Pitcher's thistle (Cirsium pitcheri) is a federally listed monocarpic plant species endemic to the shoreline dunes of Lakes Huron, Michigan and Superior. Individual plants may require 4–8 y to mature, after which they flower and die. This life history and the lack of vegetative modes of reproduction make an understanding of seed and seedling ecology critical for preservation and restoration of Pitcher's thistle. We report conditions necessary to overcome seed dormancy and the effects of light, seed mass and depth of burial (0, 2, 4 or 8 cm) on seed and seedling success from laboratory experiments in controlled growth environments. Seeds of Pitcher's thistle are dormant when dispersed, but a combination of low temperatures and afterripening can break dormancy. Germination over 30% was obtained with at least 24 wk of low temperature moist stratification; 25% germination occurred after 6 mo of storage at room temperatures. Light suppressed germination of nondormant seeds. Although burial is required for germination, only 8% of seedlings emerged from a depth of 8 cm, associated with longer time to emergence. Achenes of this taxon lack endosperm; seedlings probably lack the reserves to overcome excessive burial. Logistic regression was used to compare the relative effects of the presence or absence of light, burial depth, seed mass and differences between populations from Michigan's upper and lower peninsulas. All three variables were significant predictors of germination and emergence when tested alone. Seeds from the upper peninsula were more likely to germinate, provided they were buried, although their probability of emergence decreased at greater burial depths. Although seeds from the upper peninsula were significantly heavier on average than those from lower peninsula populations, seed source was an even better predictor of seed and seedling success than seed mass. This suggests genetic differences among populations even within this geographically restricted endemic. Seeds of Pitcher's thistle can remain viable 1–2 y in the laboratory, suggesting this species can maintain a seed bank, although it is ephemeral. Seeds and seedlings of Cirsium pitcheri successfully exploit the dynamic nature of their dune habitats. Our results suggest that conservation efforts must consider seed storage conditions, genetic source of seeds and seed size, as well as maintenance of natural sand erosion and accretion regimes for preservation and restoration of this taxon.
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Vol. 143 • No. 1