Recovery Strategy for Pitcher's Thistle in Canada [Final Version]: Background
Scientific Name: Cirsium pitcheri (Torr.) T. & G.
Common Name: Pitcher's Thistle
COSEWIC status: Endangered
Date of Assesment: May, 2000
Canadian Occurrence: Ontario
Reason for Designation: An endemic of the Great Lakes shorelines found at only a few sites. It has a very limited area of occurrence with recent population losses and continued risk from low seed set and habitat degradation. It faces additional loss because of recreational use and development of its habitat.
Status history: Designated Threatened in April 1988. Status re-examined and designated Endangered in April 1999. Status re-examined and confirmed in May 2000.
Pitcher's Thistle is listed as Endangered and is on Schedule 1 of the federal Species at Risk Act (SARA). Pitcher's Thistle is listed as Endangered on Schedule 1 of the federal Species at Risk Act (SARA). In Ontario species is listed as Endangered on the Species at Risk in Ontario list under the Endangered Species Act, 2007 (ESA 2007). The global range of Pitcher's Thistle is completely restricted to the shores of Lakes Huron, Michigan and Superior, occurring in four U.S. states and Ontario. The global rank of Pitcher's Thistle is Vulnerable (G3) (NatureServe 2010). It is currently listed as Critically Imperilled (S1) in Illinois, Imperilled (S2) in Indiana, Wisconsin, and Ontario, and Vulnerable (S3) in Michigan. The species is federally listed as Threatened in the United States. The species was extirpated in Illinois but has been successfully reintroduced at one site. Some Indiana populations are re-introductions as well. (NatureServe 2010). The Canadian range of Pitcher's Thistle probably accounts for less than 1/3 of the global geographic distribution.
Pitcher's Thistle is a perennial plant with a distinctive whitish-green colour. It usually appears as a ring of basal leaves (a "rosette") that are finely divided into narrow segments and spineless except at the tips. Rosettes are generally 15-30 cm in diameter.
Pitcher's Thistle spends the first year as a seedling, and then may spend the next 2 to 11 years as a rosette (Loveless 1984, Stanforth et al.1997, Maun 1999). At maturity the plants produce an upright stem (~50-100 cm tall) with one to many spiny, urn-shaped thistle heads of many white or pale pink flowers. After pollination, each flower produces a seed-like fruit that may be blown from the plant by wind due to a fluffy attachment that serves as a parachute. After setting seed the plant dies. Pitcher's Thistle cannot spread vegetatively.
Dunes, and the biota that inhabit them, are different from most other ecosystems and their biota. They are an exceptionally dynamic and constantly changing ecosystem and the plants and animals that live in them are remarkably mobile. Hence, if we are to protect them, individually or collectively, we must try to protect the whole ecosystem - the complete dune system from lake shore (which itself is constantly changing) to the belt of mature vegetation behind the dunes. We must strive to protect sufficient areas within each location to ensure that the whole ecosystem can continue to flourish, as well as Pitcher's Thistle within it.
In Canada, Pitcher's Thistle habitat is only found on sand dunes and beach ridges on the shorelines of Lake Huron, and on two beach ridge sites on Lake Superior. Optimal habitat is open, dry, loose sand with little other vegetation or duff. Pitcher's Thistle is typically found in Little Bluestem - Long-leaved Reed Grass - Great Lakes Wheat Grass Dune Grassland (Lee et al. 1998, NHIC 2003). It also occasionally occurs in atypical habitats, such as on the upper portion of a relatively steep, gravelly beach, or persisting in fairly dense surrounding herbaceous vegetation (Jones 2001). Greater numbers of Pitcher's Thistles are usually found in the early-successional foredunes (e.g. nearest the lake), but the species also commonly occurs on backdunes where there are areas of open, loose sand. At large dune sites, Pitcher's Thistle may be found 100 m or more away from the water. Where a storm event, tree-removal, or tree-fall creates a blowout in a forested dune, the exposed loose sand may be colonized by Pitcher's Thistle or other rare dune species (Jones 2003).
Normal habitat consists of several distinct zones (Jones 2001-2003; Otfinowski 2002). At the water's edge there is a hard-packed, wet strand which is bare of vegetation and is unsuitable habitat. Immediately landward of this there is usually a slope or ridge of sand predominantly covered with Marram Grass (Ammophila breviligulata) or Long-leaved Reed Grass (Calamovilfa longifolia var. magna) and little else. There may be a few scattered Pitcher's Thistle plants here. Inland of this, usually in a trough or a few smaller ridges, is open, loose sand where the majority of thistles are found. From here, moving back towards the forest, the vegetation tends to increase in density, covering the sand more completely, with greater frequency of shrubs. Pitcher's Thistle may be found in this zone, even as far back as the forest edge, if there are patches of open, loose sand.
