We sampled mollic epipedons in adjacent forests and grasslands and under and beyond the crowns of three old Douglas fir (Pseudotsuga menziesii) trees in southwestern Montana. Forest fire history was determined by removing cross sections from fire-scarred trees. Grassland soils had three times as many grass opal phytoliths as forest soils, and Douglas fir opal was found only in forest soils. Forest and grassland soil organic carbon contents did not differ significantly, and the old Douglas fir trees had no significant effect on soil organic carbon levels during their lifespan. Mean fire interval was calculated as 58 yr; frequent ground fires swept these forests. Opal data indicate that the forest-grassland ecotone has been stable and that forests have persisted long enough for mollic epipedons to have developed beneath them. Apparently, these forest soils have duplicated grassland Mollisol genesis because of the abundance of sylvan graminoids and the cool, dry climate. Because fire has become rare in modern times, stands are closing and graminoid undergrowth is reduced.
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