Spatial Variation in Flowering Age in Cirsium pitcheri: Does This Imperiled Dune Thistle Hedge Its Bets?

For perennial plants, the optimal timing of flowering is a tradeoff between survivorship and fecundity, a classic example of bet-hedging. For many monocarpic perennials, there is an advantage in delaying flowering until the plant reaches a larger size and is thus capable of a higher fecundity. However, in delaying flowering, there is a risk that the plant will die before ever reproducing. Here, we investigate this fitness trade-off in the imperiled dune thistle, Cirsium pitcheri (Torr. ex Eaton) Torr. & A. Gray (Asteraceae) (Pitcher’s thistle). Using a long-term, spatially explicit demographic data set with yearly censuses, we found that C. pitcheri individuals on the foredune flowered at a younger age compared to individuals in other dune habitats. We tested two alternative hypotheses: (1) younger flowering plants were able to reach a flowering threshold size more quickly, and (2) these plants were somehow stressed, and slow growth was a signal for high likelihood of dying, resulting in early reproduction at a smaller size. Our results support the former hypothesis: plants in all areas of the dune did not differ in size the year prior to flowering, despite spatial differences in age of flowering individuals. Therefore we conclude that the foredune microenvironment where plants flowered at younger ages facilitates rapid growth and reproduction for this species and may represent optimal habitat. This result provides insight into the demographic patterns of C. pitcheri that may inform the conservation and recovery of this threatened species. Although adapted to the dynamic dune environments of the Great Lakes, C. pitcheri may be vulnerable to climate change-induced changes in its optimal foredune habitat.