"This study is the first to accurately predict the distribution of nutrients across a complex tropical forest landscape, and then to detect these shifts in nutrient status using airborne sensors," says Stanford University graduate student Stephen Porder, lead author of the study, which will be published in this week's online edition of the Proceedings of the National Academy of Sciences (PNAS).

Porder's co-authors are Carnegie Institution scientist Gregory P. Asner, an assistant professor (by courtesy) of geological and environmental sciences at Stanford; and Peter M. Vitousek, the Clifford G. Morrison Professor in Population and Resource Studies at Stanford.

"Tropical soils often are assumed to be highly weathered and thus nutrient depleted," the authors write. But the study, which focused on the island of Kauai, revealed a complex "biogeochemical patchwork with almost equal areas of high and low nutrient availability."

Phosphorous, calcium and other minerals essential for plant growth are derived from the breakdown of bedrock as it is converted to soil. "As soil ages, this bedrock source is thought to be depleted, and these elements become increasingly scarce," Porder explains. "However, this process has been studied almost exclusively on uneroded surfaces, and thus it is uncertain how well it applies to actively-eroding landscapes."

In previous studies, Porder, Vitousek and their colleagues discovered that the erosion of mineral-rich rocks actually rejuvenated soils along several slopes in Hawaii, producing higher phosphorous concentrations.

The PNAS study was designed to determine the extent of nutrient rejuvenation in old Hawaiian forests. To find out, the researchers focused on a 7.7-square-mile area of Kauai that includes stable ridgetops, eroding slo
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Contact: Mark Shwartz
mshwartz@stanford.edu
650-723-9296
Stanford University
19-Jul-2005