Sensitive Joint-vetch

Aeschynomene virginica

Description

Sensitive joint-vetch, is an annual legume of the pea family that attains a height of 3-6 ft (1-2 m) in a single growing season. The stems are single, sometimes branching near the top. Leaves are even-pinnate, 0.8-4.8 in (2-12.2 cm) long, with entire, gland-dotted leaflets. The irregular, legume-type flowers are about 0.4 in (1 cm) across, yellow, streaked with red, and grow in racemes (elongated inflorescences with stalked flowers). The fruit is a loment with six to 10 segments, turning brown when ripe.

Flowering begins in late July and continues through September. Fruits are produced from July to the first frost. Some observations indicate that seedlings may germinate only in "flotsam" (plant material) that has been deposited on the riverbank.

Sensitive joint-vetch requires the unique growing conditions occurring along segments of the river system that are close enough to the coast to be influenced by tidal action, yet far enough upstream to consist of fresh or slightly brackish water.

The present distribution of sensitive joint-vetch includes New Jersey (two occurrences), Maryland (one occurrence), Virginia (six occurrences), and North Carolina (three marginal occurrences).

Habitat

A rare and specialized ecological community type occurs a short distance upstream of where certain rivers in the coastal plain of the eastern United States meet the sea. Referred to as freshwater tidal marshes, these communities are close enough to the coast to be influenced by tidal fluctuations, yet far enough upstream to consist of fresh or only slightly brackish water. Plants that grow in this environment are subjected to a cycle of twice-daily flooding that most plants cannot tolerate. Sensitive joint-vetch is a plant of such freshwater tidal communities.

Distribution

The number of sensitive joint-vetch populations has declined significantly throughout the species' range, and this plant has been extirpated entirely from Pennsylvania and Delaware. At present there are two known populations in New Jersey, one in Maryland, six in Virginia, and three in North Carolina.

Whether due to causes mentioned above, or to other as yet unidentified threats, the range of sensitive joint-vetch along river systems in Virginia is contracting. On both the Rappahannock and the James Rivers, sensitive joint-vetch was collected historically some 10 mi (16.1 km) farther upstream and downstream than it is currently known to exist. It remains on only one section of the Chickahominy River, where it once had a much broader distribution.

The currently known distribution of sensitive joint-vetch is as follows. New Jersey: one small occurrence (approximately 50 individuals) on the Wading River in Burlington County and one large occurrence (approximately 2,000 individuals) on the Manumuskin River in Cumberland County. Maryland: one occurrence of several hundred individuals on Manokin Creek, in Somerset County. North Carolina: sensitive joint-vetch was known to occur in two ditches in Hyde County and one ditch in Beaufort County. Virginia: it is believed that the total number of plants in the state is about 5,000.

Observations in North Carolina have indicated severe predation of seeds by tobacco budworms and corn earworms. However, it is unlikely that these predators will prove to be a problem in other populations throughout the species' range, as they do not occur in typical wetland habitats.

It has been speculated that the existence of sensitive joint-vetch may be threatened over the long-term by sea level rise. This phenomenon could result in merely "pushing" the species' habitat upstream from its present position. However, the location of major cities and other developed area upstream from the fresh/brackish water interface in many locations may block the migration of natural freshwater marsh communities and their component species, including sensitive joint-vetch.

Threats

The extirpation of sensitive joint-vetch from Delaware and Pennsylvania and its elimination from many sites in other states can be directly attributed to habitat destruction. Many of the marshes where it occurred historically have been dredged and/or filled and the riverbanks bulk-headed or stabilized with riprap. This is most evident in historic locations around Philadelphia. Other sources of potential or actual habitat destruction include impoundments and water withdrawal projects, road construction, commercial and residential development, and resultant pollution and sedimentation.

The remaining stronghold of sensitive joint-vetch is in Virginia, along the relatively narrow band of freshwater tidal sections of several river systems on the coastal plain. These river sections are quite pristine, despite their proximity to the major metropolitan areas of Washington, D. C., and Richmond, Virginia. As the suburbs associated with these cities expand, the impacts to these river sections from residential and commercial development, shoreline stabilization activities, point and non-point source discharges, recreational use, water development projects, and sedimentation from building and road construction are all expected to increase greatly.

Certain of these factors are known to be harmful to sensitive joint-vetch; others require further study to determine their effects. Shoreline stabilization, as in placement of riprap, can destroy the species' habitat directly. Increased motorboat traffic is known to be detrimental to freshwater tidal systems. In addition to direct toxic effects from fuel leaks, the wave action from boat wakes can rapidly erode the mud-flats and banks where sensitive joint-vetch grows.

Sedimentation could effect sensitive joint-vetch by inhibiting germination, smothering seedlings, and/or promoting the invasion of weedy species. Establishment of phragmites or other invasive species could be especially detrimental to sensitive joint-vetch, which has evolved to thrive in an environment with little competition from other plants.

Two specific projects could threaten New Jersey's large population of sensitive joint-vetch. One is the extension of a major highway, which is proposed to cross the Manumuskin River in the vicinity of the population. The plants and their habitat could be destroyed directly, during the construction process, or indirectly, through input of sediments, road salt or petrochemicals. The other project is a coal-fired electric generating facility, proposed to be upstream from the population. There is concern that the disposal of by-products from this facility could degrade the species' habitat.

Maryland's one known sensitive joint-vetch population is in an area heavily affected by humans, adjacent to a major highway, a sewage treatment plant, and a residential development. The population is also flanked by invasive weeds, including Phragmites australis and multiflora rose. Fortunately, a larger segment of this population was discovered nearby in 1991, in a less heavily impacted setting.

Conservation and Recovery

Because sensitive joint-vetch occurs in wetland habitats, many projects potentially affecting it would be within the permitting authority of the U. S. Army Corps of Engineers.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
300 Westgate Center Dr.
Hadley, Massachusetts 01035-9589
Telephone: (413) 253-8200
Fax: (413) 253-8308
http://northeast.fws.gov/

Ecological Services Field Office
Bldg D, 927 North Main St.
Pleasantville, New Jersey 08232
Telephone: (609) 646-0620
Fax: (609) 646-0352

Ecological Services Field Office
1825-B Virginia Street
Annapolis, Maryland 21401
Telephone: (410) 269-5448
Fax: (410) 269-0832

Ecological Services Field Office
Mid-County Center, U. S. Route 17
P. O. Box 480
White Marsh, Virginia 23183
Telephone: (804) 693-6694
Fax: (804) 693-9032

Reference

U. S. Fish and Wildlife Service. 20 May 1992. "Endangered and Threatened Wildlife and Plants; Determination of Threatened Status for the Sensitive Joint-vetch (Aeschynomene virginica )." Federal Register 57 (98): 21569-21574.