Rare Species Guide

  Basis for Listing

Cypripedium arietinum (ram’s head orchid) has always been considered biologically rare in Minnesota, and over much of its range in northeastern North America. Reasons or its rarity in Minnesota are not entirely known. However, it has suffered a general decline largely because of habitat loss due to changes in land use. For example, recent clearing of Pinus banksiana (jack pine) forests on public and private land in northwestern Minnesota, where some of the largest populations of C. arietinum occur, could compromise the conservation of this species in the state. This rare orchid originally ranged throughout most of the forested region in northern and central Minnesota, but recent surveys of potential habitat and historic sites have failed to document any extant populations south of Aitkin County. Cypripedium arietinum was originally listed as an endangered species in Minnesota in 1984 and was reclassified as threatened in 1996.

  Description

Like all members of this genus, C. arietinum has 3 petals, 1 modified into a pouch or "slipper." In C. arietinum, the pouch is densely pubescent and white with purple markings, which, with a little imagination, looks like the head of a charging ram. It has 2 or more alternate leaves borne nearly to the top of the stem. The lateral sepals are free rather than fused, which distinguishes C. arietinum from all other Cypripediums (Smith 2012).

  Habitat

Perhaps the most common habitat of C. arietinum in Minnesota is lowland conifer forests. They tend to be dominated by Thuja occidentalis (northern white cedar) and Picea mariana (black spruce), and have a substrate of saturated peat overlain by a carpet of Sphagnum moss. The moss cover will, in places, be broken by shallow, water-filled hollows that have soft, mucky bottoms. The water in these hollows usually marks the top of the water table. Expect to find C. arietinum in somewhat raised mossy areas where its roots are some distance (inches) above the water table. These swamp habitats are usually influenced, to some extent, by subsurface water that has flowed through nearby uplands. That is how water picks up mineral ions, which are plant nutrients that are in short supply in peatlands.

Upland habitats are also important for C. arietinum, and they vary quite a bit. Some are in mesic broad-leaved forests, but most are in mature pine forests of natural origin, especially those dominated by Pinus banksiana or Pinus resinosa (red pine). In this type of habitat, the rooting zone of C. arietinum is usually in sandy soil or in thin mineral soil over bedrock. Although these soil types are considered “droughty”, the rooting zone of C. arietinum would, under most conditions, be relatively cool and moist. This is because the tree canopy creates a shady, humid environment, and the ground itself is covered by mosses and lichens. Populations of C. arietinum in upland habitats are typically larger and denser than those in swamps and sometimes contain several hundred stems.

It appears that the underlying habitat elements that are of greatest importance to C. arietinum are probably the health of the mycorrhizal community and the composition of the pollinator community.

  Biology / Life History

Cypripedium arietinum is an obligate mycorrhizal species, at least during some portion of its life cycle. In other words, it obtains its nutrients from soil fungi that have a symbiotic relationship with trees. Maintaining a healthy mycorrhizal community is largely dependent on a stable and healthy population of trees and healthy soil conditions. Minimum habitat size required to maintain a stable, diverse mycorrhizal community is not known and likely varies depending on species and local conditions. It is reasonable to conclude that a minimum habitat requirement includes a functioning, reproducing population of native trees, though quantifying this requirement is difficult.

Likewise, quantifying a healthy soil environment is difficult. Visual evidence of soil compaction, rutting, and erosion would indicate severe damage, but more subtle damage must be inferred from indirect evidence, such as the perceived changes in the vigor of associated biological communities.

Maintaining a healthy pollinator community is dependent on maintaining a healthy community of food plants and nesting sites suitable for the pollinators (small solitary bees, including those in the genera Dialictus [Halictidae] and Megachile [Megachilidae]). Delineating such a habitat would be difficult without a careful site evaluation, and even then, it might rely on professional judgement.

In essentially all situations that can be anticipated, the habitat of a population of C. arietinum would be greater than the spatial extent of the plants themselves. In other words, drawing a polygon that connected the outermost individuals of the population would likely delineate only a portion of the minimum habitat needed to sustain the population.

  Conservation / Management

A population of C. arietinum that contains a favorable demographic structure, including sexually reproducing individuals, juveniles, sub-juveniles, emerged seedlings, protocorms and seeds, can be assumed viable as long as the important elements of its habitat remain intact. These important elements were described in previous paragraphs. Stochastic events such as wild fire, drought, insect outbreak, or windstorms will likely not affect the long term survival of a population of C. arietinum as long as the habitat remains unfragmented and functional.  Such episodic and naturally occurring events are considered part of normal variability within the habitat. They generally do not result in significant fragmentation of the habitat unless the scale and duration of the events are exceptional.

That said, a single biological population of any plant species cannot be considered permanent. It will, over time, explore previously unoccupied niches (through seed dispersal) and may appear to move. The seeds of C. arietinum are small, airborne, and have the potential to travel long-distances on wind currents. However, it is believed that viable seeds of C. arietinum usually travel short distances, rarely more than a few meters. In the case of C. arietinum, residency at any one particular place is likely to be decades or centuries. At least one Minnesota population of C. arietinum that was discovered as much as 100 years ago is known to remain in place and viable (as of 2012).

Given the sedentary nature of plant populations, management recommendations cannot be linked to a particular season of the year. Unlike certain animal species that might use a habitat only during one well-established season of the year, plants are essentially rooted in one place.

In the case of a viable population of C. arietinum, no management will be necessary. Enhancement of an existing habitat through artificial means may be impractical and, to the best of my knowledge, has never been successful. 

  Best Time to Search

The best time to search for Cypripedium arietinum is when it is in flower from late May through mid-June.

  Conservation Efforts in Minnesota

Several C. arietinum sites occur on publicly owned land where they are protected from most types of land conversion, but several are on private property or public property where they receive no special consideration. The status of several populations have been monitored, providing useful information that can help guide future conservation efforts.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

Brower, A.E. 1977. Ram's-head lady's-slipper (Cypripedium arietinum R. Br.) in Maine and its relevance to the Critical Areas Program. Planning Report 25. State Planning Office, Augusta, Maine. 14 pp.

Catling, P. M., and V. R. Catling. 1991. A synopsis of breeding systems and pollination in North American orchids. Lindleyana 6(4):187-210.

Curtis, J. T. 1943. Germination and seedling development in five species of Cypripedium. American Journal of Botany 30:199-206.

McCormick, M. K., D. F. Whigham, D. Sloan, K. O'Malley, and B. Hodkinson. 2006. Orchid-fungus fidelity: a marriage meant to last? Ecology 87(4):903-911.

Pilj, L. van der, and C. H. Dodson. 1966. Orchid flowers: their pollination and evolution. American Orchid Society and University of Miami Press, Coral Gables, Florida. 214 pp.

Smith, W. R. 2012. Native orchids of Minnesota. University of Minnesota Press, Minneapolis, Minnesota. 400 pp.

Stoutamire, W. P. 1967. Flower biology of the lady?s-slippers (Orchidaceae: Cypripedium). Michigan Botanist 6(4): 159-175.