Biodiversity Portrait of the St. Lawrence - Biological diversity: Flora

Flora of the St. Lawrence:
priority vascular species

Flora

[Plant diversity]
[Vascular species richness]
[Priority vascular species]

Species at risk

A knowledge of the diversity of species in the different regions along the St. Lawrence, although interesting from a theoretical viewpoint, is not enough to preserve biodiversity. Conserving biodiversity requires taking account of the number, distribution and status of the populations of each species in these areas. Among the 1703 species of vascular plants that occur along the river, 66 have been assigned maximum priority in Quebec (S1) because their presence is very precarious, while 103 are considered at risk (S2) and 30 as unusual (S3) (CDPNQ 1999). These species are considered to be a priority for conservation due to their rarity or because their situation is precarious. In Quebec, such species are mainly rare plants restricted to a single terrestrial province: in most cases, their rarity is due to the fact that Quebec represents the limit of their range or, less frequently, that their habitat is rare.

Distribution of 199 high-priority
vascular species (S1, S2, S3)

Most high-priority vascular species along the St. Lawrence are concentrated in the St. Lawrence Lowlands, including 14 species with temperate affinities that are restricted to this region. The occurrence of several so-called thermophilic species, which reach the edge of their range around the Hochelaga Archipelago or Lake St. Pierre, resulted from southerly specimens that colonized areas along the natural migration route north of the Richelieu River and the upper St. Lawrence. Squares with the highest value for the composite prioritization index are located mainly in the Montreal region (Mount Royal: 0.290; Senneville: 0.182; Île Perrot: 0.172; Île Saint-Hélène: 0.164).

Map of prioritization of squares

Map of prioritization of squares Montréal area

Table 1.
Squares with the highest composite prioritization index values

Square
No. Location Natural
region Composite
index Number of species

S1 S2 S3

1 Mount Royal B1 0,290 14 39 7

2 Senneville B1 0,182 9 22 11

3 Perrot Island B1 0,172 7 24 8

4 Sainte-Hélène Island B1 0,164 13 13 6

5 Lake Saint-François (Fraser Point) B1 0,160 9 19 5

6 Châteauguay B1 0,120 7 14 3

7 Lachine B1 0,112 5 15 5

8 La Prairie B1 0,108 5 15 2

9 Varennes B1 0,106 3 17 5

10 Bécancour B2 0,105 4 13 11

11 Laval B1 0,103 3 16 6

12 Baie-du-Febvre (north) B1 0,096 4 12 8

13 Lake des Deux Montagnes B1 0,096 4 12 8

14 Parc du Bic A3 0,090 5 11 2

15 Cap Tourmente C8 0,082 5 9 3

16 Côteau-du-Lac B1 0,082 4 10 5

17 Saint-Anicet (north) B1 0,079 6 7 2

18 Longueuil B1 0,076 4 10 1

19 Sainte-Foy B2 0,069 1 12 5

20 Repentigny B1 0,067 3 10 0

21 Tourelle A4 0,063 5 4 5

22 Îles de la Paix B1 0,063 2 10 2

23 Niapiskau Island (Mingan) E2 0,060 5 4 3

24 Havre-Saint-Pierre E2 0,060 4 6 2

25 Saint-Anicet–Cazaville B1 0,058 1 10 4

26 Laval-North B1 0,054 2 8 2

27 Forillon A4 0,052 4 5 0

28 Hopkins Point B1 0,052 3 6 2

29 Dundee B1 0,051 2 8 0

In addition, the restricted distribution of some species seems to be closely linked to the presence of Ordovician limestone outcrops, which make up most of the bedrock in the St. Lawrence Lowlands. Outcrops are particularly frequent in the Hochelaga Archipelago. Lastly, the freshwater estuary, which extends roughly from Grondines to Montmagny, is characterized by the presence of a certain number of species endemic to the freshwater estuary, such as the brevis variety of annual wildrice (Zizania aquatica var. brevis) and Victorin's fringed gentian (Gentianopsis victorinii). Only three species observed along the St. Lawrence, Robinson's hawkweed (Hieracium robinsonii), Connecticut beggartick (Bidens heterodoxa) and Eaton's beggartick (Bidens eatonii), have a high priority status in North America (G1, G2) which indicates that their populations may be endangered.

