Synonyms

Sagina nodosa var. borealis

  Basis for Listing

Sagina nodosa ssp. borealis (knotty pearlwort) is a low tufted or matted perennial plant characteristic of sea cliffs and rocky shores and bays on the Atlantic Coast of North America. Occasional inland occurrences are found on the shores of large subarctic lakes in Canada as well as the shores of Hudson Bay, Lake Manitoba, and Lake Superior (Crow 1978; Porsild and Cody 1980). This scattered distribution pattern has no simple explanation but is probably a result of the glacial history of North America. According to Crow (1978), the ability of S. nodosa ssp. borealis to colonize cold rocky shores and gravel beaches made it well adapted to habitats that were common during Pleistocene glaciation, and it is now confined to the relatively few places where periglacial-like conditions still exist. The current populations are likely relicts of an earlier, more widespread distribution. However long S. nodosa ssp. borealis has been at this latitude, it is clearly restricted to places that are too harsh for most temperate-climate plants to tolerate. Ongoing climate change makes this climate-dependent habitat extremely vulnerable.

In Minnesota, S. nodosa ssp. borealis occurs in a few locations along the rocky shores of Lake Superior (North Shore Highlands Subsection). Prior to 1999, it was known only from Cook County. But between 1999 and 2003, the Minnesota Biological Survey discovered three previously unknown occurrences in Lake County. Botanists also confirmed one of the Cook County sites in 1998. The native North American populations of S. nodosa have been segregated by Crow (1978) as the subspecies borealis. The typical subspecies is apparently native to Europe, with a few adventive populations on the Atlantic Coast of North America. Because of its rarity in Minnesota and its dependence on a highly specialized and vulnerable habitat, Sagina nodosa ssp. borealis was listed as an endangered species in Minnesota in 1984.

  Description

There is no other species in Minnesota that could be easily confused with S. nodosa ssp. borealis based on a few readily visible features. The plant itself is a low tufted mat-forming perennial. The lower leaves are triangular in cross-section and taper to a point. Upper leaves are paired and scale-like. At the base of some leaf-pairs there may be a tiny sterile (non-flowering) shoot. The small delicate white flowers have 5 petals, each about 4 mm (0.16 in.) long, and 5 tiny sepals, each about 2 mm (0.08 in.) long. Flowers are borne singly at the ends of slender branches. Capsules produce about 125 black seeds that have a pebbly-textured surface (Crow 2005).

  Habitat

In Minnesota, this subarctic species is known to occur only along the shore of Lake Superior. It is rooted in rock crevices within about 3 m (10 ft.) of the lake, where it is sometimes subjected to spray from waves but in settings where it is not exposed to direct wave action or severe ice-scouring. The crevices are in rhyolite (in/near calcite veins), slate, and basalt. The rocky habitats surrounding the crevices are typically bare, have some mosses and crustose lichens, or support a few other small plant species such as Campanula rotundifolia (harebell) and Primula mistassinica (Mistassini primrose). Typical populations of S. nodosa ssp. borealis in Minnesota are composed of isolated tufts or clumps ranging from 10 to over 100 stems in number.

  Biology / Life History

Sagina nodosa ssp. borealis is a low-growing, mat-forming perennial plant. Floral morphology would suggest the species is pollinated by very small flying insects, though no specific pollinator has been reported in the scientific literature. The seeds are very small, no more than 0.5 mm (0.02 in.) across and possess no specialized dispersal structure. Speculatively, these seeds would fall from the ripened fruit capsule and lodge among mineral grains in a rocky crevice; be eaten by small animals; or carried off by birds, wind, or water. In addition to reproduction by seeds, it is capable of rooting at the nodes of decumbent stems. This may lead to the establishment of new root crowns that produce additional ramets, which are, in this case, vegetatively produced individuals genetically identical to the parent plant.

  Conservation / Management

Of the sites at which S. nodosa ssp. borealis has been confirmed since 1998, two are on land owned by the Grand Portage Band of Ojibwe, one is on private land, and one site has uncertain ownership or management status. Even in protected areas, rocky habitats are often in some danger of inadvertent disturbance because people are attracted to rocky shores for recreation. It may be most beneficial for long-term conservation to increase the awareness of the concentration of rare features on Minnesota's beautiful but fragile Lake Superior shoreline. Populations of S. nodosa ssp. borealis may also be threatened by increasing numbers of Herring Gulls (Larus argentatus) that nest on the bedrock islands and shores, often upslope from plant occurrences. The high concentration of gull droppings that result in excessive nutrient enrichment, as well as direct trampling by birds, cause the elimination of most native shoreline plants. At least one S. nodosa ssp. borealis population is located alongside a Herring Gull nesting colony.

  Best Time to Search

The best time to search for Sagina nodosa ssp. borealis is from mid-June to mid-July, when it is in flower. With careful searching, this plant can be found until the end of the season and distinguished by vegetative features.

  Conservation Efforts in Minnesota

A thorough inventory by the DNR Minnesota Biological Survey has been accomplished for rare plants along much of Minnesota's North Shore of Lake Superior, which is a great contribution to conservation. Botanists agree that potential locations of this species have been well documented except for a small portion of Cook County. While the discovery of new populations is possible, it is unlikely there will be many more because of the exceedingly limited amount of suitable shoreline habitat. The survey results increase our knowledge of this species status and establish a sound basis for developing conservation plans.

  Authors/Revisions

Welby R. Smith (MNDNR), 2020

(Note: all content ©MNDNR)

  References and Additional Information

Crow, G. E. 1978. A taxonomic revision of Sagina (Caryophyllaceae) in North America. Rhodora 80:1-91.

Crow, G. E. 2005. Sagina. Pages 140-147 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 5. Oxford University Press, New York, New York.

Gleason, H. A., and A. Cronquist. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. Second Edition. New York Botanical Garden, Bronx, New York. 910 pp.

Porsild, A. E., and W. J. Cody. 1980. Vascular plants of continental Northwest Territories, Canada. National Museum of Natural Sciences, Ottawa, Ontario, Canada. 667 pp.