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Small Whorled Pogonia (Isotria medeoloides)


2.1   Description 

2.1.1   Description of the species  

Small whorled pogonia (Isotria medeoloides) is a small orchid which stands between 9.5-25.0cm tall (Brownell 1982).  Five to six leaves form a single whorl at the top of the stem (White 1998).  These glaucous leaves are elliptic to elliptic-obovate and pale milky green in colour (U.S. Fish and Wildlife Service 1992).

The leaves may be overtopped by one or two flowers which are yellowish-green in colour.  The sepals of the flower tend to be slightly longer than the petals (White 1998).  When it is not in flower, the Small whorled pogonia can be mistaken for the more common Indian Cucumber-root (Medeola virginiana) (U.S. Fish and Wildlife Service 1992).

2.1.2   Populations and distribution  

The Small whorled pogonia inhabits portions of eastern North America.  In the United States, the species occurs from New England and Michigan south to Missouri and South Carolina.  The only Canadian occurrence is in Elgin County in southwestern Ontario (White 1998) (Figure 1). 

Figure 1. Canadian Distribution of Small whorled pogonia

Figure 1. Canadian Distribution of Small whorled pogonia

The Small whorled pogonia’s distribution has led to the species being referred to as an Atlantic Coastal Plain disjunct, resulting from past geological and climate events (Brownell 1982).

Currently, Small whorled pogonia is considered globally imperiled (G2).  In the United States, it is listed as nationally imperiled (N2).  The species is identified as critically imperiled (S1) in 13 states, imperiled (S2) in 4, historical (SH) in 4 and is considered extirpated (SX) from District of Columbia.  In Canada, Small whorled pogonia is listed as nationally critically imperiled (N1).  In Ontario, it is considered critically imperiled (S1) (NatureServe 2006) (Table 1).

Once considered Endangered in both Canada and the United States, Small whorled pogonia has been reclassified as Threatened in the United States due to an increase in the number of known populations from 34 in 1985 to 104 in 1993, with at least 25% of the plant’s self sustaining populations protected through public ownership or private landowner management agreements (U.S. Fish and Wildlife Service 2006).  It remains designated as Endangered within Canada.  In Ontario, the species is designated as Endangered and is regulated under the Ontario Endangered Species Act.

In Canada, the species is found at only one location in southwestern Ontario, in the Calton Swamp of Elgin County (Environment Canada 2004). This property is owned by Catfish Creek Conservation Authority (CCCA). The species has been noted in four distinct areas within a few hundred metres of one another (White 1998). 

The species was first discovered in 1977 by Mr. W.G. Stewart, who located a total of 4 plants (Brownell 1982). No thorough inventory has been undertaken to determine the extent of the occurrence, however, searches have located plants in 4 locations on CCCA property (White 1998).  In 1989, a single vegetative plant was located at the site but was not reported again until 1998 (see Table 2).  The last recorded sighting of the species was in 1998 when a single vegetative individual was noted by Mr. Glen Martin (Oldham, pers. comm. 2006).  Since the orchid could stay in dormancy for up to twenty years (see section 1.2.1), it is unknown whether it has become extirpated or still exists in low numbers within Calton Swamp (White 1998). This represents less than 1% of its global distribution. 

Table 1. Subnational ranks for the Small whorled pogonia(NatureServe 2006)

S1- Critically ImperiledConnecticut, Delaware, Illinois, Massachusetts, Michigan, New Jersey, North Carolina, Ohio, Ontario, Pennsylvania, Rhode Island, South Carolina, Tennessee, West Virginia.
S2 – ImperiledGeorgia, Maine, New Hampshire, Virginia.
SH – HistoricalMaryland, Missouri, New York, Vermont.
SX – ExtirpatedDistrict of Columbia.

