Western spiderwort (Tradescantia occidentalis) COSEWIC assessment and status report: chapter 6

Biology

General

Western spiderwort flowers from May to July. Each flower lasts only one day. In 1996, the western spiderwort flowered from June to mid-August, and released seed from early August until the first snowfall in October. The species produces aboveground shoot buds in the fall, and overwinters in this vegetative state. Flower color ranges from white to pink to the most common form, deep purple. The rare pink variety occurs at only two of the five sites and is distributed in patches within these sites (Goulet and Kenkel 1997, Smith 2001B).

Reproduction

While self-sterility is common within the genus Tradescantia, previous reports have speculated that selfing may occur in western spiderwort. The plant reproduces by seed and by vegetative propagation. Moist stratification is required for seed propagation. The species may propagate vegetatively by producing root buds from stem portions (Smith and Bradley 1990).

In Manitoba, a comparison of flowering plants in 1996 (Goulet and Kenkel 1997) and 1993 (Hohn 1994) found the proportion of plants flowering in 1996 was much lower than in 1993 (Hellman pasture, 72% in 1993 vs. 30% in 1996; MHHC, 52% in 1993 vs. 27.5% in 1996). These results may reflect differences in the definition of an 'individual'. In 1996, about half of all plants flowered in the ungrazed ESA (Routledge Sand Hills) and the eastern population in the Lauder Sand Hills. The lower level of flowering at the grazed portion of the Routledge Sand Hills (30%) was attributable to cattle grazing. Results from both the 1993 and 1996 surveys indicate that flowering is lowest at the MHHC site (Lauder Sand Hills), which may indicate suboptimal conditions at this site (Goulet and Kenkel 1997, Hohn 1994).

Life History

The following account is based on Goulet and Kenkel (1997). On average, most plants produce one or two stems (mean = 1.5). The species invariably produces a single, multi-stemmed shoot. In fact, only one plant with two shoots was found, about half the typical size, in a highly trampled area in the pastures at the Routledge Sand Hills. The root systems are slender and somewhat fleshy. These fleshy roots extend 20-100 cm into the soil, the finer roots extend deeper still.

Plants located in early May of 1996 were already 5-15 cm in height, and consisted of two or four leaves branching at the ground. Flowering buds first appeared on June 12, 1996 in a tight cyme. Each day, a single pedicled bud is raised and forms a flower the following morning. The first flower was seen on June 28, 1996 about a week later than in 1993 and 1994 (Hohn 1994). Each flower lasts a single day, and usually closes by mid-day. At the same time, a second flowering bud is raised to flower the following day. This process continues until all the buds of the cyme have flowered. The closing of a pollinated flower results in the production of a fleshy structure hanging out from the closed sepals. This structure hardens the following day. Flowering peaked in mid-July, but continued into mid-August of 1996. Some flowering shoots continued to produce new floral buds even after the first capsule in the cyme had dehisced. Following initiation of the first cyme, many plants produced one or more additional cymes.

Plants occurring in small clumps were often at the same stage of floral development. By contrast, some plants in larger clumps failed to flower, and those that did were often at different stages of floral development. Like many perennial species, western spiderwort may not flower until it is sufficiently large and/or old enough.

After flowering, the developing capsules hang in a loose cluster. About 22% of flowers failed to set seed, either because they were sterile or because they were not pollinated. Non-fertilized flowers produce small and cylindrical shaped capsules, whereas fertilized flowers form an ovoid-shaped fruiting capsule.

About three weeks after flowering, the dry, papery capsule dehisces to release 1-6 seeds, averaging 4 seeds. Values were highest at the Routledge Sand Hills and at the eastern population of the Lauder Sand Hills. They were lowest at the MHHC site (Lauder Sand Hills). Seed release was first observed on July 22, 1996 and continued into late September.

After flowering, the above-ground parts of the plant become chlorotic and dry up. At the same time, new shoots are initiated from buds at the root-shoot interface. This fall 'regrowth' occurs as early as late July and continues well into late fall. By late September, plants are very similar in appearance to those seen in early spring. It is apparent that western spiderwort overwinters in this form. Such a strategy has the apparent advantage of allowing the species to resume growth in early spring, prior to being shaded by potential competitors, but may also be a limiting factor in the northward expansion of the species. Some plants were observed to produce two or even three 'regrowth' shoots in the fall. Since only single-shoot plants were found during the summer survey, it appears that only one of the multiple shoots survives the winter. Alternatively, multiple shoot development may be a method of vegetative propagation, if each of the shoots produces an independent 'daughter' plant the following spring.

