Plants of Concern

Citizen Science Rare Plant Monitoring Program

Cypripedium candidum

White Lady’s Slipper Orchid
Cypripedium Candidum

Illustration by Kathleen Garness; reproduced with permission

A Case Study

This small, elegant wildflower was once abundant in Illinois, but its range is now limited primarily to the northeastern portion of the state.5

Family: Orchidaceae
Life History: Perennial
Illinois Status: Rare (not listed)

Populations monitored: 54
Subpopulations monitored: 121
Counties monitored: 6

Introduction

The white lady’s slipper orchid, Cypripedium candidum, is a distinctive but vulnerable member of the Illinois flora. Unlike many tropical orchid species that are epiphytic and grow on trees, the white lady’s slipper orchid is a terrestrial orchid that is native to the calcareous prairies and fens of Illinois1, 8, 10.  At only 20-36 cm tall1, it is one of the smallest of the 12 North American lady’s slipper orchid species (Cypripedium spp.), all of which are characterized by petals that form a pouch-like lower lip resembling the toe of a slipper. Of these, the white lady’s slipper orchid is distinguished by flowers with a white, magenta-spotted lip, twisted pale green-to-yellow petals and sepals, and ribbed leaves whose surfaces are covered in dense, short hairs9,14.

Habitat And Range

The distribution of the white lady’s slipper orchid extends from southern Canada in the provinces of Ontario and Manitoba into the United States, from New York west to North Dakota and south to Missouri11. Isolated populations occur as far south as Alabama1, 11. It has a coefficient of conservatism of 10, a designation that indicates a high degree of fidelity to undisturbed, remnant habitats10. The white lady’s slipper orchid almost always grows in wet, alkaline soils, ranging in pH from 7.0– 8.21, 8.  Unlike many lady's slipper orchids that grow in shaded woodlands, it favors open, sunny areas8.

Species Biology

In Illinois, the white lady’s slipper orchid first flowers in mid-April and blooms through early June in some areas10.  It is a long-lived perennial, reproducing vegetatively by rhizomes, as well as by seed; the time from seed germination to flowering ranges from 10 to 16 years3, 4. Pollinators of the white lady’s slipper orchid include small native Andrenid and Halictine bees, also known as miner bees and sweat bees1, 2, 12. Although this orchid produces a fragrance to entice its pollinators, it supplies no nectar, a strategy known as “deceptive pollination”1, 12.  This may contribute to the species’ high levels of pollen limitation13.  Fungi in the soil known as mycorrhizae provide nutrients, and in some cases carbon, to this orchid6, 7. These mycorrhizae also promote the germination of the small, dust-like seeds, and support the development of seedlings1.  

Threats

An analysis of historic county records demonstrated that, as of 1983, the white lady’s slipper orchid had declined by as much as 52 percent across its range1. This species has suffered substantial habitat alteration and loss because of agricultural development, peat and gravel mining, hydrological change, and urban expansion1, 5, 8.  In addition, the white lady’s slipper orchid is shade intolerant and vulnerable to threats from woody species1, 8. Plants of Concern (POC) monitors most commonly cite woody brush encroachment as the biggest threat to C. candidum populations.

Conservation Status

The white lady’s slipper orchid is listed as endangered or threatened throughout much of its range, and is afforded protected status in nine out of the 13 states where it occurs11. In Illinois, the species was first listed as endangered by the Illinois Endangered Species Protection Board in 1980. In the 1980s, nine populations were known. This number increased to 21 in the 1990s. In 1998, the species was downgraded to threatened, and in 2014 it was de-listed entirely.

Monitoring & Research

Since 2001, POC has monitored populations of the white lady’s slipper orchid at a total of 54 sites encompassing 121 subpopulations (distinct groupings of plants) throughout northeastern Illinois. In addition, intensive demographic monitoring occurred for 13 years at six sites in Illinois, where data regarding the number of individuals, percentage of reproductive individuals, seed set, insect herbivory on fruits, and damage to plants by deer and rodents were collected from 2001-13. Plants of Concern’s extensive data collection has drawn attention to the decline of this species, resulting in the discovery and management of populations that were not previously tracked, and contributing to an increased local and regional understanding of how populations are faring over time.

One outcome of this data collection effort has been to elucidate the effects of management on the white lady’s slipper orchid populations. For example, POC data demonstrated that populations had significantly more plants when prescribed burns and brush removal were employed as management practices. This may be because burning suppresses the growth of woody species. Burning may also stimulate flowering in the white lady’s slipper orchid, perhaps by enhancing the activity of soil mycorrhizae1. However, an analysis of data from POC-monitored populations indicated that in the year directly following a burn, the percentage of flowering plants was reduced significantly by approximately 14 percent6. It is possible that the timing of the prescribed burn—for example, whether it takes place in spring or fall—may be a critical factor in determining flowering response.

In addition to contributing to our awareness of management impacts on the white lady’s slipper orchid, data collected by POC volunteers benefit our understanding of other ecological factors that impact population dynamics. For example, site-based differences in soil nutrients and soil fungi have been shown to affect the reproductive effort of plants in populations monitored by POC, emphasizing the importance of edaphic factors in determining the species’ success.

Impacts

Because of the efforts of POC’s network of volunteers, we have delivered yearly reports to nearly 30 landowners across seven counties, providing them with critical information about the status of the white lady’s slipper orchid populations, including the response of these populations to management practices. Managers use these reports to prioritize management activities and adjust practices if populations are declining.

Data collected by POC are intended to promote the recovery of rare species. In 2014, the Illinois Endangered Species Protection Board recommended delisting the white lady’s slipper orchid because it considered the species to be “recovered and/or more common than originally thought.” Although the white lady’s slipper orchid remains uncommon, POC has helped document the 49 populations and 30 protected occurrences currently known in the state. POC volunteers have collected critical data documenting the extent of the white lady’s slipper orchid distribution, and highlighted the status of the remaining populations of a species once thought to be in danger of vanishing from Illinois.

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