Minuartia dawsonensis (Britt.) House
Arenaria dawsonensis, Arenaria stricta ssp. dawsonensis
Basis for Listing
Minuartia dawsonensis (rock sandwort) is a small plant that grows on outcrops of sedimentary bedrock exposures in the southeastern region of the state (Paleozoic Plateau and Minnesota & Northeast Iowa Morainal sections) and on sand and gravel deposits in the northwestern corner (Lake Agassiz, Aspen Parklands Section). Habitats tend to be small, sometimes only a few square meters in size. They also tend to be isolated from one another and are somewhat fragile in that the plants are shallowly rooted. For unknown reasons, the vast majority of habitats that appear to be suitable do not harbor this species.
At the time M. dawsonensis was listed as special concern in Minnesota in 1984, it had not been seen for over two decades, and its status in the state was uncertain. For that reason, assigning a more protective status was delayed until additional data could be collected. By 2013, the Minnesota Biological Survey had completed its data gathering in the habitats most suitable for M. dawsonensis, and very few populations were found. For that reason, a status of threatened was assigned to M. dawsonensis in 2013.
Minuartia dawsonensis is a loosely tufted perennial that grows to a maximum height of about 20 cm (8 in.), though it is usually much shorter. The primary leaves are short and very narrow, usually stiff and bristle-like. They usually subtend a conspicuous cluster of shorter leaves. The inflorescence has 1-17 white flowers, which are borne on rigid and ascending pedicels. Each flower is 3.0-5.5 mm (0.12-0.22 in.) long and has 5 narrowly ovate bristle-tipped sepals, with prominent ribs. There are 5 petals that are somewhat shorter than the sepals, or sometimes the petals are absent.
Occurrences of M. dawsonensis in the southeast are typically found on dry sedimentary bedrock outcrops (sandstone, limestone, and dolomite), where the species grows in crevices and in very shallow accumulations of organic matter over the exposed bedrock. It should be noted that the outcrops are generally horizontal in nature; plants do not grow on the vertical walls of cliffs. Occasionally in the southeast, M. dawsonensis is also found in upland prairies on sands derived from bedrock. In the northwest, M. dawsonensis has been found on exposed sand or gravel deposits associated with beach ridges of Lake Agassiz (an extinct glacial lake). All of the habitats are sparsely vegetated, dry, and hot, and the species is generally considered to need full sunlight.
Biology / Life History
Minuartia dawsonensis occurs in habitats that tend to become very dry and hot, especially in mid- and late summer when rainfall is typically less than in spring. Minuartia dawsonensis is well adapted to these conditions. The leaves are small and bristle-shaped to conserve moisture, and they remain green even during droughts. The flowers are adapted for pollination by small flying insects. The seeds are small and possess no specialized dispersal mechanism. They appear to simply fall to the ground when the ripe capsule is shaken by the wind or by a passing animal.
Conservation / Management
The major concern for the conservation of M. dawsonensis is the small size of known habitats and the few number of individuals occurring at each site. This combination of factors allows for very little leeway when considering management options. The sites are simply too small to allow any competing use, even passive recreational use, such as hiking or camping. For this reason, foot traffic needs to be routed around such habitats. Certainly all motorized use, herbicide use, and extractive activity should be completely excluded. Minuartia dawsonensis also appears to be intolerant of grazing as it has only been found in pastures where outcrops were inaccessible to livestock. Smaller outcrops in the same pastures that were otherwise similar but accessible to livestock did not harbor this species. It would be very easy for a population of this rare species to be destroyed through some careless accident, so careful attention must be paid to all possible threats.
Minuartia dawsonensis does not do well with competition from other species, and woody encroachment due to fire suppression has likely contributed to the species' decline. The clearing of Juniperus virginiana (red cedar) and other woody species that encroach on bedrock outcrops and sand deposits should be considered as a management tool but only completed during the winter when the plants are dormant.
Best Time to Search
The best time to search for M. dawsonensis is when fruits or flowers are present, from early May through August in the south, and from early June through August in the north.
Conservation Efforts in Minnesota
Approximately 25 individual colonies of M. dawsonensis are currently known to occur in Minnesota. Several of these do occur on public land where conservation is given a high priority.
Welby Smith (MNDNR), 2008 and 2018
(Note: all content ©MNDNR)
References and Additional Information
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Maguire, B. 1951. Studies in the Caryophyllaceae V. Arenaria in America north of Mexico. The American Midland Naturalist 46:493-511.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.
Minnesota Department of Natural Resources, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.
Rabeler, R. K., R. L. Hartman, and F. H. Utech. 2005. Minuartia. Pages 116-136 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 5. Oxford University Press, New York, New York.