The western prairie fringed orchid (Platantherapraeclara) is an erect perennial herb with a showy inflorescence or flower stalk reaching up to 75 centimeters (29.6 in.) in height and producing 5–25 white flowers (Sheviak and Bowles 1986) (fig. 1, below). The orchid once grew throughout the western tallgrass prairie but is now restricted to remnant sites in six states and southern Canada. It has declined because of the drainage and direct loss of habitat resulting from agricultural expansion, and was listed as a federal threatened species in 1986 (U.S. Fish and Wildlife Service 1996).

Adnan Akyuz

Figure 1. The showy flower of the western prairie fringed orchid is a unique resource at Pipestone National Monument, where a small population of this threatened plant species persists. Scientists and managers are striving to understand factors affecting preservation of the orchid, including hydrology, precipitation, and prescription burns.

Extant populations of the western prairie fringed orchid are usually found in mesic (relatively good drainage but high moisture during most of the growing season) to wet tallgrass prairie habitats that are often subirrigated (moisture in the subsoil from a groundwater source). In these habitats, soil moisture is a critical determinant of the growth and flowering of orchid plants. Drought depresses this flowering and decreases survival (Sather 2000; Ashley 2001; Willson et al. 2006). Likewise, flooding or moisture-saturated soil depresses flowering and, if prolonged, kills orchid plants (Sieg and Wolken 1999; Willson et al. 2006). Prescribed burning of orchid habitat also influences flowering of the orchid and ultimately its survival. In a dry spring, burning can exacerbate soil moisture loss in tallgrass prairie (Knapp 1985), which causes orchid plants to abort flowers (Pleasants 1995).

Pipestone National Monument in southwestern Minnesota protects outcrops of pipestone or catlinite that American Indians have quarried from prehistoric times to the present (fig. 2). A small, isolated population of the western prairie fringed orchid occurs in the monument in mesic tallgrass prairie habitat of shallow soil overlying Sioux quartzite bedrock (Morey 1983). The orchid habitat is periodically prescription burned in spring to control invasive cool-season grasses, primarily smooth brome (Bromus inermis), but is not grazed by bison or cattle. Historical (1890s) photographs of the area that later became the monument show water flowing over a ridge of Sioux quartzite into the orchid habitat from multiple overflow channels of Pipestone Creek (fig. 3). In the early 1900s the creek bed was lowered about 3 meters (10 ft) at Winnewissa Falls on Pipestone Creek (fig. 4) and the channel upstream was straightened, which reduced surface flow of water into the orchid habitat.

Because of the shallow soil and altered hydrology of its habitat in Pipestone National Monument, the western prairie fringed orchid may be prone to greater moisture stress and lower survival than plants growing in other orchid habitat with deeper soils and intact hydrology. To determine if lower survival of orchid plants occurs in the monument, we compared the survival of orchid plants at Pipestone with the survival of another population of these plants at Sheyenne National Grassland in southeastern North Dakota (Sieg and King 1995). Sheyenne National Grassland protects one of the largest populations of the orchid in deep-soiled habitat that is usually wet but can be dry or flooded. Pipestone and Sheyenne are similar in general soil type (mesic loam), climate (total amount and distribution of precipitation), and management (prescribed burns and no cattle grazing in orchid habitat); they differ in soil depth and hydrology. Lower survival of orchid plants at Pipestone than that at Sheyenne would suggest the population at Pipestone would benefit from restoration of the natural hydrology. However, because of the extent and depth of channelization of Pipestone Creek in the monument and upstream, the only feasible response may be changes to the management regime (e.g., prescribed fire) in the orchid habitat.

Lower survival of orchid plants at Pipestone than that at Sheyenne would suggest the population at Pipestone would benefit from restoration of the natural hydrology.