SOIL-ELEVATION RELATIONSHIPS ON A VOLCANIC PLATEAU IN THE SOUTHERN CASCADE RANGE, NORTHERN CALIFORNIA, USA

The main features of an altitudinal sequence were determined from mapping soils on a late Cenozoic volcanic plateau west-southwest of Lassen Peak, and from the analyses of 27 pedons sampled from 270 to 2030 m above sea level. This sequence is characterized by thermic Rhodoxeralfs in oak-grassland below 500 m through mesic Palexeralfs, mesic Xeric Haplohumults, frigid Andic Haplohumults, and Haplocryands in conifer forest. Soil depth and clay content are greatest in the lower part of the conifer forest, but soils in the oak-grassland are slightly redder. The characteristic (not necessarily predominant) clay minerals, from low to high altitude, are smectite, kaolinite, halloysite, hydrated halloysite, and allophane. Soil reaction ranges from slightly acid at all depths in the oak-grassland to strongly acid in the surface and very strongly acid in the subsoil at higher altitude. The soil organic carbon content increases from about 9 kg/m2-m in the oak-grassland to about 11 in the lower part and 25 kg/m2-m or more in the upper part of the conifer forest. Soil organic carbon to total nitrogen ratios range from 11 to 14 in the oak-grassland and from 25 to 35 in the surface horizons in the conifer forest; increase with elevation in the forest being relatively small compared to the abrupt increase in C:N ratio from grassland to forest.