Basis for Listing

In Minnesota, Rorippa sessiliflora is a very rare species that is believed to occur only in transient habitats that develop in association with the cyclical rising and falling of the Mississippi and Minnesota rivers. When the rivers fall, they expose mud flats where a very specialized group of plant species germinate from a buried seed bank. Before the water rises again, the plants must mature and produce abundant seeds in order to rejuvenate the seed bank. The fast-moving current and the unpredictable rise and fall of the water determines where these mud flats will develop, and where the seeds end up. It appears that R. sessiliflora is an especially rare member of this specialized group of plants. In fact, R. sessiliflora has not been seen in Minnesota for many decades and may now be gone (Rosendahl and Moore 1947). It is even possible that the habitat itself no longer exists in Minnesota. The Mississippi River has been so drastically altered by dams, dredging, and siltation that many of the fine nuances of river dynamics have been obliterated, and the Minnesota River has not fared much better. Rorippa sessiliflora was listed as a special concern species in Minnesota in 1996.

  Description

Rorippa sessiliflora is a winter or summer annual that grows to a height of about 50 cm (1.6 ft.). The stems are smooth, unbranched or branched, and often ridged. The leaves are smooth and alternate on the stem. The lower leaves have short petioles or are sessile; the blades are pinnatifid, up to 10 cm (3.9 in.) long and 3 cm (1.2 in.) wide. The leaves become smaller and less divided as they ascend the stem. Each branch of the stem terminates in a smooth, slender raceme up to about 20 cm (7.9 in.) long. The pedicels are no more than 2 mm (0.08 in.) long. The flowers have 4 pale yellow sepals about 2 mm (0.08 in.) long, and 4 even smaller petals or no petals at all. The seedpod (silique) is smooth, cylindrical in shape, straight or slightly curved, about 1 cm (0.4 in.) long, and 2-2.5 mm (0.08-0.10 in.) in diameter. The seedpod splits open into 2 valves to release the seeds. The root system consists of a shallow branching taproot. There are 2 or 3 other species of Rorippa that occur in Minnesota and may be found in the same habitat as R. sessiliflora. Telling them apart should not be too difficult, but requires a careful examination of the fruit. The fruit of R. sessiliflora is 5-10 times the length of the stalk (pedicel), which is less than 2 mm (0.08 in.) long. The fruits of all the other species of Rorippa that are likely to be found in Minnesota are shorter than the stalks or at most 1.5 times as long, and the stalks themselves are 3-7 mm (0.12-0.28 in.) long.

  Habitat

There are no first-hand accounts of R. sessiliflora in Minnesota, only old herbarium specimens with minimal label information. Drawing from this fragmentary data and information from other states, it appears that R. sessiliflora occurs on exposed river sediments along the Mississippi River (Patman and Iltis 1961; Stuckey 1972), and possibly the lower Minnesota River. This habitat type is ephemeral or seasonal in nature, and usually appears as a narrow ecotonal zone between the normal low water line and the normal high water line. This zone is typically underwater for a period of weeks following spring snowmelt and occasionally during the summer after periods of heavy rain. It is characteristically occupied by annual plant species; the perennial or biennial species usually occur at a slightly higher elevation (but perhaps only a few meters distant) where the habitat is flooded less often or stays flooded for a shorter duration. Typical inhabitants of the "lower" zone include: Eragrostis hypnoides (creeping lovegrass), Cyperus squarrosus (awned umbrella sedge), Eleocharis intermedia (intermediate spikerush), and Rorippa palustris (Icelandic yellow cress). The habitat (when fully developed) has quite a bit of exposed substrate, particularly fine or coarse sediments, rocks, and woody debris. The habitat is sunny or partially shaded (Minnesota Department of Natural Resource 2005).

  Biology / Life History

Rorippa sessiliflora is an annual species that (in Minnesota) occurs in early successional habitats along the Mississippi and Minnesota rivers. It apparently relies on the germination of buried seeds that are periodically exposed by receding waters. This situation could occur at any time during the growing season, but most often in mid or late summer, possibly even autumn. During times of receding water, higher portions of a sand bar or mud flat are exposed before lower portions. As a result, seeds could be germinating over an extended period of time and plants in various stages of maturity could be present.

Rorippa sessiliflora reproduces only by seeds, which reportedly can float for a period of two months (Stuckey 1972), giving them ample time to disperse on river currents. Because of the dynamic nature of riverine habitats, the persistence of a population, or meta-population, of R. sessiliflora at any particular site along a river may be only short-term. The survival of this species, and its habitat, is dependent on dynamic forces that are often perceived as destructive (i.e., flooding, erosion, sedimentation, etc.). This is an example of one of the challenges faced by floristic botanists and land-use planners, who seek to identify and preserve native elements of biodiversity.

The best time to search for R. sessiliflora is when the pods are fully developed, usually from about mid-July through September.

  Conservation / Management

Rorippa sessiliflora has not been seen in Minnesota for at least two generations, so there is no way to assess its current status or management needs. The only remedy is a thorough botanical search of appropriate habitats conducted during a low-water stage of the Mississippi River and lower stretches of the Minnesota River. To maximize chances of success, searches should probably concentrate on habitats downstream of Red Wing (Goodhue County).

  Conservation Efforts in Minnesota

Because there are no known extant populations of R. sessiliflora in Minnesota, no conservation efforts have been directed towards this species.

  References and Additional Information

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.

Patman, J. P., and H. H. Iltis. 1961. Preliminary reports on the flora of Wisconsin. No. 44 Cruciferae - mustard family. Wisconsin Academy of Sciences, Arts and Letters 50:17-73.

Rosendahl, C. O., and J. W. Moore. 1947. A new variety of Sedum rosea from southeastern Minnesota and additional notes on the flora of the region. Rhodora 49:197-202.

Stuckey, R. L. 1972. Taxonomy and distribution of the genus Rorippa (Cruciferae) in North America. Sida 4(4):279-430.