Erosion, Geological History, and Indigenous Agriculture: A Tale of Two Valleys |
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Authors: | Peter M Vitousek Oliver A Chadwick George Hilley Patrick V Kirch Thegn N Ladefoged |
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Institution: | (1) Department of Biology, Stanford University, Stanford, California 94305, USA;(2) Department of Geography, University of California, Santa Barbara, California 93106, USA;(3) Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA;(4) Departments of Anthropology and Integrative Biology, University of California, Berkeley, California 94720, USA;(5) Department of Anthropology, University of Auckland, Auckland, New Zealand |
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Abstract: | Irrigated pondfields and rainfed field systems represented alternative pathways of agricultural intensification that were
unevenly distributed across the Hawaiian Archipelago prior to European contact, with pondfields on wetter soils and older
islands and rainfed systems on fertile, moderate-rainfall upland sites on younger islands. The spatial separation of these
systems is thought to have contributed to the dynamics of social and political organization in pre-contact Hawai’i. However,
deep stream valleys on older Hawaiian Islands often retain the remains of rainfed dryland agriculture on their lower slopes.
We evaluated why rainfed agriculture developed on valley slopes on older but not younger islands by comparing soils of Pololū
Valley on the young island of Hawai’i with those of Hālawa Valley on the older island of Moloka’i. Alluvial valley-bottom
and colluvial slope soils of both valleys are enriched 4–5-fold in base saturation and in P that can be weathered, and greater
than 10-fold in resin-extractable P and weatherable Ca, compared to soils of their surrounding uplands. However, due to an
interaction of volcanically driven subsidence of the young island of Hawai’i with post-glacial sea level rise, the side walls
of Pololū Valley plunge directly into a flat valley floor, whereas the alluvial floor of Hālawa Valley is surrounded by a
band of fertile colluvial soils where rainfed agricultural features were concentrated. Only 5% of Pololū Valley supports colluvial
soils with slopes between 5° and 12° (suitable for rainfed agriculture), whereas 16% of Hālawa Valley does so. The potential
for integrated pondfield/rainfed valley systems of the older Hawaiian Islands increased their advantage in productivity and
sustainability over the predominantly rainfed systems of the younger islands. |
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