Life History Traits and the Effects of Disturbance on the Ashy Dogweed (Thymophylla tephroleuca): A Narrow Texas Endemic
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Thymophylla tephroleuca, a federal and state endangered plant species, was monitored in an attempt to document certain reproductive and population level traits. The study was conducted from September 1999 to June 2001 on three populations in Webb and Zapata Counties, Texas. Life history traits examined were breeding system, phenology, pollen viability, floral insect visitors and achene viability. In addition, population level attributes such as recruitment, density and size structure of individuals were examined. Precipitation and temperature were obtained from the National Climatic Data Center. Plant species associated with T. tephroleuca habitat were collected. GPS mapping was performed to determine population area of the study sites. Achenes were collected for a genetic reserve of T. tephroleuca. A controlled, replicated experiment was also conducted documenting the effects of anthropogenic disturbances on T. tephroleuca recruitment and growth. The disturbances included root-plow, blade and root-plow seeded with buffelgrass (to document competitive effects).
Insects of the orders Coleoptera, Diptera, Hymenoptera and Lepidoptera were observed and captured on T. tephroleuca flowers. Fruit set mainly occurred in xenogamous crosses, with 88.8% achene set. Peak flowering months for field plants were from May to November. However, flowering appears to be initiated by rainfall events, as individuals will flower all year long with sufficient watering. Pollen viability in field plants averaged 82.5% in 2000 and 86% in 2001. Achene viability averaged 36.5% in 1999 and 8.3% in 2000. Density, recruitment and size of individuals varied significantly between two populations. In one population (previously disturbed), plants were small and density at the end of monitoring was 4.1 plants/m2. Recruitment was observed at 21 plants for the year. In a second population (not previously disturbed), individuals were larger and density was lower throughout the study period at 0.6 plants/m2. No recruitment was observed in this population. In addition to the results obtained from these two populations, the controlled field experiment suggested that soil disturbance increases plant density. A significantly higher number of individuals were recruited into the root-plow plots than in root-plow seeded with buffelgrass or control plots. No significant difference was observed between root-plow and bladed plots. Aboveground and belowground biomass accumulation was also significantly greater in the root-plow plots than in bladed or control plots.
Results indicate that T. tephroleuca is an outcrossing species with a variety of potential pollinators. Pollen viability was consistently high, while achene viability was low and variable depending on the year. Density, recruitment, and size structure of individuals was variable between the two monitored populations, presumably due to site characteristics. Results of the disturbance experiment indicate that root-plowing increases density of plants. Landowners can now make informed decisions about the effects of certain land management practices on Ashy dogweed populations that reside on their ranches.
CitationDodson, D. P. (2001). Life history traits and the effects of disturbance on the ashy dogweed (Thymophylla tephroleuca): A narrow Texas endemic (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.
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