Bracted twistflower (Streptanthus bracteatus): The ecology of a rare Texas endemic
The bracted twistflower (Streptanthus bracteatus), a member of the Brassicaceae (mustard) family, is a rare endemic of the Edward's Plateau ecoregion of Texas. There appear to be many factors affecting its distribution. This study examined plants in a controlled environment and in the field. Factors examined in this study included seed dormancy, light levels, soil nutrients, herbivory, soil water content, and soil depth. Plant response variables were shoot height (cm), number of leaves, basal diameter (mm), diameter of basal rosette (cm), above and belowground dry mass, and total dry mass (g). The light experiment demonstrated that plants responded positively to higher light levels for basal diameter. Plants responded better to higher levels of soil nutrients but the response was minimal. The herbivory experiments consisted of simulated and actual herbivory treatments. Plants showed no clear response to simulated herbivory. However, in the field, herbivory showed negative effects on bracted twistflower by a large herbivore, likely white tailed deer (Odocoileus virginianus). Plants in the full sun exclosures had higher dry mass than plants in the full sun without exclosures. Full sun plants in the exclosure had higher dry mass than shaded plants in exclosures. Populations at two field sites, Eisenhower Park and Rancho Diana Natural Area, both located in Bexar County, were monitored. Response of plants at Eisenhower Park to light levels, soil moisture, and soil depth were examined. No significant linear correlations were found for plant responses to light levels and soil water content. However, there was a positive relationship between soil depth and number of leaves, shoot height, basal diameter, and number of flowers. Plants at Rancho Diana showed a positive response in terms of shoot height and basal diameter to increasing soil water content.