Rare Species Guide

  Synonyms

Petalostemon candidus var. oligophyllus, Dalea occidentalis, Petalostemon occidentalis

  Basis for Listing

Dalea candida var. oligophylla is a Great Plains species that reaches the far eastern edge of its range in western Minnesota. There are a number of records of the species from the western tier of Minnesota counties, although most of the recent records are clustered in Lincoln and Yellow Medicine counties. Virtually all the records are from relatively high-quality dry prairies on sandy or gravelly glacial deposits. Overgrazing can eliminate this species from a habitat and is a threat.

In an unfortunate conflict between conservation and development, the prominent deposits of sand and gravel that support the habitat of this species are also the source of aggregate material demanded by road construction. The threat posed by sand and gravel mining is another serious threat to the long-term survival of this species in Minnesota, and it is a threat that is increasing every year. Dalea candida var. oligophylla was listed as a special concern species in Minnesota in 1996.

  Description

Dalea candida var. oligophylla is an herbaceous perennial with 1 or more stems reaching a maximum height of about 70 cm (27.6 in.). It is very similar to, and easily confused with, D. candida var. candida (white prairie clover), a common variety. However, while the stems of D. candida var. candida are stiffly erect, the stems of D. candida var. oligophylla tend to sprawl outwards, and they tend to be shorter than those of the common variety. The stems are either simple or the upper half of each stem may have a few branches. The leaves are alternate on the stem and are pinnately compound with 3-5 pairs of leaflets; each leaflet is 1-2 cm (0.4-0.75 in.) long, which is shorter than those of D. candida var. candida. With these smaller leaflets, leaves of D. candida var. oligophylla more closely resemble leaves of D. purpurea var. purpurea (purple prairie clover). Another difference between D. candida var. oligophylla and D. candida var. candida is that the flowers are not as densely packed into the spikes in the former as they are in the latter. This allows the axis of the spike to be easily seen between the flowers in pressed specimens. The flowers themselves are white and 4-5.7 mm (0.16-0.22 in.) long. The fruit is a dry, indehiscent pod 2.6-4.5 mm (0.10-0.18 in.) long, and it remains covered by the persistent calyx (Great Plains Flora Association 1986).

  Habitat

All of the Minnesota records of D. candida var. oligophylla are from dry prairies, especially the south and west-facing slopes of dry hill prairies. The soil in known habitats is sandy or gravelly or sometimes calcareous loam. It has been reported that where the two varieties of D. candida occur in the same area, var. oligophylla is found in the drier sites.

  Biology / Life History

Dalea candida var. oligophylla is potentially a long-lived perennial in stable prairie communities. It reproduces only by seeds, and the flowers are insect-pollinated. The seed pods and the seeds themselves contain seem to posses no specialized structures to aid in dispersal. It is likely that seeds are dispersed by one or more animal vectors. The modality of animal vectoring is most likely through consumption of the fruits (tiny pods each with a single "pea") and excretion of the seeds.

The best time to search for D. candida var. oligophylla is when flowers are present, typically during the months of June and July.

  Conservation / Management

All of the known Minnesota occurrences of D. candida var. oligophylla are in native prairies. Several of these sites are currently being grazed by cattle or have been grazed at some time in the past. It has been reported that D. candida var. oligophylla soon disappears with overgrazing so this is a matter of concern (Great Plains Flora Association 1986). The presence of plants in a grazed prairie would lead to the conclusion that at least some plants are able to survive some level of grazing intensity for some period of time, but not much can be said beyond that. For the purpose of conserving a rare species, it would seem prudent to limit the intensity and duration of cattle grazing in habitats where D. candida var. oligophylla occurs until research can clarify the species response to various grazing pressures. If some grazing is planned, careful monitoring should be incorporated into a management plan to determine if the species is being negatively impacted by the activity. In addition, habitats should not be broadcast sprayed with broadleaf herbicides.

Several programs and resources are available to land managers and landowners to help protect and manage remaining prairie parcels including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.

  Conservation Efforts in Minnesota

A number of sites where D. candida var. oligophylla is known to occur are publicly owned. This includes at least one state Wildlife Management Area and two state Scientific and Natural Areas. There are also at least two sites that are owned and managed by a private conservation organization. However, nothing is currently known about the health and viability of any of the occurrences or the habitats they occupy.

  References and Additional Information

The Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence. 1,402 pp.