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

Abstract

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Seed Dormancy and Seedling Ecophysiology Reveal the Ecological Amplitude of the Threatened Endemism Picris willkommii (Schultz Bip.) Nyman (Asteraceae)

“…However, the optimum range for the central ones (15–25 °C) was slightly higher than for the peripheral ones (15–20 °C), with the central seeds germinating faster (MGT 50 = 2.5 days) than the peripheral ones (MGT 50 = 6.1 days). These optimal germination temperatures are within the temperature ranges shown by other herbaceous plants of the Asteraceae family (15–30 °C), depending on the climatic conditions of the habitat and the germination season of each species [ 6 , 24 , 25 , 30 , 32 , 36 ]. The statistical significance of the Temperature × Seed type interaction revealed that the central seeds were much more sensitive to temperature than the peripheral ones, germinating in 80% when the temperature was close to the optimum.…”

“…In addition, the low water-use efficiency would hamper its spread into the surrounding sandy soils, as well as its ability to withstand increasingly frequent prolonged drought periods. The practically null emergence of plants during summer, even though some rainy episodes were recorded every studied year, would indicate that high temperatures (daily mean ≥ 24 °C) do not promote germination of this species [ 22 , 23 , 24 , 25 ], and/or that the seeds undergo some type of dormancy when they mature from which they are not released until late summer [ 10 , 26 , 27 , 28 , 29 ]. Similarly, once the mild temperatures of early autumn had passed (daily mean around 20 °C), the low temperatures (daily mean < 15 °C) of late autumn and winter considerably reduced germination, as occurs with many other species of the Asteraceae family [ 6 , 30 , 31 , 32 ].…”

“…The larger size of the peripheral seeds compared to the central ones implies a greater storage of energy reserves, which makes it possible to stay longer in the soil seed bank than the central ones [ 19 , 25 , 49 ]. In turn, the larger initial size of seedlings produced from the peripheral seeds makes them more competitive against other species in the colonization of both, gaps and (undisturbed) grassland with high competition, than the central ones due to their greater initial growth and their smaller specific leaf area (SLA) [ 50 ].…”

Demography and threats to population growth of Cirsium pitcheri, a threatened dune plant, in Wisconsin

“…Larinus carlinae has been observed to reduce the seed set of C. pitcheri by 60% (Gijsman et al., 2020 ). In addition, this seed‐eating weevil also significantly reduced seed mass, which is positively associated with germination probability in this species (Hamze & Jolls, 2000 ; Gijsman & Vitt, 2021 ) because larger seeds are more likely to emerge especially from areas with deep seed burial depth (Hamze & Jolls, 2000 ). When grown in a common garden, plants from our two sites (WFD and SC) were similar in their size and seed set; however, plants from SC produced seeds with higher mass than those from WFD (Gijsman et al., 2020 ).…”

“…Seeds germinate following a period of cold stratification. In the laboratory, additional seedlings flush after each of three successive cold treatments before the seed bank is exhausted (Gijsman & Vitt, 2021 ; Hamze & Jolls, 2000 ), and therefore, the species may form an ephemeral soil seed bank in situ (Hamze & Jolls, 2000 ). Because C. pitcheri does not spread vegetatively, recurring establishment by seedling recruitment is required to maintain populations.…”