Callitriche heterophylla Pursh
Larger Water Starwort
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Basis for Listing
Callitriche heterophylla (larger water starwort) is a small aquatic plant, with two disjunct ranges and habitats in Minnesota. Plants in the southwest part of the state (Big Stone, Nicollet, Pipestone, and Rock counties) occur in shallow rainwater pools on Sioux Quartzite outcrops. Plants in the northeast (Lake and St. Louis counties) occur in the shallow margins of protected bays of lakes.
When C. heterophylla was designated special concern in 1996, there were only five known records, four of which hadn’t been seen since the 1940s and 1950s. This clearly indicated C. heterophylla was rare in Minnesota. However, lack of up-to-date information prevented it from being assigned a more protective status at that time. Extensive botanical surveys were subsequently conducted in appropriate habitat within the species’ potential range, yet only five additional populations were found. In addition, one of the historical locations is suspected of having been destroyed by a quartzite mine. For these reasons, the status of C. heterophylla was changed from special concern to threatened in 2013.
There are about 12 species of Callitriche in North America. All are small and inconspicuous aquatic plants, with opposite leaves, greatly reduced flowers, and tiny nutlike fruits. Three of them occur in Minnesota: C. heterophylla (larger water starwort), C. hermaphroditica (autumn water starwort), and C. palustris (spring water starwort). Only C. heterophylla is currently listed or proposed for listing in Minnesota, and it is most likely to be confused with C. palustris. Both species have dimorphic leaves (narrow submerged leaves and wider floating leaves) and fruits no more than 1.5 mm (0.06 in.) long. The seeds of C. heterophylla are wingless, about as long as wide, and have pits that are not oriented in rows. In contrast, the fruits of C. palustris are winged at the apex, tend to be longer than wide, and have pits that tend to run in longitudinal rows.
Callitriche heterophylla plants in the southwestern portion of the state occur in shallow rainwater pools on outcrops of igneous or metamorphic rocks, primarily Sioux Quartzite. Plants in the northeastern part of the state occur in the shallow margins of protected bays of lakes.
Biology / Life History
Little is known about the biology or life history of C. heterophylla in Minnesota. However, a few things can be surmised based on its habitat. Clearly, it must be able to survive seasonal desiccation because its habitat in the southwestern counties consists primarily (perhaps exclusively) of rainwater pools. These pools routinely form in the spring, fed by snowmelt and spring rain, and then disappear by mid-June, when evaporation exceeds precipitation. It is likely that C. heterophylla populations survive such conditions as dormant rhizomes or as seeds buried in the shallow sediments at the bottom of pools. How the plants disperse from pool to pool is still uncertain; however, there may be an animal vector involved, and seed dispersal via wind is likely.
Flowers of C. heterophylla that occur above the surface of the water are designed to be pollinated by small flying insects, which can result in cross-pollination. Flowers that develop under water are self-pollinated by a unique system of internal geitonogamy (fertilization of a flower by pollen from another flower on the same plant; Philbrick and Bernardello 1992). Seeds from both types of pollination are fertile. It is uncertain whether populations of C. heterophylla in Minnesota are annual or perennial, though seed production is very important to the maintenance of populations.
Conservation / Management
Wherever it occurs, C. heterophylla is dependent on a healthy aquatic ecosystem, with high water-quality and an intact biotic community. Its lake habitats can be threatened by both the invasion of non-native aquatic species and the herbicides that are usually used to combat them. The rainwater pool habitats in the southwest are much smaller and more fragile than the lakes of the northeast, and they face different issues. Since these habitats are usually in agricultural regions, they can be subjected to the harmful effects of cattle grazing, including nutrient enrichment from manure and trampling of exposed sediments that may harbor the seed bank. Herbicide drift from the routine aerial application of agricultural herbicides is another concern. Most recently, mining of rock outcrops has emerged as a serious threat, fueled by federal highway construction standards that now require crushed bedrock instead of gravel.
Best Time to Search
The best time to search for C. heterophylla in southwestern Minnesota is typically during the last two weeks of May and the first week of June, when the species' rock pool habitats most reliably have water. In northeastern Minnesota, the best time to search for lake populations is July and the first two weeks of August.
Conservation Efforts in Minnesota
Callitriche heterophylla is known to occur at two protected sites, Blue Mounds State Park and Northern Tallgrass Prairie National Wildlife Refuge, both located in Rock County. However, no known attempt has been made to assess the conservation needs of this species within these areas or to determine the effects of current management on its status.
In 2007, the Renville and Redwood County Soil and Water Conservation Districts (SWCD), in association with state and federal agencies, coordinated an initiative to compensate interested private landowners for permanently protecting the most ecologically intact rock outcrops and associated wetlands in those two counties. A total of 86 ha (213 ac.) were enrolled into perpetual conservation easements as a result of this initiative, and C. heterophylla is one of the rock outcrop species most likely to benefit from the program. In 2009, the program was expanded to protect an additional 214 ha (530 ac.) within a five county area of the Upper Minnesota River valley. Conservation plans that outline conservation practices, such as removal of invasive species and livestock exclusion, are being developed for each of the easement sites. Habitat maintenance and improvement measures, such as prescribed burning, fencing, rotational grazing, and/or seeding are also included in the plans.
Welby Smith, MN DNR, 2008 and 2018
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Harris, F. 2009. Rock pools on the prairie. Minnesota Conservation Volunteer 72(423):30-39.
Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed 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. 352 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.
Minnesota Department of Natural Resources, Division of Fish and Wildlife. 1995. Statement of need and reasonableness in the matter of proposed amendment of Minnesota Rules, Chapter 6134: endangered and threatened species. Minnesota Department of Natural Resources, St. Paul, Minnesota. 336 pp.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis, Minnesota. 320 pp.
Philbrick, C. T., and L. M. Bernardello. 1992. Taxonomic and geographic distribution of internal geitonogamy in New World Callitriche (Callitrichaceae). American Journal of Botany 79:887-890.