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 δ13C to evaluate the contribution of C3 and C4 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 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. 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.