Population demography and seed and seedling ecology of the Great Lakes endemic, Pitcher's Thistle (Cirsium pitcheri, Asteraceae)

TitlePopulation demography and seed and seedling ecology of the Great Lakes endemic, Pitcher's Thistle (Cirsium pitcheri, Asteraceae)
Publication TypeThesis
Year of Publication1998
AuthorsHamze SIssami
DegreeMaster of Science
Number of Pages114 pp.
UniversityEast Carolina University
CityGreenville, NC

Cirsium pitcheri is a monocarpic perennial endemic to the Great Lakes dunes. I examined the population demography, seedling dispersion, and seed and seedling ecology of C. pitcheri in Michigan. I also measured the effects of removal of an invasive thistle population demography. A three-year study in a 1600 m2 plot revealed that Pitcher's thistle seedling recruitment and survival are variable among years; seedling and juvenile stages are the most sensitive. Non-destructive morphometric measurements indicated that plants remain in the vegetative stage for several years before flowering. Flowering is size-dependent; belowground biomass (taproot) was a better predictor of flowering than aboveground biomass. Seedling dispersion was measured around 106 potential parents. Approximately 50% of parents had seedlings within 5 m and successful parents had at least one seedling within 1.27 m. Despite wind dispersal of seeds, there was no directionality to seedling dispersion around parents. Most seedlings (80%) were found in areas of at least 60% bare sand. Seed dormancy was broken by time and moist-stratification. Germination was highest in the dark. Burial increased the probability of germination, but burial at 8 cm decreased seedling emergence. In the presence of an exotic, Gypsophila paniculata, C. pitcheri abundance decreased during a 15 year period. Weed removal with an ax did not appear to alter the demography of the endemic species; thistle recruitment actually increased. The proportions of seedlings and flowering C. pitcheri were low in the weed's presence suggesting competitors may delay flowering. The life history of Pitcher's thistle may buffer populations from extirpation. Plants achieve flowering size over a period of several years resulting in a continuous inputof seeds to the populations. Seedling abundance in areas of high sand cover suggests that this species is dependent on the dynamic processes of the dunes. The probability of germination increased with burial; this further suggests dependence of Pitcher's thistle on sand movement. Pitcher's thistle populations appear to be in decline where invasives decrease sand movement. Therefore, the long-term protection of this taxon is ultimately dependent on the dynamic processes that maintain open sand dunes around the Great Lakes.