Gulf of St. Lawrence aster COSEWIC assessment and status report: chapter 8
Biology
General
Gulf of St. Lawrence Aster was first collected by J. Macoun, at Brackley Point, Prince Edward Island, on September 5, 1888 and distributed as Aster subulatus Michx. (CAN, GH, NY) (Macoun, 1883-1903; Hurst, 1933). It was subsequently reported under A frondosus (Nutt.) T.&G. (Fernald and Wiegand, 1910; Britton and Brown, 1913; Hurst, 1940), and finally named A. laurentianus (Fernald, 1914). The genus Aster was recently revised by Nesom (1994), who transferred to the genus Symphyotrichum all the taxa whose base chromosome number is n = 8 and n = 7.
Fernald (1914, 1925, 1950) found that Gulf of St. Lawrence Aster had shown a marked tendency to produce insular varieties. He described three: one confined to Prince Edward Island and to the southern end of the Magdalen Islands (var. laurentianus), one to the northern end of the Magdalen archipelago (var. magdalensis) and a third to the sands of northern New Brunswick (var. contiguus).
More recently, Boivin (1966-67, 1972) used A. brachyactis as a synonym of A. laurentianus, with the latter having priority of use. Several authors have agreed with this position (Rousseau, 1968; Scoggan, 1978-1979; Jones, 1980; Semple and Brouillet, 1980; Morton, 1981; Jones and Young, 1983; Semple et al., 1983; Hinds, 1983, 1986). Jones (1984) treated Gulf of St. Lawrence Aster as a subspecies of A. brachyactis. However, after close examination of isotypes and topotypes (CAN, DAO), Catling and McKay (1980) recognized Gulf of St. Lawrence Aster as a distinct taxon. Houle (1988a) demonstrated that morphological, phenological and ecological differences between Gulf of St. Lawrence Aster and A. brachyactis, observed both in the field and in the laboratory, warrant their recognition. The stability of the diagnostic characteristics and the reproductive isolation between the taxa justify the rank of species. The geographic isolation of Gulf of St. Lawrence Aster also suggests this.
Gulf of St. Lawrence Aster belongs to a small group of annuals (Aster sensu lato, section (Conyzopsis) that are widely distributed in saline or brackish habitats and somewhat transitional in their floral structure between the true Asters and the genera Erigeron and Conyza. This has led to confusion about its generic placement and to the creation of a distinct genus, Brachyactis (Ledebour, 1845-46; Gray, 1873; Hooker, 1876; Löve and Löve, 1982; Jones, 1984, 1985). Since the revision by Nesom (1994), the name retained for Gulf of St. Lawrence Aster is now Symphyotrichum laurentianum (Fernald) Nesom.
The only close relatives of Gulf of St. Lawrence Aster are S. frondosus (synonym: A. frondosus) of the Great Basin region of western North America and S. brachyactis (synonym: A. brachyactis) of saline and brackish habitats in Central Asia and the Great Plains region of North America. Certain constant and stable characteristics clearly distinguish the three species. The key characteristics are as follows:
Marginal flowers 5.5 to 8 mm long; often regularly imbricated phyllaries, the outer series shorter than the inner.
S. frondosus
Marginal flowers 2 to 5 mm long; phyllaries generally non-imbricated, all similar in shape.
Leaves linear-attenuate to narrowly lanceolate, ciliate, thin and scabrous; stem pubescent, 15 to 60 cm high; remains in the rosette stage several months before producing its floral stem.
S. brachyactis
Leaves linear-oblong to spatulate-oblanceolate, eciliate, more or less fleshy; stem glabrous, 1 to 30 cm high; bolts as soon as the first basal rosette of leaves is formed after germination.
S. laurentianus
Ecological similarities between these three species have been revealed by the presence of common associated species (Houle, 1988b). S. brachyactis exhibits a broad ecological amplitude; it can colonize habitats similar to those of the other two species, but seems to prefer salt plains. S. frondosusis rather restricted to xeric habitats of the Rockies, while S. laurentianus is restricted to coastal habitats in the Gulf of St. Lawrence.
