Showy phlox (Phlox speciosa): COSEWIC assessment and status report: chapter 6

Biology

General

The taxon is a perennial with plants observed in the field varying from small, non-flowering, and essentially herbaceous plants, to larger plants with woody stems with showy displays of dozens of flowers. There is little information available in the literature on the biology and ecology of Phlox speciosa.

Reproduction

Because Phlox species vary in their mating systems, and no specifics were found for Phlox speciosa, it would be difficult to make any useful generalizations based on information from the genus as a whole.

The speciesdoes not have any structures specialized for asexual reproduction. Because Phlox speciosa has often been observed in flower in British Columbia (BC CDC 2002), the plant is assumed to reproduce primarily by seed. Propagation of plants can be done in the spring with ripe seed and cuttings can be made in late summer (SlabĂ˝ 2004). Pollinators were not observed in the field; however, flowering plants tend to grow in dense clusters, so reproductive individuals likely grow in close enough proximity to allow for effective pollen transfer. Site visits coincided with peak flowering, consequently no fruiting plants were observed.

Survival

The factors that affect the survival of Phlox speciosa are unknown. No diseases or herbivory of the plant were observed in the field.

Physiology

In British Columbia, the species grows within a narrow climatic range in a region with very dry, hot summers, but seemingly restricted to somewhat cooler sites based on aspect of the topography. Plants also grow within a narrow range of elevations, from 700 to 1100 m.

Like many perennial herbs, Phlox speciosa undergoes a dormant period during the winter. At most sites in British Columbia, the plant was in full bloom in the first week of June of 2003. In the sagebrush grassland habitat at the White Lake Ranch, the peak of blooming likely occurred in late May 2003, as most flowers had shriveled by early June.

Movements/dispersal

Dispersal by seed is likely the primary mode of dispersal of Phlox speciosa. In some species of Phlox, seed dispersal is aided by an explosive opening of the fruit, but it is not known whether this occurs in Phlox speciosa. The mechanism of pollen dispersal is unknown for Phlox speciosa.

Whereas Phlox speciosa is rare in southcentral British Columbia, the plant is widespread in the Pacific Northwest (Douglas et al. 1999; Hitchcock and Cronquist 1973). Due to the lack of information on dispersal abilities, if Canadian populations disappeared, it is uncertain whether populations in Washington State would eventually serve as a source for recolonization within the Okanagan valley. From the southernmost extent of occurrence of Phlox speciosa in British Columbia to the nearest population in Washington State is more than 120 km.

Nutrition and interspecific interactions

No specific information is available about the nutrient requirements or interspecific interactions of Phlox speciosa. Like many native grassland herbs, Phlox speciosa is probably vulnerable to competition from alien species.

Behaviour/adaptability

Given its limited range, Phlox speciosa is adapted to a relatively narrow range of climate but has some adaptibility to a degree of habitat disturbance.

Phlox speciosa seems to tolerate an intermediate level of disturbance within its range. The plant was often observed growing on the edge of soil and bedrock road-cuts and along roadsides. The taxon often grows in habitats grazed by cattle and deer, however, it was not observed to grow in habitats that have been severely overgrazed. Phlox speciosa habitats are often subject to natural fires, and require regular burning to maintain the plant communities. Some Phlox speciosa locations occur in areas that have been cleared or mechanically altered in the past, often growing with a higher proportion of introduced grasses and other species. Phlox speciosa also grows on slopes that are subject to natural erosion processes.

Phlox speciosa is grown as an ornamental plant in residential gardens in the United States.

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