Effect of capitulum position and weevil infestation on seed production of threatened monocarpic perennial, Cirsium pitcheri

Seed size and capitulum position drive germination and dormancy responses to projected warming for the threatened dune endemic Cirsium pitcheri (Asteraceae)

“…Larger C. pitcheri seeds have also been reported to have higher chances of infection from seed‐ and soil‐borne pathogens (Chen & Maun, 1999). These results are especially noteworthy given that L. planus oviposits eggs at higher proportions in secondary capitula at WFD (Gijsman et al., 2020). When combined with the lower germination probability of heavier seeds produced in secondary capitula at WFD, our results highlight the potential impacts of L. planus on C. pitcheri that go beyond seed predation.…”

“…The effects of seed weight on dormancy and germination, more specifically, have also been shown to depend on a seed's developmental position in a fruit or on a parental plant (Baskin & Baskin, 2014; Diggle, 1995; Susko & Lovett‐Doust, 2011). We had previously found that C. pitcheri seeds from terminal capitula are significantly heavier than those from secondary capitula and that flowering plants at both sites in 2017 produced fewer viable seeds in terminal and secondary capitula in 2016 (Gijsman et al., 2020). Following up on that work, here, we used seeds from senescing flowering plants in 2016 and 2017 to investigate the effects of C. pitcheri seed attributes on cumulative and individual seed germination, respectively.…”

“…For seeds from terminal capitula, germination probabilities increased with seed weight at 20 and 25°C and decreased at 30°C, but these effects were not significant. Given that terminal capitula at WFD produced few viable seeds in 2017 (Gijsman et al., 2020), our sample sizes for this capitulum position were small, which may have precluded our ability to distinguish any effects. For seeds from secondary capitula at WFD, seed weight significantly predicted germination probabilities at 25 and 30°C.…”

“…Seeds used in this study were collected in June 2016 and 2017 from a randomized sample of C. pitcheri flowering plants at two sites in Door County, Wisconsin: The Ship Canal Nature Preserve (SC, N = 34 plants in 2016, 15 in 2017) and Whitefish Dunes State Park (WFD, N = 43 plants in 2016, 15 in 2017). We used seeds collected from senescing flowering plants in 2016 for germination trials tracking cumulative seed germination ( N = 1,217 at SC, 1,078 at WFD) and in 2017 for trials tracking individual seed germination ( N = 1,073 at SC, 1,277 at WFD) (see Gijsman et al., 2020 for collection methods). For each site and year, we haphazardly selected a subsample of the total number of viable seeds produced by flowering plants in terminal and secondary capitula.…”

“…As a monocarpic perennial species, C. pitcheri flowers and sets seeds only once in its lifetime after growing as a rosette for 4-8 years (Havens et al, 2012;Jolls et al, 2019;Loveless, 1984). Flowering in this species is determinate, beginning with terminal capitula at the apex of main stems and progresses basipetally to axillary, secondary, and tertiary capitula that are produced at the tips of first-and second-order branches, respectively (Gijsman et al, 2020;USFWS, 2002). At the time of dispersal in late July, C. pitcheri seeds are dormant and are attached to a long pappus that helps parachute them across the dune (Chen & Maun, 1999;Hamzé & Jolls, 2000;Keddy & Keddy, 1984;USFWS, 2002).…”