The varieties of Gulf of St. Lawrence Aster described by Fernald (1914, 1950) have not been recognized. The three morphotypes can be identified in Fernald’s collection, but cannot be distinguished in Houle’s collection, which is now more representative of the species. Furthermore, the diagnostic characteristics of these varieties are influenced by culture conditions. Hence, this variability probably reflects the phenotypic plasticity of Gulf of St. Lawrence Aster.
Biogeography and phylogeny
Gulf of St. Lawrence Aster inhabits the salt marshes of the Magdalen Islands, the east coast of New Brunswick and the north shore of Prince Edward Island (Houle and Haber, 1990; Guignion et al., 1995; Godbout, 2001;). Its closest relative is Symphyotrichum brachyactis, an Aster of the margins of temporary salt ponds of the prairies and Central Asia, considered a weed in Quebec in areas where salt has been introduced by human activity. The two species are autogamous, and in this respect they differ from the other Asters (Houle, 1988a). Both derive from an annual species in the southwestern United States, S. frondosus. The latter has fine rays that attract insect pollinators and can ensure cross-fertilization, but it is also capable of self-fertilization. It inhabits the margins of temporary salt ponds in the semi-deserts of this region. In addition to the reduction of the rays and autogamy, these three species share a unique genetic characteristic within the genus Aster sensu lato: instead of 9 or 8, the haploid number of chromosomes is 7, which confirms that they are related.
The origin of the genus Symphyotrichum (Conyzopsis section of the genus Aster sensu lato), results from the reduction in the number of chromosomes from 8 to 7, from the reduction in the size of the rays and from the acquisition of the annual growth habit (Houle, 1988a). S. frondosus was the first species. It then extended its range to the temporary salt ponds of the western Rockies. A fortuitous migration event enabled a small population to cross this geographic barrier toward the east, giving rise to S. brachyactis, whose rays are no longer visible outside the head. This event probably occurred during the glaciations of the Quaternary. Subsequently, during the eastward and northward migration following the retreat of the ice, a population reached the Gulf of St. Lawrence. Gulf of St. Lawrence Aster then adapted to the conditions of the tide-influenced salt marshes. This is a case of rapid and relatively recent speciation.
The speciation process that gave rise to Gulf of St. Lawrence Aster is therefore the result of the glaciations of the Quaternary and of the accompanying upheavals and migrations. The ambient conditions that have prevailed since that time on the territory occupied by the species have shaped it to make it what it has become today.
Phenology
Gulf of St. Lawrence Aster does not flower until late August. It reaches full flower in mid-September and fruits in late September. Fruit dispersal is in late October (Houle, 1988b).
The development of Gulf of St. Lawrence Aster in the greenhouse is very rapid (2-3 months compared to 6-8 months for S. brachyactis and S. frondosus). Also, under uniform growth conditions, Gulf of St. Lawrence Aster is a generally smaller plant, with fewer and smaller basal leaves and heads (Houle, 1988a).
Reproduction
Gulf of St. Lawrence Aster and the other species of the genus Symphyotrichum are self-fertile (Houle, 1988a). The floral morphology of S. laurentianus and of S. brachyactis therefore displays characteristics of self-fertile species: ligules and nectaries small or absent, style generally included in stamen tube and pappus overtopping corolla even before anthesis. The self-pollination and floral morphology of these two species indicate a close relationship between them. They could rapidly fix morphological features within the genotype. S. frondosus, on the other hand, is frequently visited by insects in the field and can probably also reproduce by cross-pollination, conforming to the ancestral mode in Aster sensu lato.