Pollinators and other insects

Various pollinators have been observed on western spiderwort. A species of halictid or 'sweat' bee (order Hymenoptera, family Halictidae) is the most common pollinator that has been seen on western spiderwort (Goulet and Kenkel 1997). These are solitary bees that nest in the ground and consequently prefer light sandy soils.

Another species inhabiting western spiderwort is a mesh web-spider (family Dictynidae). It builds a web on a portion of a leaf and lives within a curled section of the leaf. Less commonly seen was a species of crab spider (family Thomisidae). An aphid (order Homoptera) that appears to mimic the floral buds was occasionally noted within the 'mature' cyme (Goulet and Kenkel 1997).

Survival

Western spiderwort possesses both fleshy and slender roots, which is unique within the genus. Succulent root and shoot systems are adaptation to habitats of low soil moisture, such as sand dunes (Goulet and Kenkel 1997).

Physiology

Over all survey years, the MHHC site had the greatest proportion of pink-flowering plants. Despite an extensive search, no pink-flowered plants were observed at the Loutit site in 1996. A new large population of pink-flowered plants was discovered at the north end (Crowfoot dunes) of the Routledge Sand Hills ESA in 1996, substantially increasing the number of pink-flowered plants reported from that area (Goulet and Kenkel 1997).

Manitoba populations of western spiderwort show a continuous gradation in flower color, from almost pure white through to pink and various shades of blue and purple. This observation suggests that flower color is determined by the amount of pigmentation present. Only two white-flowered individuals were observed in 1996, both in the Crowfoot dunes at the north end of the ESA. Other oddities were noticed in the 1996 survey including a single plant that produced two cymes, one with pink-colored flowers and the second with purple flowers and another plant that produced pink, purple and striped pink-purple flowers from the same cyme (Goulet and Kenkel 1997).

The comparative rarity of pink flowers suggests that 'loss of pigmentation' is a recessive trait, and/or that is selected against by pollinators. The discovery of uniquely coloured plants indicate that variation in flower color may be attributable to a mutation in the floral meristem, or a chimeric mutation. However, the fact that all of the uniquely coloured plants occur in a localized region within the Lauder Sand Hills at the MHHC (western population), and no pink flowering plants occur in the eastern population suggests that the mutation is not random in the populations in Manitoba (Goulet and Kenkel 1997).

The strongly clumped spatial distribution of pink-flowered plants suggests that seed dispersal and pollination in western spiderwort populations is limited. Observations in 1996 indicated that pollinators tend to move between adjacent plants within a dune region. This suggests that gene flow in western spiderwort populations is highly localized, which could account for the spatial distribution of pink-flowered plants (Goulet and Kenkel 1997).

Higher leaf damage in unshaded sites suggests that such micro-environments may be stressful to the plants. Higher transpiration rates under full light conditions may result in water and nutrient stress, leading to leaf chlorosis and/or necrosis (Goulet and Kenkel 1997).

The largest plants were found in shaded, lightly-grazed areas where leafy spurge was present. The robust growth of these plants may be attributable to interspecific competition for light. To survive, plants must grow higher than the leafy spurge (Goulet and Kenkel 1997).

Movements/dispersal

Western spiderwort plants show a strongly clumped spatial pattern. Such a pattern likely reflects the limited seed dispersal of the species. Vegetative propagation (from lateral buds), if it occurs, could also account for the observed clumped spatial patterns (Goulet and Kenkel 1997).

Seeds usually fall to the ground directly below the parent plant. Indeed, several seeds were observed trapped between the leaves and the stem node of the parent plants. The relatively large size of the western spiderwort seeds probably limits their dispersal. In many sites, inidividual plants were observed to form linear patterns down a dune face, suggesting that seeds may be dispersed downslope through the action of wind, rain, and/or snowmelt. Strong winds may disperse seeds upslope. Herbivores may also contribute to dispersal by consuming seeds, as they often graze flowering/fruiting plants. If western spiderwort seeds remain viable after passing through the herbivore rumen and gut, this could be an important method of long-distance dispersal (Goulet and Kenkel 1997).

Behaviour/adaptability

It may be possible to transplant greenhouse-grown western spiderwort plants into suitable habitat. However, Hohn (1994) noted that transplanted populations may be difficult to establish and maintain. Sites would have to be prepared by removing leafy spurge, thinning shrubby vegetation, and/or partially destabilizing the sand dunes. Suggested potential transplant sites in Manitoba include the Oak Lake Sand Hills, the Lauder Sand Hills Wildlife Management Area, the Carberry Sand Hills, and the Portage Sand Hills.

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