A CHRONOSEQUENCE OF SOILS AND VEGETATION ON SERPENTINE TERRACES IN THE KLAMATH MOUNTAINS, USA

Abstract

A sequence of serpentine soils (soils with ultramafic parent materials) on four Quaternary terraces was characterized in southwestern Oregon. The soils are Mollisols (Phaeozems), Inceptisols (Cambisols), and Alfisols (Luvisols and Acrisols) that range from thousands to hundreds of thousands of years old. Weathering has practically eliminated olivine from the oldest soil and greatly diminished serpentine from both sand and clay fractions. Citrate-dithionite-extractable Fe increased from 21 g kg−1 in the Bw horizon of the youngest soil to 166 g kg−1 in the fine earth (soil <2 mm) of the Bt2 horizon in the oldest soil, and the cation exchange capacity (pH 8.2) has decreased from >0.4 to about 0.1 mmol+/g of clay. Base saturation decreased from 75% in the slightly acid Bw horizon of the youngest to 17% in the moderately acid Bt2 horizon of the oldest soil. Nevertheless, the oldest soil is an Alfisol (or Acrisol), because the base saturation is 44% at 125 cm below the top of a kandic horizon. Tree growth, as gauged by timber site index, increases slightly as the estimated available-water capacity in the soils increases from 6 cm in the 100-cm depth on the youngest terrace to 10 cm on the penultimate terrace, and then tree growth increases greatly from the penultimate to the ultimate (oldest) terrace as excess Mg is leached from the soil. The timber site index indicates that the expected (from site curves) conifer height on the oldest terrace is 26 m in 50 years, or 55 m in 300 years.