Pedogenesis of a coastal climosequence and a volcanic ash-influenced elevational transect of western Haleakalā, Maui

The western slope of Haleakalā, Maui, demonstrates a wide range in soil development (eight mapped soil orders), thus providing a unique opportunity to investigate how climate and volcanic ash deposition influence soil development on basalt. The objective of this study was to examine soil chemical and physical properties across climatic gradients and elevation to address how climate and relatively recent volcanic ash deposition affect pedogenesis across western Haleakalā. Sixteen pedons were sampled: five in a coastal climosequence uninfluenced by volcanic ash (15–224 m elevation; 461–2768 mm mean annual precipitation [MAP]; 22°C–23°C mean annual temperature [MAT]; 3357–5577 mm potential evapotranspiration [PET]), and eleven in an elevational transect variably influenced by volcanic ash (73–1362 m elevation; 283–2267 mm MAP; 13°C–24°C MAT; 1555–2704 mm PET). Pedogenic thresholds for dynamic soil properties (base saturation [BS], pH, organic C, and Al/Si extracted by ammonium oxalate) occurred at about 0.4 MAP/PET (1500 mm MAP) along the coastal climosequence and at about 0.8 MAP/PET (1600 mm MAP) along the elevational transect due to an increase in soil moisture availability and leaching potential. Greater clay/Fe (extracted by citrate dithionite [CD]) and crystalline Fe (CD minus hydroxylamine hydrochloride-hydrochloric acid [HH]) in drier lowland soils more distant from the summit indicate they are likely older and occur on older landforms. The ratio MAP/PET is an effective climatic index for understanding trends in pedogenesis and indicating pedogenic thresholds in climosequences and elevational transects in Hawaiian ecosystems.