Muhlenberg's centaury (Centaurium muehlenbergii COSEWIC assessment and status report: chapter 6
Biology
There is little published information available on the biology of Centaurium muehlenbergii. The sections that follow present summary information gleaned from floras, published literature on other Centaurium species, personal communications with Centaurium specialists, and the authors’ own (unpublished) field observations.
Life cycle and reproduction
Centaurium muehlenbergii is a small herb of open moist habitats that flowers in the early summer. The fruit is a slender, 2-valved capsule containing numerous small seeds. Although successful fruit set has been observed in recent field surveys on Vancouver Island, rates of seed production and seedling recruitment have not been documented. The pollination system of C. muehlenbergii is also unknown (J. Pringle, pers. comm. 2001), although autogamy (self-pollination) and out-crossing are both known to occur within the genus (Kugler 1977). It is likely that allopolyploidy has occurred in several clades of Centaurium, and the C. muehlenbergii complex shows evidence of diverse introgressive hybridization (G. Mansion, pers. comm.).
Most authors refer to Centaurium muehlenbergii as an annual. The PLANTS Database (USDA-NRCS 2005) lists it as an annual or biennial. Hickman (1993) suggest that it may persist as a biennial if damaged.
No information is available on mortality rates or on the factors affecting the survival of Centaurium muehlenbergii at different stages of its lifecycle. However, the closely related C. erythraea is known to possess a large soil seed bank (Davies and Waite 1998), and it is possible that C. muehlenbergii also relies to some extent on a dormant seed bank to act as a buffer during periods of unfavourable environmental conditions.
Herbivory
There have been no observations of direct herbivory on Centaurium muehlenbergii. However, the Oak Bay site has been grazed by livestock in the past and is now heavily grazed by rabbits and deer, while the Gulf Island site hosts a large breeding population of Canada geese. The impact of these different herbivores on C. muehlenbergiiis unknown, although it is possible that, in some instances, grazing benefits the species by suppressing competing vegetation (M. Fairbarns, pers. comm. 2003).
Physiology
Species in the genus Centaurium are found in a wide range of habitats. Most are calcicolous, some are acidiphilous (e.g., C. scilloides), and a few are restricted to gypseous soils. However, edaphic conditions appear to be of less importance overall to Centaurium germination and growth than the amount of available moisture and sunlight (G. Mansion, pers. comm. 2001).
Dispersal
Centaurium muehlenbergii does not appear to have any specialized seed dispersal mechanisms. Most germination likely occurs within a few centimetres of the parent plant. The recent discovery of C. muehlenbergii near Nanaimo likely represents the finding of an unreported and established population that had previously been missed due to the extremely small number of plants (7). There have been no known instances of dispersal to a new area, or even of local colonizations, in the several decades since the species was first identified on southern Vancouver Island.
Interspecific interactions
Interspecific interactions are largely unknown. McGee (1985) reported that the related Centaurium erythraea was dependent on mycorrhizal infection for growth and survival under low nutrient conditions.
Most Centaurium species do not compete well with other species and therefore are generally found in disturbed or open places (G. Mansion pers. comm. 2001). C. muehlenbergiialso appears to be a poor competitor and may be susceptible to competition from invasive species, especially introduced grasses (M. Fairbarns, pers. comm. 2003).
Adaptability
Aside from the possible existence of a seed bank, and anapparent ability to tolerate seasonally fluctuating moisture levels, Centaurium muehlenbergii is not known to possess any particular specializations that would allow it to adapt readily to sudden change or to stochastic events such as fires, extended drought, or human exploitation. No information relating to successes in artificial propagation, seed germination, or horticultural use was found. The feasibility of using cultivated seeds from horticultural sources for transplanting is also unknown. A closely related species, C. erythraea(common centaury, feverwort) is widely grown as a medicinal herb and appears in numerous garden and nursery seed catalogues in Canada, the U.S., and Europe. Planting instructions for this plant generally recommend seeds be sown in situ, in “average” to sandy soils, in full sun to partial shade.
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