The discontinuous distribution and local concentrations of priority species in Quebec outside the St. Lawrence Lowlands seems to suggest that geological rather than climatic influences are at play. As was explained in the section on vascular species richness, the Bic, Forillon, Anticosti Island and Mingan Islands regions are characterized by an abundance of marginal calcareous habitats: small, permanently exposed, unstable habitats such as talus slopes and cliffs which have probably not been recolonized by forest since the end of the last ice age. Among the priority species restricted to the estuary and Gulf (n = 56), calcareous species are predominant, making up 63% of the species found in these habitats. Another noteworthy characteristic of the priority species of the Gulf is their origins: most of these species have ranges that encompass mainly the Rocky Mountains (cordilleran species) or the Arctic, where open calcareous habitats are common. During the last ice age, these species survived by colonizing the tundra that stretched across North America south of the glaciers. At the end of the ice age, forests invaded most of the habitats freed from the ice, except for a few marginal calcareous habitats where these species persist to this day. The Gulf of St. Lawrence is the main refugia for these species east of the Rockies and south of the Arctic.

The Gulf of St. Lawrence is also characterized by the presence of some endemic species, the best-known of which is the Mingan thistle (Cirsium minganense). The thistle is restricted to a few islands in the Mingan Archipelago. Although this plant is closely related to the leafy thistle (Cirsium foliosum) common in the Rockies, isolation for thousands of generations has led the Gulf species to diverge ecologically and physiologically from the original population. On the Mingan Islands, the thistle's habitat is extremely restricted. In addition, the plant is self-fertilizing, while the species in the Rockies requires cross-pollination (Morisset 1971). Among the sites in the Gulf supporting priority species, the Magdalen Islands are noteworthy in that they contain temperate species from the Atlantic Coastal Plain such as the broom crowberry (Corema conradii). Most species in this group that occur in Quebec are restricted to the Magdalen Islands. In Eastern Canada, aside from their presence in the Magdalen Islands, these species are found only in southern Newfoundland, Prince Edward Island and Nova Scotia.

Protected species

Among the 19 species now protected under the Act respecting threatened or vulnerable species in Quebec, nine have been recorded along the St. Lawrence, eight of them in the freshwater section.

Table 2.
Protected species

Species Scientific name Status
according to
the Quebec Act Suscep-
tible¹ Priority rank Rare⁴(Quebec) CSEMDC
(1997) Rare⁵(Canada) Canad.
priority⁵ Number
of squares

Queb.² Contin.³

False Hop
Sedge Carex
lupuliformis Threatened X S1 G3G4 X Threatened X 2 2

American
Water-Willow Justicia
americana Threatened X S1 G5 X Threatened X 2 12

Dragon-root Arisaema
dracontium Threatened X S2 G5 X Vulnerable X 3 11

May Apple Podophyllum
peltatum Threatened X S1 G5 X - - - 5

Dwarf Arctic
Ragwort Senecio
cymbalaria Threatened X S1 G5 X - - - 1

Autumn
Coralroot Corallorrhiza
odontorhiza Threatened X S1 G5 - - - - 2

Ram's Head
Lady's-slipper Cypripedium
arietinum Vulnerable X S2 G3 X - - - 2

Woodland
Sunflower Helianthus
divaricatus Vulnerable X S2 G5 X - - - 1

Wild Leek Allium
tricoccum Vulnerable X S3 G5 - - - - 28

¹ Lavoie, 1992
² CDPNQ, 1999
³ The Nature Conservancy, 1999
⁴ Bouchard et al., 1983
⁵ Argus and Pryer, 1990

These eight species are found in the Montreal-La Prairie physiographic unit, including four in the Mount Royal square. Three species have been designated priority species by the St. Lawrence Vision 2000 species subcommittee. Some of the sites may have been obtained from historical records relating to populations that have since disappeared or have not been relocated.

Table 3.
Squares where the greatest numbers of protected species are found (T: Threatened species;
V: Vulnerable species)

Square
No. Location Number
of species Species

1 Mount Royal 4 Dragon-root (T), American Waterwillow (T), Wild Leek (V), Woodland Sunflower (V)

2 Repentigny 3 Dragon-root (T), May Apple (T) American Waterwillow (T)

3 Varennes 3 Dragon-root (T), American Waterwillow (T), Wild Leek (V)

4 Sainte-Hélène Island 3 May Apple (T), American Waterwillow (T), Wild Leek (V)

5 Oka (south) 3 Autumn Coralroot (T), May Apple (T), Wild Leek (V)

6 La Prairie 2 Dragon-root (T), American Waterwillow (T)

7 La Grosse Île 2 American Waterwillow (T), Wild Leek (V)

8 Bécancour 2 American Waterwillow (T), Wild Leek (V)

9 Dupas Island 2 Dragon-root (T), Wild Leek (V)

10 Laval-North 2 False Hop Sedge (T), Ram's Head Lady's-slipper (V)

11 Senneville 2 False Hop Sedge (T), Wild Leek (V)

12 Pointe-Claire 2 Autumn Coralroot (T), Wild Leek (V)

13 Les Coteaux 2 May Apple (T), Wild Leek (V)

14 Sainte-Foy 2 Wild Leek (V), Ram's Head Lady's-slipper (V)

Taxonomic uniqueness

Among the 1919 species of vascular plants found along the river, only four are the sole representative of their order, while 20, 335 and 1560 species received taxonomic uniqueness scores of 3, 2 and 1 respectively. The total of the taxonomic uniqueness scores for the species present in each 100 km² square is closely related to observed total vascular plant richness.

