Regularity: Regularly occurring
Regularity: Regularly occurring
Global Range: This species' range is centered around the Great Lakes; it occurs from southern Ontario, through Michigan, Wisconsin, Minnesota, Iowa, Illinois, and northern Indiana. Reports from Ohio, Pennsylvania, and New York are currently believed to be erroneous; they were probably based on the similar C. pumilum (Oldham 2004). Using GIS tools, total range extent was calculated to be approximately 300,000 square km.
From Moore and Frankton (1966): Cirsium hillii can be distinguished from Cirsium pumilum by several characteristics. C. hillii is a monocarpic perennial (plants flower then die and therefore only reproduce once even though they are perennial) (Oldham 2004) whereas C. pumilum is a biennial. The stem leaves of C. hillii are less deeply lobed with finer and shorter marginal spines than those of C. pumilum. In addition, in C. hillii, the outer involucral bracts are glandular and have shorter, narrower spines than the non-glandular outer involucral bracts of C. pumilum. Plants of C. hillii are also shorter than those of C. pumilum, and less often branched. Belowground, C. hillii has a single hollow, tuberous root, whereas C. pumilum has a solid taproot. Finally, the achenes of C. hillii are usually larger than those of C. pumilum, although exceptions have been noted.
Comments: Occurs in open fire-prone habitats on dry sandy to gravelly soils; open or relatively open conditions appear to be a key requirement. Habitats include dry-mesic prairies (including gravel hill/bluff prairies and sand prairies), pine barrens, oak savannas and oak barrens, open woods, limestone barrens (alvar and limestone "pavement" communities), prairie-like sand dunes and dune edges, and pastures. 200-400 m.
In Michigan, it occurs principally in jack pine or oak savannas, where plants occur in grassy openings. Also occurs in alvar communities on Drummond Island, where plants form scattered, large, loose colonies. Additional small populations occur in scattered prairie remnants and on sandy banks. In Wisconsin, it is typically found on dry-mesic blacksoil sand and gravel prairies with pH around 7.1 (circumneutral to calcareous soils). Other habitats in Wisconsin include thin dryland peat soils of limestone "pavement" or alvar communities, with pH 6.7-7.0. In Ontario, it has been collected in woodland alvar habitats as well as in sandy, dry prairie-like habitats on established dunes and pine/oak forested dune edges. Natural disturbances such as drought and fire historically played a significant role in shaping the woodland alvars. In Minnesota, it is apparently restricted to the transition zone between prairie and forest biomes, where habitats include thin, dry, sandy or gravelly soil in prairies, savannas and open woods. In Illinois, it is found in upland, drained, dry-mesic prairie with gravelly to sandy soil, or loam soils that possess at least some coarse element. In Iowa, it occurs on mesic to dry-mesic prairie and in Indiana, it is known from dry upland prairie. This species seems to respond positively to disturbance or at least needs open bare soil for seedling establishment.
Associates in Michigan jack pine savannas include Pinus banksiana, Festuca scabrella, Danthonia spicata, Rubus spp., Solidago spp., Astragalus neglectus, Deschampsia flexuosa, Koeleria macrantha, Comptonia peregrina, Vaccinium ssp., Gaylussacia ssp., Carex pensylvanica and Polygala polygama. Associates in Wisconsin prairie habitats include Andropogon gerardii, Bouteloua curtipendula, Sorghastrum nutans, Muhlenbergia cuspidata, Anemone patens, Dodecatheon meadii, Viola pedata, Aster spp., Solidago spp., Liatris spp., Lesquerella ludoviciana, Artemisia frigida, Delphinium virescens, Astragalus crassicarpus, Psoralea esculenta and Gentiana sp. Associates in Ontario alvars include Danthonia spicata, Arctostaphyllos uva-ursi, Cladina rangiferina, Pteridium aquilinum, Schizachyrium scoparium, Pinus banksiana, Picea glauca, Thuja occidentalis, Juniperus communis and Juniperus horizontalis. Associates in Minnesota prairies include Amorpha canescens, Andropogon gerardii, Bouteloua curtipendula, Schizachyrium scoparium, Solidago rigida, Liatris aspera, L. punctata, Anemone cylindrica, Zizia aptera, Echinacea angustifolia, Petalostemum purpureum, Coreopsis palmata, Monarda fistulosa, Anemone patens, Aster ericoides, A. sericeus, Helianthus rigida, Potentilla arguta, Comandra umbellata, Psoralea esculenta, P. argophylla and Sporbolus heterolepis. Associates on Illinois prairies include Amorpha canescens, Schizachyrium scoparium, Sporobolus heterolepis, Zizia aptera, Arenaria stricta, Pedicularis canadensis, Asclepias viridiflora, Bouteloua curtipendula, Gentiana puberula, Lithospermum incisum, Solidago nemoralis, Aster sericeus, Petalostemum purpureum and Silphium terebinthinacium.
