Rare Species Guide

  Synonyms

Habenaria leucophaea var. praeclara

  Basis for Listing

Platanthera praeclara (western prairie fringed orchid) was first documented by the John C. Fremont expedition in what is now Wyoming in 1842 (Fremont 1845). It is presently known to occur in Manitoba, Canada and in Minnesota, North Dakota, Iowa, Missouri, Nebraska, and Kansas in the United States. It is believed to be extirpated from Oklahoma, South Dakota and Wyoming. Platanthera praeclara is rare because of the widespread conversion of native prairie for agricultural uses (Minnesota’s Remaining Native Prairie). Today it is known from ten Minnesota counties scattered throughout the prairie regions of the state. Historical collections from Douglas, Faribault, Freeborn, Fillmore, Goodhue, Hennepin, Houston, Kandiyohi, Martin, Nicollet, and Nobles counties are presumed to represent extirpated populations. The species’ complex relationship with soil fungi and pollinators means protecting wild plants in their natural habitat is the best option for ensuring the survival of the species. The species’ decline has become so critical that it was listed as state endangered in 1984 and federally threatened in 1989. Platanthera praeclara was subsequently assessed for the International Union for Conservation of Nature and Natural Resources (IUCN) Red List in 2008 as endangered and listed as endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2016.

  Description

Platanthera praeclara has a showy, open raceme of up to 24 white to creamy-white flowers, each with a long nectar spur. The lip, or lower petal, of each flower is deeply 3-lobed and fringed. The single smooth stem is up to 89 cm (35 in.) tall. Flowering plants have 3 or more smooth elongate leaves. Nonflowering plants have 1-3 leaves of similar appearance (Sheviak 2002; Smith 2012). The sepals of P. praeclara are deflected downward behind the lip, and the lateral lobes of the lip are raggedly fringed almost to the base. A more common member of the same genus is P. lacera (ragged fringed orchid). This species has a narrower, more compact spike of pale green-white flowers about half the size of P. praeclara, and its petals are more linear than those of P. praeclara.

  Habitat

The species is found almost exclusively in remnant native plant communities. In northern Minnesota (Aspen Parklands and Red River Prairie subsections), these are typically northern wet prairie, northern mesic prairie, and occasionally prairie wet meadow/carr. In southern Minnesota (North Central Glaciated Plains, Minnesota and Northeast Iowa Morainal, and Paleozoic Plateau sections), most populations are found in southern mesic prairie and occasionally southern wet prairie. The majority of sites occur in full sunlight on moist calcareous till or sandy soils. None of the sites have had a significant history of cattle grazing, though a few have a history of intermittent mowing for hay.

  Biology / Life History

More is probably known about Platanthera praeclara than most of the listed plant species in the state. The state’s Natural Heritage Program and later the Minnesota Biological Survey have studied this species since 1985. Platanthera praeclara has been documented emerging as early as late-March in southwestern Minnesota and early-April in northwestern Minnesota (Anderson 2018; Biederman et al. 2018). Plant senesces occurs in late September, or earlier if soil moisture is abnormally low. Over the growing season, a single bud is produced on the fleshy rhizome. That bud will remain dormant until the following spring, when it will develop into the aerial stem. If this bud is damaged in the interim, or fails to develop, no stem will be produced that year. Even without the stem, the rhizome may survive and produce another bud in late summer, which will develop into a stem the following year. If the emerging shoot of a flowering stem (which contains the apical meristem) is damaged by fire, there are no dormant buds in reserve, and it cannot resprout that year. The rhizome must survive that year without any photosynthetic capacity, and it must produce another bud for the following year. If the bud or emerging shoot is damaged again the second year, mortality will likely ensue.

Our demographic monitoring studies of the species has found that an individual is usually 4-6 years old between the time it first appears above ground and its first time flowering. Individual plants in Minnesota monitoring plots have persisted for as long as 25 years, including reappearances after presumed dormancy (Sather and Anderson 2010). Episodes of presumed dormancy of 1-2 years is common for both sterile and reproductive individuals.

