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Oliver A. Chadwick and others Department of Geography, University of California, Santa Barbara, CA 93106; USA Abstract Humans colonized Hawai’i about 1200 years ago, and have progressively modified vegetation, particularly in mesic to dry tropical forests. We use 13C to evaluate the contribution of C3 and C4 plants to deep soil organic matter, in order to reconstruct pre-human contact vegetation patterns along a wet to dry climate transect on Kohala Mountain, Hawai‘i Island. Precontact vegetation assemblages fall into three distinct zones; a wet C3 dominated closed canopy forest where annual rainfall is > 2000 mm, a dry C4 dominated grassland with annual rainfall < 500 mm, and a broad transition zone between these communities characterized by either C3 trees with higher water-use efficiency than the rainforest trees or C3 trees with a small amount of C4 grasses intermixed. The likelihood of C4 grass understory decreases with increasing rainfall. The behavior of rock-derived nutrients differs in these vegetation zones. Nutrient losses are driven by leaching at high rainfall, and by plant cycling and wind erosion at low rainfall. By contrast, nutrients are best preserved in surface soils of the intermediate rainfall zone, where rainfall supports abundant plant growth but does not contribute large amounts of water in excess of evapotranspiration. It was this intermediate rainfall zone with adequate rainfall and fertile soils that Polynesians exploited to develop an extensive dryland agricultural complex. |
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