Light Levels and Herbivory Partially Explain the Survival, Growth, and Niche Requirements of Streptanthus bracteatus A. Gray (Bracted Twistflower, Brassicaceae), a Rare Central Texas Endemic

ABSTRACT 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. Pl...