Natural Resources, School of



Soil morphology of a debris flow chronosequence in a coniferous forest, southern California, USA

Date of this Version



Turk, J.K., B.R. Goforth, R.C. Graham, and K.J. Kendrick. 2008. Soil morphology of a debris flow chronosequence in a coniferous forest, southern California, USA. Geoderma 146:157-165.




Soils on a series of debris flow deposits, ranging from < 1 to 244 years old, were described and sampled in order to investigate the early stages of soil development. The parent material at the site is debris flow regolith, composed mainly of gneiss, the soil moisture regime is xeric, and the vegetation is mixed coniferous forest. Ages of the deposits were assessed using dendrochronology. Morphologic trends in the organic horizons included a thickening of the humus form over time, along with the development of Fm and Hr horizons. The humus forms underwent a progression from Mormodors (20 years old), to Hemimors (26–101 years old), and finally Lignomors (163 years old) and Resimors (184–244 years old). Changes in physical properties of the uppermost mineral horizons as a function of increasing age included a decrease in the volume of coarse fragments, a linear decrease in bulk density, and a darkening and reddening of the soil color. No significant soil development took place in the subsoil during the time span of this chronosequence. The soils described were classified as Typic Xerofluvents and Typic Xerorthents (Regosols and Leptosols). Buried A horizons were observed in many of the soils. Where the A horizons could be linked to dendrochronology to assess the age of the buried surface, we found that the properties of the buried A horizons do not serve as a good indicator of the age of the surface. This study suggests rapid development of the humus form profile (organic horizons and A horizon) following debris flow deposition and rapid degradation of these horizons when the debris flow surface is buried.