Table 4.
Taxonomic uniqueness of vascular plant species

Species Scientific name Native status Single representative of Phenotypic
unicity

Order Family

Lizard's-tail Saururus cernuus Native Piperales Saururaceae 4

Hornleaf Riverweed Podostemum ceratophyllum Native Podostemales Podostemaceae 4

American Witch-hazel Hamamelis virginiana Native Hamamelilales Hamamelidaceae 4

Pincushion Plant Diapensia lapponica Native Diapensiales Diapensiaceae 4

Canada Yew Taxus canadensis Native Taxaceae 3

Flowering Rush Butomus umbellatus Introduced Butomaceae 3

Mountain Xyris Xyris montana Native Xyridaceae 3

Asiatic Dayflower Commelina communis Introduced Commelinaceae 3

Poet's Narcissus Narcissus poeticus Introduced Amaryllidaceae 3

Small Mistle-toe or Eastern Dwarf Mistletoe Arceuthobium pusillum Native Loranthaceae 3

Green Carpetweed Mollugo verticillata Introduced Aizoaceae 3

Hornwort or Coon's Tail Ceratophyllum demersum Native Ceratophyllaceae 3

Common Moonseed Menispermum canadense Native Menispermaceae 3

Roughseed Clammyweed Polanisia dodecandra Native Capparidaceae 3

Purple Pitcherplant Sarracenia purpurea Native Sarraceniaceae 3

False Mermaidweed Floerkea proserpinacoides Native Limnanthaceae 3

American Bladdernut Staphylea trifolia Native Staphyleaceae 3

American Basswood Tilia americana Native Tiliaceae 3

Threestamen Waterwort Elatine triandra Native Elatinaceae 3

Narrow-leaved Collomia or Narrowleaf Mountaintrumpet Collomia linearis Native Polemoniaceae 3

Water-leaf or Shawnee Salad Hydrophyllum virginianum Native Hydrophyllaceae 3

American Waterwillow Justicia americana Native Acanthaceae 3

American Lopseed Phryma leptostachya Native Phrymaceae 3

Sweet Mockorange Philadelphus coronarius Introduced Hydrangeaceae 3

St. Lawrence species

St. Lawrence species are defined as those whose distribution along the St. Lawrence makes up a significant part of their North American range. This group of plants is very small (a few dozen taxa) compared with the entire flora of the St. Lawrence. It contains endemic species of the freshwater estuary and the Gulf of St. Lawrence. Although there is still controversy over the taxonomic status of these species, all of which are related to species with broader distributions, they have gradually diverged, through isolation or adaptation, since the last ice age, from their original populations. Among the Gulf endemics, the Mingan thistle (Cirsium minganense) is the best known. Endemics of the freshwater estuary include several species that have developed morphological adaptations to the mechanical action of the freshwater tides. For example, the coast willowweed (Epilobium ecomosum), closely related to the ubiquitous glandular willowweed (Epilobium glandulosum), produces seeds without comas (plumes) but with papillae, which facilitates water-borne dispersal. Its lack of coma makes the coast willowweed unique among members of its genus that have wind-borne seeds. According to Marie-Victorin (1964), the coast willowweed occupies estuary beaches not by preference but because of the competitive disadvantage associated with its poor ability to disperse seed beyond this habitat.

The small number of plant species that are restricted to the St. Lawrence attests to the low number of endemic species that is typical of northern regions affected by the last ice, compared with the flora of more southerly regions of North America, which have long benefited from more favourable climatic conditions. The vascular flora of southern regions often reflects their long history, not only because of the large number of species but also because of the significant number of endemics present. The rate of endemism, or the proportion of species restricted to a given area in relation to the total flora, is an index that is often used to determine the amount of time over which a flora has evolved. Although the freshwater estuary of the St. Lawrence is the largest estuary of its type in the world, the low number of endemic plants observed is directly related to its recent geomorphological history rather than to factors such as area. This is also true for the endemics of the Gulf, which owe their survival to the presence of habitats similar to those occupied by their original populations in western North America and the Arctic.

Benoît Jobin, Jean Deshaye and Bernard Tardif

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