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Interaction between soil acidity and vegetation phenolic concentration was investigated to identify mechanisms by which forests sustain productivity on extremely acidic, infertile soils. Contrasting soils on well-preserved marine terraces of the "Ecological Staircase" near Mendocino, CA comprise an extreme edaphic gradient. Pygmy forests of dwarf (< 5m) Mendocino cypress (Cupressuspygmaea), Bolander pine (Pinus contorta var. bolanderi), and Bishop pine (Pinus muricata) are found on the oldest, most acidic soils, and along a gradient that includes three distinct levels of soil acidity, with pH(CaC12) ranging from 5.0 to 3.0 in the upper mineral soil and from 4.0 to 2.0 in the litter layer where fine roots are concentrated. Mature foliage was collected from five sites on this edaphic gradient. Aqueous methanol extracts of the samples were analyzed for concentrations of total phenols and condensed tannin by the Prussian blue and acidified vanillin assays, respectively. There were significant differences (p < 0.05) in foliar condensed tannin and phenolic concentrations within each species and concentrations were inversely related to soil pH, approximately doubling along the gradient. Natural selection for soil-regulated variation in polyphenol concentration is interpreted in the context of plant-litter-soil interactions as an adaptation that permits these conifers to survive in extremely acidic soils.

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