Streptanthus bracteatus A. Gray, bracted twistflower (Brassicaceae), is a rare, endemic of the Edwards Plateau ecoregion of central Texas. We examined effects of light levels on the growth of S. bracteatus in a field experiment and also the combined effects of a canopy ( /-) and exclosure ( /-, herbivory) in a 2 × 2 factorial field experiment. Plants in the light experiment in high light had significantly greater growth compared to plants in the low light treatments. For most variables, plants in the factorial field experiment were negatively affected by herbivory (likely white-tailed deer, Odocoileus virginianus) and the presence of woodland canopy with significant interactions between herbivory ( /exclosure) and canopy treatments ( /-). Survival was 100% in exclosures and 58% when unprotected. Plants protected from herbivory and in the open had the greatest basal diameter, height, aboveground, belowground, and total dry mass, while those not protected from herbivory had much lower responses. Plants in the open, full sun, but not protected from herbivory, were lower in basal diameter, shoot height, aboveground, belowground, and total dry mass. Response variables were dependent on the presence or absence of an exclosure and canopy. Streptanthus bracteatus appears to be a sun-adapted plant, but is susceptible to herbivory by large ungulate herbivores. Both factors appear to determine, in part, the niche of this rare species.
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Vol. 33 • No. 3