Precontact vegetation and soil nutrient status in the shadow of Kohala Volcano, Hawaii

Humans colonized Hawaii about 1200 years ago and have progressively modified vegetation, particularly in mesic to dry tropical forests. We Use delta C-13 to evaluate the contribution Of C-3 and C-4 plants to deep soil organic matter to reconstruct pre-human contact vegetation patterns along a wet to dry climate transect on Kohala Mountain, Hawaii Island. Precontact vegetation assemblages fall into three distinct zones: a wet C-3 dominated closed canopy forest where annual rainfall is > 2000 mm, a dry C-4 dominated grassland with annual rainfall < 500 mm, and a broad transition zone between these communities characterized by either C-3 trees with higher water-use efficiency than the rainforest trees or C3 trees with a small amount Of C-4 grasses intermixed. The likelihood Of C-4 grass understory decreases with increasing rainfall. We show that the total concentration of rock-derived nutrients in the < 2-mm soil fraction differs in each of 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. Polynesian farmers exploited these naturally enriched soils as they intensified their upland agricultural systems during the last three centuries before European contact. (c) 2006 Elsevier B.V. All rights reserved.