A significantly greater proportion of brush-cleared plots had the Zapata bladderpod Physaria thamnophila than did uncleared plots. However, density of P. thamnophila, especially density of seedlings, was correlated positively with a canopy-cover index on the four sites studied. These apparently contradictory results may have been due to facilitation of seeds and seedlings of P. thamnophila by litter. Litter, normally associated with shrubs, was spread across the site by brush-clearing. Beneficial effects of brush-clearing (without disruption of soil) on P. thamnophila suggest that fire may have been part of its environment in the past. The four populations of P. thamnophila that we studied fluctuated widely in size and in rates of reproduction and establishment from year to year.
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Vol. 56 • No. 3