American columbo (Frasera caroliniensis) COSEWIC assessment and status report: chapter 8
Biology
- Life Cycle and Reproduction
- Herbivory
- Physiology
- Dispersal/Migration
- Interspecific Interactions
- Adaptability
Frasera caroliniensis was the focus of a Masters thesis completed by Paul F. Threadgill in 1979. Much of his research focused on the unusual reproductive behaviour of this species: F. caroliniensis is a monocarpic perennial. Monocarpic perennial plants live for many years, but flower only once and then die (Harper, 1977). In eastern North America this life history strategy is usually associated with weedy biennial or short-lived perennial species that thrive in frequently disturbed habitats. Frasera caroliniensis differs from such species in its much longer life span and affinity for stable habitats. Threadgill and his supervisors published a series of papers on the biology of F. caroliniensis, and nearly everything that is known about this species was first documented in these publications (Threadgill et al., 1979, 1981a, b, c; Baskin and Baskin, 1986). It should be noted that this research was conducted in Kentucky. The results should therefore be generally applicable to Ontario populations, but specific details may vary. Phenology in particular would be expected to vary between Ontario and Kentucky populations; the flowering times reported by Threadgill et al. (1981a) are slightly earlier than those observed in Halton (personal observation).
Several other more recent papers on this species were reviewed for information on the biology but contained little significant data (Pringle, 1993; Horn, 1997; Floyd and Huneycutt, 2000).
Many floras refer to F. caroliniensis as a biennial, triennial, or short-lived perennial (Threadgill et al., 1981a). However, McCoy (1949) noted that it requires six or seven years of growth before flowering, but provided no evidence to support his statement. Steyermark reported that a plant transplanted to his garden had not flowered in 15 years (1963 - cited by Threadgill et al., 1981a). Following several years of intensive research, Threadgill et al. (1981a) could not determine how old a plant needed to be to flower. His evidence suggested that size was likely an important factor. However, the largest juvenile plants were bigger than the smallest flowering plants, indicating that size was not the only limiting factor.
Indeed, Threadgill’s data revealed a strong tendency for populations to bloom synchronously, echoing the observations of earlier workers. Threadgill hypothesizes that this may be part of an evolutionary response to pollinator competition. By storing up resources over many years prior to flowering, plants can produce massive inflorescences when they finally reach reproductive maturity. By flowering synchronously the population provides an overwhelming abundance of flowers for local pollinators. As a consequence the pollinators can temporarily ignore other nectar sources, and in the process maximize the intraspecific transfer of F. caroliniensis pollen.
This reproductive strategy depends on the presence of generalist pollinators, as any pollinator specializing in F. caroliniensis would face local extinction in the years between flowering events. Understanding pollinator dynamics is an important component of plant conservation (Leigh, 2003; Morris, 2003), but the available evidence for F. caroliniensis suggests it should not be the source of major concern in this case. Threadgill et al. (1981b) found a number of hymenopterans in the family Apidae to be the most effective pollinators for F. caroliniensis, including the common, widespread generalist pollinator Apis mellifera and several Bombus spp. In light of this information, it is unlikely that F. caroliniensis is vulnerable to the loss of its most important pollinators.
Threadgill et al. (1981b) also discussed the possible role of irregular synchronous flowering in F. carolinensis as a strategy to avoid seed predation. They found 25% of the seed crop in 1976 was lost to invertebrate seed predators, but more research is needed to determine how important seed predators are in this species. We observed gastropods feeding on F. caroliniensis foliage during the 2004 field season, but could not determine if they presented a serious threat to the plants.
Little is known of the physiology of this species, other than what can be inferred from the climatological conditions that occur across its range (Threadgill et al., 1979).
Frasera caroliniensis has a peculiar form of seed dormancy, described by Threadgill et al. (1981c) and Baskin and Baskin (1986). Seeds remain dormant until they have imbibed water and undergone a period of embryological development at about 5oC. Embryological development does not occur at higher temperatures. As a consequence seeds that drop in the fall or early winter will imbibe moisture from the soil, undergo the necessary development cycle, and germinate the following spring. Seeds that remain within the capsule until late winter or the following spring are kept dry, preventing them from completing the embryological development until the following winter, finally germinating the second spring after flowering. This effectively spreads the germination of a single year’s seed crop over two (or potentially three) years. As Baskin and Baskin (1986) explained, “such a germination pattern may be important in maintaining a wide spread of distribution of sizes and ages of plants in the population...[buffering] this long-lived monocarpic perennial with synchronous flowering against extinction at the local population level by ensuring that many plants remain vegetative in a flowering year.”
No data are available regarding the dispersal of this species. It is apparently gravity dispersed, making it extremely unlikely to disperse across areas of unsuitable habitat, such as between existing Canadian populations or between Canadian and American populations.
Threadgill’s (1981b) study of floral ecology, discussed above, is the only known investigation of interspecific interactions involving Frasera caroliniensis.
As mentioned above, the long lifespan of this species may allow it to persist temporarily in degraded habitats. This species was propagated experimentally as part of a conservation seed bank program at Royal Botanical Gardens in Hamilton, but no ex situ stock remains.
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