Flower-Visiting Insects of Hill's Thistle in Illinois
(Long-tongued bees suck nectar & collect pollen; short-tongued bees collect pollen & are non-pollinating; beetles feed on pollen or knaw on the flowers & are non-pollinating; other insects suck nectar; observations are from Robertson)
Apidae (Bombini): Bombus pensylvanica sn, Psithyrus variabilis sn; Megachilidae (Megachilini): Megachile montivaga sn cp, Megachile pugnatus sn cp
Halictidae (Halictinae): Lasioglossum pectoralis cp np
Nymphalidae: Danaus plexippus sn, Speyeria cybele sn fq, Speyeria idalia sn; Papilionidae: Battus philenor sn, Papilio cresphontes sn, Papilio troilus sn; Pieridae: Colias philodice sn
Hesperiidae: Polites peckius sn, Thorybes bathyllus sn
Cerambycidae: Typocerus sinuatus fp np; Scarabaeidae (Cetenonniae): Trichiotinus piger fp np gnw icp fq
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 81 to >300
Comments: Rangewide, approximately 425 extant occurrences are known, with an additional 44 of unknown status (unclear whether extant or historical). A further 105 occurrences are ranked historical, extirpated, or failed to find. Additional extant occurrences are expected with further inventory; Michigan Natural Features Inventory notes that "numerous additional colonies [are] found annually", M. Oldham (pers. comm. 2008) suggests that there are "undoubtedly some additional undiscovered sites in the Bruce Peninsula and Manitoulin Island areas" of Ontario, and Penskar (1997) indicated that "there are likely additional large to moderately large populations that will discovered through ongoing inventory in Michigan and elsewhere." Michigan supports the largest number of extant occurrences, followed by Minnesota and Wisconsin.
Hill's thistle is a perennial with large, fragrant, pink-purple flower heads which bloom from June through August. In Minnesota, C, hillii flowers from late June through July (Van Norman 1990), while flowering occurs from mid-July through August in Ontario (Moore and Frankton 1974).
Betz (pers. comm.) described C. hillii as a relatively short-lived perennial, generally persisting two or three years but usually no more than four or five. Basal rosettes are produced from adventitious buds along the lateral roots as well as from seed (Ownbey pers. comm.). Several lateral shoots may be produced by a single plant. Primary tap-roots of die with the remainder of the plant shortly after flowering (Wade pers. comm.). In some instances, lateral shoots are produced prior to death and these grow in the succeeding years. Wade (pers. comm.) observed an instance where a plant produced three lateral shoots after flowering. One resulting shoot produced two additional shoots.
Flower production may occur one or two seasons after the appearance of the rosette (Reznicek pers. comm.) and most typically occurs in three-year-old plants (Wade pers. comm.).
Seed production in individual heads is abundant, but insects and perhaps fungi attack the flowers and seeds (Ownbey pers. comm.). When the head is mature, it breaks off and blows across the prairie scattering the seeds (Betz pers. comm.). Poor seedling establishment may be the primary cause for the apparent rarity of Hill's thistle (Betz pers. comm., Crispin pers. comm.). Excessive litter accumulation may interfere with successful germination and seedlings may be poor competitors for the available light and space. Loss of the historic, natural fire regime has enhanced the degree of litter accumulation on historic habitats.
Seed germination within the species is apparently low. Henderson (pers. comm.) recorded a 10-20% germination success upon sowing seeds in flats in a greenhouse. Wade (pers. comm.) stated that seeds sown 1/4 inch in the soil germinated poorly. Naturally-dispersed seeds, on the other hand, produced a significantly higher degree of germination. The low rate of germination in C. hillii may be an artifact of low seed viability (Henderson pers. comm.), but may also reflect a light requirement (Wade pers. comm.).
