Rare Species Guide

  Synonyms

Decodon verticillatus var. laevigatus

  Basis for Listing

A status of special concern was assigned to Decodon verticillatus (water-willow) in 1984 to highlight two aspects of its occurrence in Minnesota. First, it is quite rare in the state and second it occurs in a sensitive habitat. Its rarity, in part, can be attributed to the "edge of range" effect, which is basically an artifact of geography. Minnesota occurs at the northwestern edge of the species' range, and plant species often become more scarce as they approach the edge of their range. Decodon verticillatus' sensitive habitat includes lake shores in the east-central part of the state, where lake shores in general are under intense development pressure.

  Description

Decodon verticillatus is a perennial herbaceous plant, although it is sometimes considered a shrub or subshrub because the roots are somewhat woody. The stems may reach 1.8 m (5.9 ft.) in length and often arch over the water. If a stem touches the water or moist soil, it may root at the tip. The leaves are arranged opposite on the stems or in whorls of 3 or 4. They are lanceolate in shape, have short petioles and entire (without teeth) margins, and are 5-20 cm (2-8 in.) long and 1-5 cm (0.4-2.0 in.) wide. The flowers are clustered in the axils of the upper leaves and have 5 reddish purple petals. The massive submerged parts are thickened with conspicuous, spongy and soft, brown aerenchyma (air filled tissues).

  Habitat

Decodon verticillatus has been reported to occur in a variety of wetland habitats across its range in northeastern North America, but in Minnesota it appears to be restricted to boggy or marshy margins of lakes and slow-moving streams. It is typically found in a narrow fringe of shoreline vegetation with Typha spp. (cattails) or Schoenoplectus spp. (bulrushes). These vegetation zones may be on floating root mats or "grounded" in peat, muck, or sand. To a certain extent, water quality and pH do not seem to be restrictive.

  Biology / Life History

There is only one species of Decodon (a monotypic genus), and it is restricted to eastern North America. Although the flowers are visited by bumble bees (Bombus spp.) and honey bees (Apis mellifera), flowers in the northern part of the species' range reportedly produce few seeds (Dorken and Eckert 2001). Reproduction is primarily clonal, and it is accomplished by a process called tip-rooting. This happens when a branch arches over and the tip touches moist ground or water. At that point, the branch tip thickens and produces specialized stem tissue that is made of buoyant air-filled cells. At the same time, it produces adventitious roots. In this way, the branch tip can essentially create a new plant. The branches are herbaceous so when they die in the autumn, the connection between the parent plant and the clonal offspring is severed. The clonal progeny may disperse widely on the water surface, or it may remain rooted in the substrate near the parent plant, to which it is no longer connected (Dorken and Eckert 2001). The establishment of true seedlings (as opposed to clonal progeny) is chancier, and may be limited to patches of bare mud where competition is reduced (Eckert and Barrett 1993). Clonal progeny, on the other hand, usually attain larger size and develop extensive root systems before their first winter, giving them a distinct advantage over seedlings (Eckert and Barrett 1993).

Although D. verticillatus can be found throughout the growing season, perhaps the best time to search is when flowers or fruits are present, which should be anytime after about the first of July.

  Conservation / Management

The specific habitats where D. verticillatus has been found in Minnesota are ecologically stable plant communities along the undeveloped shores of certain lakes. These habitats are characteristically subjected to high wave action during storms, ice-heave in the winter, and seasonal water level fluctuations that correspond to precipitation patterns. These events may seem destructive but are actually normal, and may even be necessary for the long-term survival and reproduction of D. verticillatus. In fact, intact habitats of D. verticillatus should require no regular intervention or management. Yet as persistent as these habitats may appear, they can sustain very little, if any, active disturbance. Activities such as shore fishing or launching a canoe would cause no harm, but physical alterations to the shoreline such as the installation of rip-rap or a sand blanket would be destructive. Any nearby upland development such as residential or commercial construction or road building must leave a substantial buffer of native vegetation between the lake and the development. Also, any activity that would alter the normal season fluctuations in water level or the normal high water line should be avoided.

  Conservation Efforts in Minnesota

A number of state, county, and local authorities are making efforts to protect native vegetation along shorelines within their jurisdictions, and several resources and tools regarding shoreland management, including lakescaping and shoreland restoration, are available for landowners and land managers who want to improve degraded shorelines. Such efforts are done primarily to protect or improve water quality and fish habitat, although the benefit to a wider range of plant and animal species cannot be denied. To the extent that shoreline protection measures are being implemented, they are generally effective. However, given the increasing demand on lake shores for recreational and residential development, the rate of shoreline loss is probably accelerating beyond all current protection efforts.

  References and Additional Information

Dorken, M. E., and C. G. Eckert. 2001. Severely reduced sexual reproduction in northern populations of a clonal plant, Decodon verticillatus (Lythraceae). Journal of Ecology 89:339-350.

Eckert, C. G., and S. C. H. Barrett. 1993. Clonal reproduction and patterns of genotypic diversity in Decodon verticillatus (Lythraceae). American Journal of Botany 80(10):1175-1182.