The artificial interfertility of S. brachyactis (f) X S. frondosus (m), S. laurentianus (f) X S. frondosus (m) and S. laurentianus (f) X S. brachyactis (m) has been demonstrated by Houle (1988a) with the production of 7, 7 and 45 fertile F1 hybrids respectively and the production of F2 hybrids by natural self-pollination (Houle, 1988a). The hybrid status of the progeny is confirmed by the inheritance of paternal characteristics. A closer genetic affinity is indicated between A. laurentianus and A. brachyactis by the higher degree of crossability.
Reciprocal crosses have been impossible to perform, suggesting unidirectional interfertility from A. frondosus to A. brachyactis and to A. laurentianus. It appears that in this genus, only ancestral species can cross-breed with derived species. Although unidirectional interspecific hybrids can be produced experimentally, Gulf of St. Lawrence Aster is reproductively isolated by two factors: geographic isolation and self-fertility.
However, the recent expansion of S. brachyactis along salted roads in New Brunswick within the range of S. laurentianumcould eventually complicate the situation through hybridization (Sean Blaney, Atlantic Data Base, pers. comm., 2002). S. brachyactis is still rare, but known in Saint John, Sussex and the Moncton area. It seems to be only a matter of time before it reaches the Northumberland coast, where it could easily invade the salt marshes.
Survival
Boudreau and Houle (1998) and Houle et al. (2002) have demonstrated that interspecific competition appears to play a significant role in the population dynamics of Gulf of St. Lawrence Aster. Vegetation canopy closure is not conducive to either its reproductive effort or its survival rate. Thus, disturbance appears to play a role in maintenance of populations.
In addition, anthropogenic disturbances such as cottage construction and fill operations completely destroy the species’ habitat. This is believed to have been the cause of the loss of at least two sites, though the potential for such pressure on current populations is less clear. While the impact of all-terrain vehicles (ATVs) may be temporarily favourable (a population has been observed colonizing ruts in Clarke Bay (Île de l’Est, Magdalen Islands [MI])), the long-term effect of this disturbance would depend on the extent of immediate damage to the plants and their habitat, as well as the subsequent potential for severe erosion events.
Physiology
Seed germination is inhibited by salinity greater than 20 g salt/L (Houle et al., 2001 and 2002; Reynolds et al., 2001). However, the seeds conserve their viability when subjected to salinities of up to 40 g salt/L for a period of 30 days. The salinity of the substrate has a significant negative effect on the germination and emergence of the seedlings. Salinities as low as 1% reduce emergence by a third, although an additional increase of up to 5% has no additional effects. The growth of the plants is significantly reduced by salinities of 10 to 40%. However, the number of inflorescences per plant does not appear to be affected by salinity. Therefore, in the stages following emergence and establishment, Gulf of St. Lawrence Aster has a very great tolerance to high salinities.
Of the factors of salinity, nutrient availability and light, only light has a significant effect on growth and reproduction. Hence, in a situation of interspecific competition, light rather than nutrients is the resource for which the various species compete with Gulf of St. Lawrence Aster. On the other hand, Gulf of St. Lawrence Aster exhibits certain characteristics associated with opportunistic plants, such as excessive nutrient absorption, even if it behaves like a plant tolerant to saline and nutritional stresses.
Water stress during the juvenile stage, which is the stage of reproductive bud differentiation, reduces inflorescence production by 83%. Such stress at the seedling or adult stage affects only the total biomass and the leaf surface (Houle and Belleau, 2000; Houle et al., 2002).
Movements/dispersal
According to the various surveys carried out since the publication of the original status report Houle and Haber (1990), namely those of Gagnon et al. (1995b and 1996), Houle et al. (2002), Godbout (2000 and 2001), the Piper Project (Sabine Dietz, pers. comm., 2002) and those conducted under the aegis of the Prince Edward Island National Park (Denyse Lajeunesse, pers. comm., 2002), it appears that, apart from the new populations discovered at previously unexplored or little explored sites, its distribution has not changed and that it remains very faithful to the known occurrence sites. Within a site, its movements are subject to regular fluctuations of topography, salinity and areas of eelgrass deposition (Houle et al., 2002; Reynolds and Houle, 2002), which also determine its density. The local distribution of Gulf of St. Lawrence Aster is also influenced by interspecific competition in the upper portion of the topographic gradient.