Placement of seeds overnight in a 10-3 molar gibberellic acid solution resulted in low germination (Bruckart pers. comm.). Betz (pers. comm.) recommended moist stratification at 40 degrees Fahrenheit for two months followed by sowing seeds in soil. Ingels (pers. comm.) contended that surface sowing with warm stratification followed by cold stratification worked best. Under such conditions however, germination was slow and non-uniform. See Roberts and Chancellor (1979) for information on seedling emergence and achene survival in other grassland Cirsium species.
Wade (pers. comm.) excavated a three-year-old plant with complete root system from a poor-soil nursery environment and found that the root had grown 1.5 feet below the surface of the soil. In addition, the root was much wider in size at the base (0.75 inches) than at the soil surface (0.25 inches). The plant was grown in the Driftless Area of southeastern Minnesota, but it was not known whether the heavy clay soils of the region influenced this behavior. Plants growing in morainic or sandy soils may possess deeper, differently-shaped root systems.
National NatureServe Conservation Status
Rounded National Status Rank: N3 - Vulnerable
Rounded National Status Rank: N3 - Vulnerable
NatureServe Conservation Status
Rounded Global Status Rank: G3 - Vulnerable
Reasons: Approximately 400 occurrences are scattered through a range centered around the Great Lakes, from southern Ontario through Michigan, Wisconsin, Minnesota, Iowa, Illinois, and northern Indiana; additional populations are likely to be found through ongoing inventory. Although around 100 known occurrences are thought to have excellent or good viability, many nevertheless contain small numbers of reproductive plants; the total reproductive population may not be more than 3500-4000 individuals. Thought to have declined significantly from likely historic distribution and to still be declining currently. The primary threat is habitat loss/degradation, due to conversion for agriculture and development and to fire suppression which contributes to canopy closure and litter build-up, reducing habitat suitability. Active management (e.g. prescribed fire) may be necessary to ensure long-term persistence.
Environmental Specificity: Narrow. Specialist or community with key requirements common.
Comments: Occurs predominantly in alvar, prairie, and savanna vegetation types, many of which are globally rare, and may be fire-dependent at many sites (Oldham 2004).
Global Short Term Trend: Decline of 10-30%
Comments: Probably still decreasing due to habitat conversion and lack of proper management. Penskar (1997) felt that this species "remains vulnerable to continued decline in many portions of its range without determining the most appropriate management strategies... without intervention through active management and a better understanding of the natural history of this species and its habitat, it is very possible that [it] could significantly decline over the next several decades." Allen (2003) also noted past and expected future declines of the species in Ontario, stating that "a downward trend in Ontario populations appears inevitable... to control, let alone reverse this trend, with respect to development pressures in the shoreline alvars, lack of fire disturbance in the species' habitat, and successional closure of open habitats supporting Cirsium hillii, will pose a considerable challenge." Allen (2003) also stated that "declines in area of habitat have been noted" for at least six Ontario sites. Flora of North America (2006) also mentions "declining populations."
Global Long Term Trend: Decline of 30-70%
Comments: Approximately 20% (105 of 530) of known, ranked occurrences are currently ranked historical, extirpated, or failed to find. Penskar (1997) found that habitat loss and degradation through fragmentation, conversion, and widespread fire suppression have "resulted in the elimination or severe diminishment of populations over most of the range." Allen (2003) also notes declines over the past 100 years in Ontario for similar reasons (fire suppression and resultant forest succession, conversion of land to agricultural and residential uses, and sand extraction).