Regardless of where in the habitat it occurs, Pitcher's Thistle can be subjected to extremes of heat, light, drought, wind, and lack of nutrients. As well, it may experience shifts of substrate, sand-blasting, and burial. Otfinowski (2002) found no direct correlation between the amount of Pitcher's thistle growth and location within the dunes; or with other environmental factors such as moisture or light.
The habitat of Pitcher's Thistle is dynamic due to sand movement caused by wind, water, and ice actions, and winter storm activities (McEachern 1992). Suitable habitat exists in a balance between the processes that keep sand open and loose, and succession, the natural process that causes a gradual increase in vegetation. Through succession, open areas naturally grow up into forests. In the absence of dynamic processes, sand may become too vegetated for Pitcher's Thistle.
Pitcher's Thistle is a monocarpic perennial, meaning it flowers and produces seed only once and then dies. Flowers are bisexual and self-fertile, but self-pollination produces lower rates of seed set than out-crossing (Keddy 1982). A variety of insects visit Pitcher's Thistle including bumblebees, megachilid bees, anthophorid bees, small and large helictid bees, as well as butterflies, skippers, flies, wasps, honey bees and several types of beetles and true bugs (Keddy and Keddy 1984; Loveless 1984). Pollination is likely not a limiting factor.
The trigger for flowering is still not understood. Rosette size is probably not the main factor since flowering stalks may be produced from rosettes as small as 15 cm in diameter, and non-flowering rosettes as large as 40 cm diameter have been observed (Jones, unpublished data). Pitcher's Thistle has mycorrhizal fungi associated with its roots (Maun 1999), but it is not known what species are involved or how necessary they are for survival of Pitcher's Thistle plants.
Pitcher's Thistle has the largest seeds among thistles in eastern North America (USFWS 2002, Gleason 1952, Montgomery 1977), perhaps to maximize root growth for seedlings in dry sandy habitat (USFWS 2002, Loveless 1984, Hamze and Jolls 2000). Seeds are thought to be viable for up to 3 years (Maun 1999, Rowland and Maun 2001). Studies suggest the quantity of viable seeds may be low (Bowles et al. 1993, Maun et al. 1996). However, when the seed coat was scarified or removed in the laboratory, total germination increased to above 95%, suggesting that seed viability may be high under proper germination conditions, and that a complex dormancy mechanism may be involved (Chen 1997, Chen and Maun 1998). Pitcher's Thistle appears to have a small year-to-year seed bank (Loveless 1984, Bowles and McBride 1993, McEachern 1992, Hamze and Jolls 2000). Thus, seed dispersal may be more necessary for the stability of Pitcher's Thistle populations than reliance on seed banks.
Seeds of Pitcher's Thistle are wind-dispersed, and most fall within 0-4 m of the parent plant (Keddy 1982, USFW, 2002). In a genetic study of Pitcher's Thistle in the Manitoulin Region, Coleman (2007a) found identical genotypes at widely separated sites, showing that dispersal over distances of as much as 99 km has occasionally occurred.
Seedling mortality can be related to microhabitat (Keddy 1982). Mortality was shown to be highest in open sand and lowest in debris-covered sand, but there was a trade off between germination and mortality because germination was also highest in open sand areas. Sand erosion contributes to mortality, so dune grasses that stabilize sand and ameliorate the effects of wind and sand blasting may help Pitcher's Thistle (D'Ulisse and Maun 1996). Delayed maturity also increases the probability of mortality prior to reproduction because rosettes live for a number of years.
Genetic evidence indicates that Pitcher's Thistle derived directly from Prairie Thistle (Cirsium canescens) through a series of genetic bottlenecks (Loveless and Hamrick 1988) following migration and isolation after glaciation. Pitcher's Thistle probably dispersed to its present range through sandy habitats formed by Wisconsinan glacial meltwaters (Moore and Frankton 1963, Johnson and Iltis 1963). Studies show Pitcher's Thistle has a genetic diversity lower than other species of Cirsium (Loveless 1984, Loveless and Hamrick 1988). DNA analysis of Pitcher's Thistle in the Manitoulin and Lake Superior Regions also demonstrated low genetic variability (Coleman 2007a). Six genetically distinct populations were found in the total Canadian population. Despite the possibility of dispersal up to 99 km, this distance is not sufficient to create a meta-population across the Canadian range.