Interspecific interactions
Interspecific competition is an important factor controlling the distribution of Gulf of St. Lawrence Aster at a given site (Houle, 2002). When competition is eliminated, it becomes more abundant, particularly in the upper portion of the topographic gradient, where abiotic conditions are less limiting (e.g. lower salinity and reduced exposure to waves and to eelgrass debris and sand). Similarly, the overall reproductive success of the plants (number of fruits produced) increases when interspecific competition is absent. The main effect of the competitors and the one that has the greatest impact on Gulf of St. Lawrence Aster is the reduction in available light.
Houle and Legault (1986) as well as Boudreau and Houle (1998) determined the species frequently associated with Gulf of St. Lawrence Aster. According to Houle and Legault, they are, in order of prevalence: Juncus bufonius, Atriplex patula, Rumex maritimus var. fueginus, Agrostis alba var. palustris, Potentilla anserina ssp. egedii, Plantago maritima, Bidens cernua, Ranunculus cymbalaria, Spergularia marina var. leiosperma and Glaux maritima var. obtusifolia. The complete list of the 50 species associated with Gulf of St. Lawrence Aster is in Houle (1988a). According to Boudreau and Houle, the most abundant species of two communities of the Magdalen Islands are Spartina pectinata, Juncus balticus, Atriplex hastata, Ranunculus cymbalaria, Eleocharis smallii, Sonchus arvensis, Agrostis stolonifera and Calystegia sepium. To these are added Scirpus acutus and Lomatogonium rotatum (Dietz and Chiasson, 2001).
Lepidoptera larvae have been observed on Gulf of St. Lawrence Aster individuals at Havre-aux-Basques (Boudreau and Houle, 1998). The larvae appeared to be feeding on the leaves. They could have an impact on the plants’ reproductive performance and on population dynamics. In 1994, a few individuals of a beetle, Trirhabda borealis, were found feeding on Gulf of St. Lawrence Aster on a small island in the Havre aux Basques lagoon (Jean Gagnon, pers. comm.). The impact of this predator on the plant is not known.
Behaviour/adaptability
Since Gulf of St. Lawrence Aster is an annual that colonizes a relatively denuded substrate, the density of the vegetation cover and competition with other species undoubtedly represent a major constraint to its establishment and maintenance. The dynamic process whereby the habitat is reshaped, denuded (for example by summer storms) and recolonized more or less cyclically could largely explain the significant population fluctuations as well as the apparent disappearance of some populations. Since the seeds of Gulf of St. Lawrence Aster can probably survive for about 10 years (Houle, 1988b), they could persist in the soil until ecological conditions again become favourable, when new gaps appear in its habitat.
Gulf of St. Lawrence Aster has a high seed set (50-60% in greenhouse, Houle, 1988a). The presence of dispersed fruits has been noted in all fruiting populations. Fruit dispersal is by wind and water. The cultivation of the species does not require a dormancy period, scarification or salt concentrations. Common conditions for germinating seeds in vermiculite can be used. After two weeks, 80% of achenes germinate. However, the species’ survival rate in nature appears to be highly dependent on favourable seasonal conditions, as its rarity attests. Storm events resulting in the deposition of eelgrass debris, burial under sandy deposits or submergence of the habitat have occurred in the last two years in New Brunswick and Prince Edward Island, and appear to have had an impact on several populations (Kate MacQuarrie and Éric Tremblay, pers. comm., 2002). The situation will be monitored in order to determine whether the affected populations will reappear or recover.
Under laboratory conditions, the achenes are still viable after three years. Expected longevity is around 10 years, according to culture data of a related taxon, S. brachyactis, using achenes collected by Hudson (1979, in Houle, 1988b).
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