and Hawaii? After all, the Polynesians first arrived in the Hawaiian archipelago around 800 A.D., so they had hundreds of years to develop potato fields throughout the islands.

The answer, according to an international research team, may lie in the soil. Writing in the June 11 edition of the journal Science, the researchers conclude that relatively recent volcanic eruptions on Maui and the island of Hawai'i produced a handful of sites with soil nutritionally rich enough to raise large quantities of sweet potatoes. What's remarkable, say the authors, is that early Polynesian settlers found these fertile farmlands, which were originally covered by thick tropical forest, and successfully exploited them for hundreds of years.

"The public perception of the early Polynesians focuses on their greatness as navigators – and they were the greatest navigators of their era," said Peter Vitousek, a professor of biological sciences at Stanford University and lead author of the Science study. "But another way they were equally great was that they were fantastic farmers. They were able to develop and sustain intensive agricultural systems in tropical environments that are very difficult to farm."

Volcanic history

The idea that volcanoes played a crucial role in early Hawaiian agriculture should come as no surprise. After all, the Hawaiian archipelago is actually a chain of volcanoes that has been forming for more than 30 million years.

The volcanic islands are created when the Pacific tectonic plate slowly passes over a region of hot magma located deep in the Earth's mantle. As a result, the islands get progressively younger as you move from west to east. Kauai, in the western end of the chain, emerged some 4.7 million years ago, followed by O'ahu, Moloka'i, Maui and the newest island, Hawai'i, where the magma hotspot continues to produce lava from Kilauea Volcano.

Spectacular rainfall

In the Science study, Vitousek and his co-authors focused on Kohala – a comparatively young volcano on the northwest tip of the island of Hawaii. Early Polynesians created a vast agricultural complex on the volcano's southwest flank, where they raised sweet potatoes and other dryland crops. Remnants of long-abandoned fields – including rows of earthen walls once used as windbreaks – are still visible on the 25-square-mile site, most of which is now on private ranchland.

Kohalas last eruption occurred about 150,000 years ago – long before the Polynesians arrived. "Human settlement and farming in the region began [around] 1200 to 1300 A.D, and the most intensive farming probably took place in 1400 to 1800 A.D," the authors wrote. Standing nearly 5,400 feet above sea level, Kohala Volcano is a place of extremes, experiencing "what may be the most spectacular rainfall gradient on Earth," according to the authors, with annual precipitation on the southwest flank ranging from about 180 inches near the summit to just seven inches near sea level – a distance of less than nine miles.

One of the most puzzling aspects of Kohala agriculture is that, for almost 600 years, intensive sweet potato farming was confined to a narrow, diagonal strip of land about 10 miles long and 2.5 miles wide. This fertile region, which stretched from sea level to an altitude of 3,000 feet, only covered a fraction of the volcano's total surface.

Violent geology

Sweet potatoes do best with at least 20 inches of rainfall a year and at elevations below 3,000 feet. Although such conditions exist on all of the Hawaiian Islands, evidence of large-scale sweet potato farming has only been found on Kohala and a half-dozen other regions of Hawaii and Maui. This fact led researchers to suspect that there was something in the soil of these young volcanic islands that made them conducive to sweet potato cultivation.

Kohala's recent geological history offered several tantalizing clues. The volcano was built 150,000 to 400,000 years ago by a series of violent eruptions that spewed out rock rich in phosphorus, calcium and other minerals essential for plant growth. To find out if large concentrations of these nutrients were present in the narrow agricultural system, Vitousek and his colleagues decided to analyze the soil in and around Kohalas abandoned fields.

"We collected soil samples every 200 meters [600 feet] – from well above the agricultural system to well below it," Vitousek explained. "It turns out that, within the boundaries of the system, phosphorus levels are high, but above and below the system, the levels are low."

The researchers also measured the concentration of several chemical bases in the soil – calcium, magnesium, sodium and potassium. "We found that base conditions were moderately high within the agricultural system, but very low in the higher elevations above it," Vitousek said.

According to the researchers, these results provide strong evidence that the early inhabitants of Kohala had discovered a "sweet spot" of high soil fertility – a swath of land, rich in phosphorus and bases, which received enough precipitation to harvest vast quantities of sweet potatoes.

That rare combination of rainfall and fertile soil did not exist on Kauai, Oahu and Molokai, however. According to the research team, the soils on those ancient islands had lost their nutrients through weathering and other factors – thus precluding the development of large-scale sweet potato farming.

The research team also dug beneath the earthen walls, which the Hawaiians used as windbreaks, and collected samples of virgin soil that had never been farmed. "We found higher phosphorus levels under the walls than in the abandoned fields," Vitousek said. "This suggests that the amount of phosphorus originally in the soil may have been depleted by agriculture and/or subsequent ranching."

Agricultural collapse

"Clearly, the Hawaiians were pushing agriculture to its limits," said Patrick V. Kirch, a professor of anthropology at the University of California-Berkeley and co-author of the Science study. "We can see that the fields on Hawai'i were getting smaller and smaller, and that there was no place for them to expand geographically."

According to Vitousek, the shortage of arable land probably played a role in the rise of aggressive chiefdoms on Maui and Hawaii in the 18th century.

"The Hawaiian Islands had a true class system led by chiefs who enjoyed elite privileges," Kirch noted. "To maintain the social order at the level they were accustomed to, the chiefs had to go into a mode of aggressive action. It's interesting that the really aggressive chiefdoms came from the highly intensified dryland systems on Maui and Hawai'i, where per capita yields were declining. They probably looked up the chain of islands toward Moloka'i and O'ahu and said, 'I'd love to get hold of those taro paddies.'"

By the early 1800s, just a few decades after European contact, the remarkable agricultural system that once prospered on Maui and Hawaii had collapsed. Most historians blame its demise on the introduction of human diseases, which caused a drastic crash in the Hawaiian population. As a result, there were no longer enough people to work the labor-intensive fields, and as the population fell, the demand for sweet potatoes also declined.

According to Vitousek, the successes and failures of the early Polynesians offer important lessons for tropical forest management today.

"Just as land influenced Polynesian societies, Polynesian societies influenced land," he wrote. "Those societies faced the challenge of making a transition from intensive, exploitative use of their island's obviously limited resources, to more sustainable use of those resources. We ourselves now face the challenge of global transition to sustainability; can we learn from societies that succeeded or failed on smaller worlds?"

Other coauthors of the Science study are Shripad Tuljapurkar, professor of biological sciences at Stanford; T.N. Ladefoged of the University of Auckland; O. A. Chadwick and A.S. Hartshorn of the University of California-Santa Barbara; M.W. Graves of the University of Hawaii-Manoa; and S.C. Hotchkiss of the University of Wisconsin. The research was supported by a Biocomplexity in the Environment grant from the National Science Foundation.