Spalding's campion (Silene spaldingii) COSEWIC assessment and status report: chapter 6

Biology

General

Silene spaldingii is a perennial herb arising from a simple or branched caudex formed above a long, slender taproot. Its shoot tips are buried in the ground during the plant’s dormant stage (typical of “geophytes”). Rhizomes or other means of vegetative propagation are lacking.

Reproduction

Silene spaldingii is a perennial. In the plant’s first year (if seeds germinate in the spring), only rosettes are formed; aerial shoots are produced in subsequent years. If seeds germinate the previous fall, the new plantlets may only produce vegetative stems in their first summer but flowering stems in subsequent years. Existing plants send up new vegetation in mid-May and become senescent by early September (Lesica 1999).

Silene spaldingii is protandrous, with the anthers maturing and dehiscing pollen before the styles extend and stigmas become receptive. Most plants flower for the first time when two years or older. Flowers are borne in a branched, terminal, inflorescence and, in Montana, bloom in July and set seed in August (Lesica 1999). Each flower persists for two to several days, and two or more flowers may be in bloom on the same plant. This arrangement promotes outcrossing, while also allowing selfing (Lesica 1993, Lichthardt and Gray 2002). Seeds are dispersed by being shaken from an orifice on the top of the mature ovary. Seeds will germinate with as little as four weeks of cold treatment, so germination likely occurs in fall as well as spring (Lesica 1993).

Research by Lesica (1993) indicates that Silene spaldingii is dependent on insects for pollination. At Lesica’s Montana study site (on Tobacco Plains), only Bombus nevadensis appeared to be an important pollinator.

Survival

Silene spaldingii is a moderately long-lived perennial, with at least 72% of plants surviving for at least five years in a study at Dancing Prairie Preserve. Survival of first-year plants is high, indicating that most mortality occurs in early seedling stages. This study also found recruitment of S. spaldingii to be sporadic, although the population growth rate remained stable over the seven-year period of the study. Since the plant is long-lived, populations are able to persist many years without recruitment (Lesica 1997).

The maximum and minimum temperatures for survival of S. spaldingii plants are unknown, although the maximum recorded summer temperature in the area (around Eureka, Montana) over the last 30 years is about 29°C, and the minimum winter temperature is -9.4°C (Eureka Ranger Station Normals, 2005). Climatic fluctuations are considered a threat to S. spaldingii populations, particularly drought conditions (USDI-FWS 2001, Lorain 1991).

Loss of habitat, as a result of heavy grazing, agricultural cultivation, chemical spraying, and invasion of introduced species, is the primary factor affecting survival and recruitment in S. spaldingii in the U.S. (Lorain 1991, Lesica 1999). In addition, it has been shown that fire enhances recruitment in S. spaldingii. Reduction of the litter layer, increased available nutrients and/or warmer soil temperatures, as a result of fire, increase seedling germination and/or establishment (Lesica 1999). Predation does not appear to be a significant factor affecting the survival of S. spaldingii, although insect predation has been observed in Oregon (seed weevils) and Montana (caterpillars) (Lorain 1991).

Physiology

Silene spaldingii plants exhibit prolonged summer dormancy and may not produce above-ground vegetation for 50% of their growing season (Lesica and Steele 1994, Lesica 1997). In Montana’s Tobacco Plains population, Lesica (1997) found that 41% of S. spaldingii plants exhibited prolonged dormancy each year between 1989-1994. In essence, this means that in a given population of S. spaldingii, there may be only about 40-50% of the population with above-ground shoots visible in a particular season. The causes of dormancy in S. spaldingii are unknown, although it can be theorized that it is a physiological response to environmental stresses such as drought or flooding (Lesica 1999).

Movements/dispersal

Due to the close proximity (approximately 1.3 km) of the Canadian Silene spaldingii population to a population in Montana, it is possible that some genetic dispersal could occur between the two populations, at least over longer periods of time. It is unknown, however, whether seed dispersal between populations actually occurs. The British Columbia and Montana populations are separated by a distance of 190 km from other S. spaldingii populations in Idaho and Washington (USDI-FWS 2001). Considering the localized means of seed dispersal from capsules, spread of invasive weeds that reduce habitat quality, and the relatively dry conditions within the region in the last few years, it is unlikely that seed dispersal and seedling establishment from nearby US sub-populations would occur. At best, the rescue effect would have a very low probability.

Nutrition and interspecific interactions

Nonnative plants compete with Silene spaldingii for water, nutrients and light (USDI-FWS 2001). In addition, nonnative flowering species such as Hypericum perforatum can compete with S. spaldingii for pollinators (Lichthardt and Gray 2002).

Research by Lesica (1993) indicates that S. spaldingii is dependant upon insect pollination, most likely by one species of Bombus. Without adequate pollinators, enforced selfing would result in inbreeding depression, which in turn could lead to extirpation of S. spaldingii populations after only a few generations (Lesica 1993).

Although there is evidence of predation/pests on S. spaldingii, it is unknown whether these have a significant direct impact on the plant in British Columbia. Grazing of S. spaldingii inflorescences by livestock or native herbivores is considered a serious threat in U.S. populations. In eastern Washington, rodent activity is affecting the persistence of S. spaldingii. Insect predation of seeds also reduces the reproductive success of the species (USDI-FWS 2001).

Behaviour/adaptability

The specialized habitat requirements of Silene spaldingii suggest that the species is poorly adapted to forest invasion (resulting from fire suppression), and is adapted to moderate disturbance. Forest invasion is not presently a threat at the British Columbia site. The species is a poor competitor with nonnative vegetation. Although S. spaldingii can be found in disturbed grasslands, such as the British Columbia population, it has disappeared in many degraded sites in the United States. The species tolerates, and may benefit from, moderate periodic disturbance such as fire or moderate grazing but not heavy grazing or trampling, which encourages growth of nonnative species (Lesica 1999).

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