Basis for Listing

Viola nuttallii (yellow prairie violet) is chiefly a species of the western plains and grasslands. It is largely absent from the tallgrass prairie region of the Midwest, and occurs in Minnesota only in dry, semiarid habitats in the southwestern corner of the state. First discovered in 1981 on a series of four isolated gravel hills in Lac Qui Parle County, the species was subsequently found in 1999 in adjacent Yellow Medicine and Traverse counties. One of the previously known sites has been largely destroyed by gravel mining, and this activity poses a threat to remaining populations. Viola nuttallii was originally listed as a special concern species in Minnesota in 1984, but given its extreme rarity, was reclassified as threatened in 1996.

  Description

Viola nuttallii is a perennial herb that flowers locally in May. The species is the only yellow-flowered violet found in Minnesota prairies. It has short, aboveground stems, or only subterranean stems. The aerial structures are 2-12 cm (0.8-4.7 in.) long. Flowers are deep yellow with 5 sepals, 5 petals, and 5 stamens. The 2 lateral petals are bearded, and the lower 3 have brownish-purple lines. Leaves are often 5 cm (2 in.) long or longer, ovate or lanceolate. Solitary flowers arise from the axils of the upper leaves. The fruit is a dry, 3-valved, many seeded capsule.

  Habitat

The known sites of V. nuttallii are on gravelly kame and morainic formations; more specifically, on slopes and summits of dry prairie. The species prefers well-drained, loose, exposed soil where competition is minimal. It occurs with other rare species such as Astragalus missouriensis var. missouriensis (Missouri milk-vetch) and Xanthisma spinulosum var. spinulosum (cutleaf ironplant). Associated common species include Geum triflorum (prairie smoke), Heuchera richardsonii (alumroot), and Lomatium orientale (desert parsley).

  Biology / Life History

The characteristic flowers of the genus Viola are highly specialized for insect pollination and are attractive to bees and ants. The spurred lower petal produces nectaries, which extend into the lower spur (Zomlefer 1994). At maturity, the seed capsules open explosively, scatter the seeds from the parent plant, and are subsequently foraged by ants and rodents (Turnball and Culver 1983).

  Conservation / Management

Viola nuttallii occurs only in dry prairies that develop on prominent glacial deposits of sand and gravel (kames and moraines). These deposits are in high demand for use in road construction; in fact several have already been mined. Mining the gravel substrate obviously destroys the prairie habitat and any populations of V. nuttallii that might be present. Habitats supporting this species must be protected from aggregate mining if this species is to survive in Minnesota. Other activities potentially causing direct damage to vegetation are livestock grazing and off-road vehicle use. An indirect consequence of habitat disturbance is the establishment and spread of non-native species such as Poa pratensis (Kentucky bluegrass). Apart from habitat protection, management of this species is relatively straightforward. Occasional dormant season fire conducted in early April, before V. nuttallii appears above ground, is the primary management activity necessary.

  Best Time to Search

The best time to search for this species is when V. nuttallii is in flower from late April to late May.

  Conservation Efforts in Minnesota

At least one site where V. nuttallii occurs has been purchased by the state and is presently managed as a Scientific and Natural Area.

  References and Additional Information

Fabijan, D. M., J. G. Packer, and K. E. Denford. 2011. The taxonomy of the Viola nuttallii complex. Canadian Journal of Botany 65(12):2562-2580.

Ownbey, G. B., and W. R. Smith. 1988. New and noteworthy plant records for Minnesota. Rhodora 90:369-377.

Turnbull, C. L., and D. C. Culver. 1983. The timing of seed dispersal in Viola nuttallii: attraction of dispersers and avoidance of predators. Oecologia 59:360-365.