Arid to humid serpentine soils, mineralogy, and vegetation across the Klamath Mountains, USA

• Serpentine is extensive at about 42°N latitude in the Klamath Mountains.
• It has arid steppe with Mollisols, and wooded and forested Alfisols and Ultisols.
• Major clay minerals are smectite in dry soils to goethite in moist, weathered soils.
• Some serpentine soils with ≥ 27% “free” iron have net positive charges.
• Net positive charges are highly unusual in nontropical soils lacking tephra.

Ultramafic rocks are extensive in the Klamath Mountains of California and Oregon and there is a great diversity of climate, soils, and vegetation. Soils were sampled and vegetation described over serpentinized peridotite at sixteen low altitude, well drained sites from arid to humid parts of the Mountains receiving from 400 to 3200 mm/year of precipitation. The soils are dry Mollisols and Alfisols, moist Alfisols, Ultisols, and a moist Mollisol. All of the soils have subsoil exchangeable Ca:Mg ratios < 0.5 mol/mol. Subsoil dithionite extractable, or “free”, iron (Fed) ranged from 1.5% at a dry site about 130 km from the Pacific Ocean to 27% at a much wetter site near the coast. With “free” iron increases from 1.5 to 27%, soil pH differences in molar KCl and in distilled water decrease from about 0.7 to − 0.1, indicating net positive charge in the soils with very high “free” iron contents. Net positive charges in soils lacking tephra are unique for nontropical soils. The main clay minerals, other than serpentine and chlorite inherited from the soil parent materials, are smectite in the drier soils and goethite in the wetter soils with more “free” iron. A warm dry site at 40.2°N had chamise chaparral with scattered gray pine trees. Plant communities on the cooler remainder of the transect, near 42°N latitude, from arid to humid, were sagebrush steppe, open conifer forest with shrubs and grass, semidense conifer forest with shrubs, and dense conifer forest.