Evolution of soil properties and plant communities along an extreme edaphic gradient

Plant-litter-soil interactions were investigated along a soil chronosequence occurring on five coastal terraces in northern California that range in age from about 100 000 to 500 000 YBP. Soil and plant communities vary dramatically and consist of slightly acidic, fertile soils supporting highly productive mixed-conifer forests on the youngest terrace to highly acidic, infertile soils supporting pygmy forests of dwarf (< 3 m) conifers and Ericaceae species on the older terraces. Soil genesis, soil nutrient status, soil acidity and plant community structure reach a steady state after approximately 250 000 years. All major species of the pygmy forest are polyphenol-rich, and those species that also grow elsewhere on the gradient have their highest concentrations of extractable polyphenols when growing on the most highly acidic, infertile sites. Polyphenols provide several beneficial effects on highly acidic and infertile soils, such as nutrient conservation, detoxification of soluble aluminum and enhanced phosphorus availability. Polyphenols may also provide some plant species with a competitive advantage by altering nitrogen cycling, allowing these plants to alter forest succession. The convergent evolution of polyphenol-rich plant communities on highly infertile soils throughout the world may be due to the ability of polyphenols to ameliorate acid soil infertility factors and regulate the nitrogen cycle. (C) 1999 Editions scientifiques et medicales Elsevier SAS.