Table 2.  Last reported sightings of Small whorled pogonia (White 1998)

StationLast observed (#)
Station 11981 (3)
Station 21980 (1)   * Destroyed by bike trail
Station 31982 (2)
Station 41989 (1)

Note:  The 1998 report did not specify the station location

2.2   Description of the species’ needs 

2.2.1   Ecological role, biological needs, and limiting factors 

Populations of Small whorled pogonia typically consist of plants that may be in any of four life stages: dormant, vegetative, with abortive bud, or flowering (U.S. Fish and Wildlife Service 1992).  Flowering plants tend to be taller and have larger whorls than those plants with abortive buds or which are vegetative (U.S. Fish and Wildlife Service 1992).

Small whorled pogonia generally flowers from mid-May to mid-June with flowers lasting from a few days to almost two weeks (U.S Fish and Wildlife Service 1992; U.S. Fish and Wildlife Service 2006).  The plants are thought to be primarily self pollinated, as the flowers lack nectar guides and fragrance (U.S Fish and Wildlife Service 1992; U.S. Fish and Wildlife Service 2006).  If pollination occurs, a seed capsule containing thousands of tiny seeds may be formed and it is estimated that approximately 83% of flowers form capsules (Brownell 1982).  The species only occasionally reproduces vegetatively but this is not its primary reproductive strategy (U.S Fish and Wildlife Service 1992). Other aspects of the plant’s life history are not fully understood including seed dispersal and seed germination.

Like other orchid species, Small whorled pogonia has an interdependence on mycorrhizal fungi.  Such fungi have been noted in the tissues of mature individuals although no specific mycorrhizal fungi have been identified.( U.S Fish and Wildlife Service 1992).

Questions still remain around dormancy.  Some studies suggest that the plant may undergo periods of dormancy lasting up to 20 years (U.S Fish and Wildlife Service 1985 in Difazio 2003).  However, other studies have found that no plants emerged after 3 consecutive years of dormancy (Mehrhoff 1989).  Research into this aspect of the plant’s biology is necessary.

No specific information is available related to the ecological role of the Small whorled pogonia at the Calton Site.  The fact that the plant has only been identified at one site in Canada would suggest that studies are required to determine if the plant is an ecological indicator of a rare habitat type or ecological pattern on the landscape (Difazio 2003).

Shading is a limiting factor for the species.  One study, conducted in the United States, found that sites where colonies of Small whorled pogonia had become extirpated were more shaded by vegetation than were the extant sites (Mehrhoff 1989).  In New Hampshire, canopy defoliation from Gypsy Moths, preceded the discovery of the orchid at several sites in the 1980s (Difazio 2003).   In addition, the populations at Calton are located within a few metres of an old logging road that had resulted in reduced canopy cover and an associated decrease in shading (Difazio 2003).  Alterations in the microclimate of the site, such as increase in temperature or a decrease in moisture due to natural processes, human disturbance, or climate change, may also limit the species ability to survive (Brownell 1982).  Such changes may cause plants to become dormant or die out completely.

Small whorled pogonia is rare throughout North America.  The possibility that the occurrence represents very few plants implies that the genetic diversity of the population may be very low, and this may limit its long term viability.  The species susceptibility to site disturbance, low reproductive rates, wide population fluctuations, complex life history and extended periods of dormancy all contribute to limiting the plants survival throughout North American (Difazio 2003).

2.2.2   Habitat needs  

Small whorled pogonia inhabits damp mixed woods with an acidic soil (White 1998).  There is usually abundant leaf mould and limited shrub and herbaceous cover (Brownell, 1982).  Various types of decaying vegetation are usually found in Small whorled pogonia habitat, including fallen trunks and limbs, leaf litter, bark, stumps, and roots of dead trees (U.S Fish and Wildlife Service 1992).  The orchid prefers a habitat with flatter terrain and a canopy with small openings to provide light (Brownell 1982).

When openings in the tree canopy allow more light to reach the forest floor, Small whorled pogonia plants typically respond favourably, at least in the short term.  One study reported exceptional vigour in plants adjacent to a recent clear cut, and smaller, less vigorous plants away from the clearing. (NatureServe 2006). 

2.3   Threats 

2.3.1   Trampling 

Due to the plant’s high profile as a very rare orchid, the site has been visited by many naturalists, orchid enthusiasts, and photographers.  This puts the plant at considerable risk from inadvertent trampling (Difazio 2003).

