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ABSTRACT-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.
RESUMEN-Physaria thamnophila estaba presente en un número significativamente mayor de parcelas con remoción de arbustos que en las parcelas sin remoción de arbustos. Sin embargo, la densidad de P. thamnophila, especialmente la de las plántulas, estuvo positivamente correlacionada con el índice de cobertura arbustiva en los cuatro sitios de estudio. Ambos resultados, aparentemente contradictorios, pueden deberse a que la broza ofrezca un efecto facilitador para las semillas y plántulas de P. thamnophila. La broza, normalmente acumulada bajo los arbustos, estaba esparcida a través del sitio debido a la remoción de los arbustos. El efecto positivo para P. thamnophila de la remoción de los arbustos (sin perturbar el suelo) sugiere que los incendios formaban parte de su ambiente en el pasado. El tamaño y las tasas de reproducción y establecimiento de las cuatro poblaciones de P. thamnophila incluidas en este estudio fluctuaron ampliamente año con año.
Understanding ecological requirements of an endangered species of plant often is essential to its preservation and successful recovery (Menges and Dolan, 1998; Pfab and Witkowski, 2000; Brys et al., 2004; Colling and Matthies, 2006). It is especially important when the species has become restricted or nearly restricted to suboptimal sites. For example, railroad right-of-ways are often the only habitat leftfor prairie species, but are not optimal for them (Harrington and Leach, 1989). When the mismatch between current and optimal habitat is not obvious, ecological requirements of the species may be misunderstood, reducing effectiveness of conservation actions. This is especially likely to be the case where grazing, suppression of fire, hunting, and other...