Characteristics of the vascular flora

Although the reported richness of the flora in the St. Lawrence Lowlands reflects in part the accessibility of sites and the greater surveying effort, the species richness of a given area depends mainly on local ecological conditions, followed by historical, climatic and anthropogenic factors. Therefore, a combination of these different factors determines the presence of species in a region. In addition, since Quebec is situated at a fairly northern latitude, the flora of the St. Lawrence consists mainly of perennial herbaceous plants.

These species are able to survive the winter cold since their buds are buried in the ground or near the surface. The number of tree and shrub species is also relatively low throughout the study area, as is the number of annual species, although the latter are more abundant in the St. Lawrence Lowlands, mainly due to the presence of weedy species. Most of these weeds are plants restricted to open habitats and not forest species. Most originate from Europe or Asia, and were introduced relatively recently in North America, most often by accident. Such exotics (505 introduced species, 322 of which are georeferenced in our database) are found mainly in open areas modified by man along the St. Lawrence and therefore increase the plant diversity in the St. Lawrence Plain, somewhat artificially but albeit significantly.

Most introductions of exotic plants occurred in connection with the intensive clearing of land for agriculture, following the massive influx of Europeans in past centuries. Although most introduced species have now become naturalized, they probably would disappear completely should the forest reinvade the cleared areas.

The native flora of Quebec reflects the climatic conditions that have prevailed over a geological time scale. During the last glacial epoch, for nearly 100,000 years and until roughly 15,000 years ago, the territory of Quebec was covered by continental ice sheets, which encompassed roughly two thirds of North America. All forms of life were absent from the area along the St. Lawrence during that period. Although climatic conditions have more or less stabilized since then, colonization of the St. Lawrence by plants is still occurring today. Overall, the area along the St. Lawrence is dominated by two groups of species: species of the Eastern temperate zone and boreal species.

The first group is typical of the deciduous forests of eastern North America, with most of these species reaching the northeastern limit of their range in the St. Lawrence Lowlands, which explains the greater species richness in this area. Eastern temperate species probably survived the last glaciation in areas like Louisiana, Georgia and northern Florida, and then spread north and northeast after the ice age ended. The fact that they are restricted to eastern North America can be explained by the presence of the Prairies or Great Plains, which halted their expansion westward. Readers should note that the species listed in the group "other affinities" in Figure 3 are mainly temperate species with a continental distribution.

Boreal species, on the other hand, are uniformly distributed along the St. Lawrence. This is due to Quebec's northerly position, giving it a large share of the vast boreal forest that stretches across North America from the Arctic to the temperate zone. Among the species recorded in each of the six natural provinces, those with the greatest distributions were all boreal species: the Canada beadruby (Maianthemum canadense) (327 squares), balsam fir (Abies balsamea) (325 squares), bunchberry dogwood (Cornus canadensis) (320 squares), paper birch (Betula papyrifera) (319 squares) and American starflower (Trientalis borealis) (318 squares). These observations show the lack of individuality of northern flora compared to more southerly flora, with the same species being found everywhere. It is well known that native flora become less and less individualized, or specific to a given region, as latitude increases.

The regional distribution of some species can be explained by the nature of the bedrock. Limestone formations in the Bic, Gaspé, Mingan Islands, Anticosti Island and Blanc-Sablon regions have allowed calcareous plants to become established. Unlike boreal species—which are also very widespread in these regions, making up most of the flora—calcareous species do not compete well with other species, particularly forest species, and therefore are only found in permanently exposed limestone habitats such as cliffs, talus slopes and river flats.



Bernard Tardif and Jean Deshaye