2.3.2   Habitat degradation 

A possible threat to the Canadian population of Small whorled pogonia is lack of suitable habitat in the heavily modified Carolinian Zone of southwestern Ontario.  In the past, a trail used by dirt bikes and ATVs passed through the Calton Swamp site, with at least one subpopulation being destroyed by these activities (Difazio 2003).   However, Catfish Creek Conservation Authority (CCCA) has implemented initiatives to close trails in the area and prohibit the operation of motorized vehicles on the property to eliminate further destruction of habitat and populations. (Difazio 2003).

Events causing dramatic increases in the amount of light reaching the forest floor might cause the herbaceous layer to flourish.  This would result in more competition and increased shading which would in turn reduce the suitability of the habitat (U.S. Fish and Wildlife Service 1992).

2.3.3   Herbivory 

Herbivory by deer is a known threat to Small whorled pogonia populations in the United States and may be a threat in Canada.  In New England, herbivory by slugs is considered by some to be a serious threat to the orchid (U.S. Fish and Wildlife Service 1992).  It has been suggested that “touching the plants may leave salts on the leaves that are, in turn, attractive to slugs” (U.S. Fish and Wildlife Service 1992), so there may be a compounding effect from visitors touching the orchids, resulting increased susceptibility to slugs.  Slugs have not been identified as a known threat to the species in Canada, however it should be considered a potential threat to the Canadian population. 

2.3.4   Collection 

Although collection has not been recorded within the Canadian population, this activity is a potential threat.  The release of locational information on Small whorled pogonia population increases the potential for collection.  For example, “within days after a newspaper article was published revealing the location of a site in Connecticut, the plants had been dug up and removed” (U.S. Fish and Wildlife Service 1992).

2.3.5   Loss of genetic diversity 

Reductions in population size can decrease the genetic viability of populations over time and this can, in turn, lead to lower individual fitness and population viability.  The Canadian population of Small whorled pogonia may be vulnerable to genetic loss due to its small size.  As well, pollen transfer and seed immigration from populations in the United States is very unlikely to occur due to the distance separating the populations. 

2.4   Actions already completed or underway

In August of 1994, the Catfish Creek Conservation Authority (CCCA) completed an “Interim Operational Plan” for properties near Calton Swamp.  The property containing the Small whorled pogonia has site specific policies in place to protect the plant and supporting habitat.  Policies state that the property is not to be altered in any way without an Environmental Impact Assessment being conducted to ensure that no harmful impacts to the Pogonia sites will result.

In July of 2001, the CCCA completed the “Small Whorled Pogonia Biological Inventory Project”.  A report was prepared to summarize local biological information on the Small whorled pogonia population at Calton, including all known sightings and occurrence reports.  The report also provided recommendations on future recovery efforts and management strategies.

2.5   Knowledge Gaps 

The biological needs of the Canadian population are not well understood.  Therefore, information on the topography, soil conditions, associated species, seed dispersal, and light conditions must be studied to fill in these information gaps.

It is believed that too much shading may be a limiting factor for the species, however it is unknown if this is a current limiting factor to the population in Ontario.  It is also unknown if the species would benefit from forest management to maintain optimum light levels.  Research into these aspects of the plant’s biology should be undertaken to fill these gaps and guide site management.

As only one occurrence has been noted for this species in all of Canada, studies should be undertaken to determine if the species exists in similar habitat in the other parts of southwestern Ontario.

The issue of dormancy in Small whorled pogonia still remains a matter of debate among researchers.  Early research suggested that dormancy could extend from 10-20 years while other studies suggested dormancy periods of up to 4 years (U.S. Fish and Wildlife Service 1992).  Dormancy for this species should be studied as it has direct impact on the recovery of this species.

Small whorled pogonia, like other orchid species, has a relationship with mycorrhizal fungi.  Mycorrhizal fungi have been seen in the tissue of mature individuals; however no specific mycorrhizal fungus has been identified.  This aspect of the species biology should be investigated.  Such information may aid in the reintroduction of the species if it is deemed necessary in the future.