Platanthera praeclara flowers produce a fragrant scent from the evening into the night to attract pollinators. While this scent may attract myriad insects, all known pollinators of this orchid are Sphingid moths (family Sphingidae), whose proboscis length must match the length of the nectar spur on the flower, which is 3-6 cm (1-2 in.) long. In the northern part of the species' range, known pollinators include the Galium Sphinx (Hyles gallii), the White-lined Sphinx Moth (Hyles lineata), the Wild Cherry Sphinx Moth (Sphinx drupiferarum), the Achemon Sphinx (Eumorpha achemon) and the non-native Leafy Spurge Hawkmoth (Hyles euphorbiae) (Fox et al. 2013; Harris et al. 2004; Westwood and Borkowsky 2003). Not all flowers are pollinated in any given season. In Minnesota, pollination rates have ranged from 8%-30% of flowers.

The seed pods of P. praeclara mature by autumn, at which time they split open and the tiny seeds are dispersed by wind. Some of the seeds will germinate the following spring and a few might wait another year. It is not known how many years pass between germination and when the seedling first appears above ground (Smith 2012).

Another important aspect of the species’ life history is its interaction with soil fungi. Seed germination and seedling development cannot progress without mycorrhizal fungi (Sharma et al. 2003). Additionally, the recent work of Kaur et al. (2019) found P. praeclara selects a narrow group of mycorrhizal fungi across its range, over its phenological development, throughout the growing season, and over multiple seasons of study. Their work also found that fungi associated with the orchid are patchy and localized in distribution in the soil as well. Their work concludes that this combination of factors creates a bottleneck that limits this species distribution and recruitment.

  Conservation / Management

Conservation efforts for this species should be directed toward protection of its high quality, intact, native habitat. The complex relationship that exists with mycorrhizal fungi for germination and seedling establishment and the highly specific pollinators are best maintained in these settings. Prolonged droughts lasting several years can also severely increase mortality and reduce flowering of surviving plants. It is also known that plants do not persist when flooded for extended periods during the growing season.

Cattle grazing is a management tool that should be avoided at sites with P. praeclara. Cattle will not only graze directly on the orchids, they will also trample the sensitive habitat and facilitate the invasion of non-native species that may be impossible to control (Alexander et al. 2010).  Annual haying can present another problem to long-term survival if done prior to seed capsule maturation and dehiscence. Platanthera praeclara reproduces entirely by seed, and annual mowing could lead to reproductive failure and eventually population collapse.

The effects of fire must also be considered. Before settlement, the prairie habitats of P. praeclara were exposed to wildfire at intervals of perhaps 2-4 years. While fire was necessary for the proper function of the prairie ecosystem, native prairie has now been reduced to small isolated fragments that no longer function as an ecosystem. In an effort to restore ecological function to these isolated fragments, fire is often reintroduced on a prescription basis. Prescribed fire can be very useful in managing P. praeclara habitats, but timing is critical. Fire should be limited after the orchids have emerged (as early as late March in southwestern Minnesota and early April in northwestern Minnesota). If the growing tips of P. praeclara are even 5-7 cm (2-3 in.) above the ground, they may be damaged or killed by fire. The species is not able to resprout if the growing tip is severely damaged by fire. At least two poorly-timed burns have resulted in severe damage to important populations of P. praeclara. It will take years for these populations to recover. Fall burns may have similar effects as haying. If done too early (before mid- to late September), they may destroy the seed capsules before the seed is fully dispersed.

  Best Time to Search

The best time to search for Platanthera praeclara is when plants are at peak flowering, from early to mid-July. Identification is possible after these dates; however, plants are more difficult to detect.

  Conservation Efforts in Minnesota

Several sites of P. praeclara occur on publically owned property, on lands owned by private conservation organizations, and on privately owned sites with conservation easements. This is a positive step in protecting this rare species. Many land managers and agencies consult with Minnesota Biological Survey experts in an attempt to minimize negative impacts to P. praeclara as it relates to their management activities. It is essential that core populations of P. praeclara be designated and managed in accordance with practices that are known to enhance populations and habitats of this species. Additionally, Minnesota plays a critical role in collaboration with range-wide research efforts studying P. praeclara.

  Authors/Revisions

Derek S. Anderson and Welby R. Smith (MNDNR), 2020

(Note: all content ©MNDNR)