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Stanford Report, February 17, 1999

Dust and sea spray nourish Hawaiian rainforest

BY LISA TREI

Dust from Asia and sea spray have been responsible for helping Hawaii sustain its lush rainforests for millions of years, according to a new study published in Nature on Feb. 11.

In a paper written by Oliver A. Chadwick from the University of California-Santa Barbara and four colleagues, including Stanford biological sciences Professor Peter M. Vitousek, the researchers explain how the Hawaiian Islands, the most remote archipelago on earth, are closely dependent on natural processes taking place both nearby and thousands of miles away.

"This shows that no place on earth is really isolated," said Vitousek. "Everywhere is connected to areas that are upwind or upstream of it, even if those connections aren't always apparent."

In a wet environment like Hawaii, minerals such as phosphorus, calcium and magnesium get leached out as the soil becomes weathered. The team of scientists measured this by sampling six sites across the archipelago that were similar except for their ages, which ranged from the 300-year-old rocks on the Big Island to the 4.1-million-year-old soils on Kauai. They analyzed the rocks' and soils' chemical makeup to distinguish soil-derived nutrients from those supplied by sea spray or dust.

The scientists discovered that plants in the youngest sites, mainly the Ohia tree (Metrosideros polymorpha), took nutrients that came from the lava that built the islands. After about 100,000 years, however, salty sea spray carried into the atmosphere became the main source of calcium, magnesium and potassium.

"The conventional way of thinking is that minerals wash out of soils and into streams and eventually go to the ocean where they feed the productivity of ocean organisms in coastal waters," said Vitousek. "But in these systems, we can trace . . . that most of the calcium, magnesium and probably potassium found in forests have come from the ocean."

Phosphorus is lost from soils more slowly than calcium but, after a million years, soils had lost their stocks of this mineral as well. In the oldest soils, plants relied almost entirely on phosphorus in the atmospheric dust blown 6,000 kilometers from arid central Asia.

"Scientists in Hawaii have known for a long time that there has been dust from Asia [in Hawaii] because it contains minerals that can't be formed in Hawaiian soils," said Vitousek, a terrestrial ecologist. "Like rock, dust breaks down chemically and becomes unrecognizable from its minerals after 100,000 years. But there are some tracers in the dust, some elements that are present in different ratios in the Asian continent to those found in the rock in Hawaii. So even though we can't measure all of the dust now, we can see its ghost in the soil."

Vitousek said that the research shows that the atmosphere is more critical to ecosystems than previously thought. "It shows that dust from very distant sources and distant times can be important," he said.

The six-year study relied on participation from a range of different scientific disciplines. "This is the kind of work no individual could do," Vitousek said. In addition to Chadwick, a soil scientist, participants in the study included Lou A. Derry, an isotope geochemist, and Lars O. Hedin, an aquatic ecologist, both from Cornell University; and Barry J. Huebert, an atmospheric chemist from the University of Hawaii. SR