Degree of Threat: High
Comments: The primary threat to this species is past and continuing loss, fragmentation, and degradation of its once considerably-more-extensive habitat (Penskar 1997). Significant habitat conversion has occurred for both agricultural (especially prairie areas) and residential/commercial uses (currently an acute threat along shorelines, e.g. Ontario alvars). Perhaps the most significant current threat is the absence of natural fire regimes and other landscape processes that maintain open habitat, resulting in encroachment of woody vegetation and increased litter buildup in once-suitable areas. Natural wildfire has been suppressed for at least 100 years throughout this species' range (Allen 2003). Consequently, preferred, open substrates are scarce and plants are often shaded out, preventing key processes such as seedling establishment. Fire suppression continues to be a threat even to many ostensibly protected populations; for example, in the Escanaba State Forest, failure to manage protected savanna habitats through prescribed burning threatens their C. hillii occurrences. The small C. hillii patches that remain rangewide are also threatened by various less severe issues. Habitat loss/degradation due to railroad, pipeline, and road construction and upkeep (including mowing at inappropriate times of year, compaction and destruction by heavy equipment, and skidding) and oil and gas exploration are threats in some areas (Penskar 1997). Ontario's largest occurrence is threatened by aggregate extraction, a threat exacerbated by restrictions on aggregate extraction in other areas of Ontario (Allen 2003, Oldham 2004). In some areas, monocultural forest plantings within savanna habitat pose an acute threat (e,g, red pine planting in Shakey Lakes Savanna) (Henson pers. comm.); other forest management practices, such as furrow planting, have less certain consequences (Penskar 1997). Confusion with noxious thistle species, and consequent herbiciding, has likely also caused non-trivial population loss over the past 50 years, and continues to be a threat. Populations may also be vulnerable to exotic seed-eating weevils introduced for biological control of noxious thistles (e.g. musk thistle); for example, Sauer and Bradley (2008) have recently documented the occurrence of the biological control weevil Rhinocyllus conicus on C. hillii in Wisconsin, with unknown effects. Excessive grazing, particularly of prairie lands, is an additional past and present threat, through direct damage to individual plants as well as compaction and disturbance of the soil. However, note that light, well-managed grazing may actually enhance C. hillii germination and establishment by preparing suitable germination sites. Other threats include ATVs and trail bikes, competition with invasive plants, deer browsing, siltation due to agricultural runoff, and prolonged, severe drought (Penskar 1997, Allen 2003). Low genetic diversity and inbreeding depression may be a further concern for small isolated populations (C. Anderson pers. comm. 2008).
Restoration Potential: Recovery potential of C. hillii populations is presently uncertain. Cirsium hillii seeds apparently germinate poorly (Henderson pers. comm., Wade pers. comm.). Transplantation success has been mixed, with many efforts ending in failure (Henderson pers. comm.). Wade (pers. comm.) found transplantation to be typically successful if done while still in a rosette stage. In his experience, ten transplant attempts (both with bare-root stalk and with intact soil) have been successful without a single failure. Henderson (pers. comm.) transplanted several plants rescued from a freshly plowed prairie. One plant that was transplanted to a south-facing gravelly dry slope has flowered during the past four seasons. Others planted in partial shade in a dry to mesic site are persisting vegetatively. Further research is needed to refine propagation techniques and to determine the potential for natural recovery.
Preserve Selection and Design Considerations: Preserves should of sufficient size to protect viable C. hillii populations and insect pollinators. Large buffer zones for safe and effective management (i.e., burning, removal of woody growth, mowing or grazing), and protection from pesticide drift and exotic species encroachment should surround each preserve.
Management Requirements: Populations within jack pine, oak savanna, alvar (Crispin 1984) and prairies communities must be managed in order to provide adequate protection. Stephenson (pers. comm.) stated that the harsh environmental conditions present in alvar communities tend to keep woody species in check. Consequently, active management (eg., prescribed fire and mowing) was not absolutely necessary. Removal of encroaching shrubs may need to be undertaken in some portions of the community, and fire management would not be detrimental.
In prairie, jack pine and oak savanna communities, management through prescribed fire is recommended. Fire management effectively removes the accumulating duff layer and opens the ground layer as germination sites. In shallow soil habitats (crests of gravel hill prairies, sand prairies and savannas), bare soil areas may naturally occur in relatively high frequency. Under these situations, management practices other than prescribed fire need not be implemented.
In mesic situations, management of the prairies through prescribed fire may have negative effects on C. hillii populations under some instances. The lush prairie growth produced through successive annual prairie fires, although enhancing the aesthetic beauty of the landscape, may ultimately destroy existing populations through shading and competition for available resources (Nuzzo pers. comm.). Research has shown that gaps in the vegetation are needed for germination and successful seedling development.
In small, relatively flat habitats, management of C. hillii through mowing may be an option. Betz (pers. comm.) suggested mowing as a management practice after seed dispersal has occurred. Typically, mowing is done in early to mid-July. Mower blades typically pass over the rosettes and do not damage the living plants. The reduction in vegetative cover admits more light and opens up space that may enable seedlings to become established. One or two mowings may be sufficient at most sites, but tests are being conducted to determine the actual number (Betz pers. comm.). Mowing has sustained the plant in an Indiana cemetery (Bacone pers. comm.). Bowles (pers. comm.) has observed that mowing in cemeteries usually begins around Memorial Day (late May). Bowles (pers. comm.) has also noted that mowing and grazing appeared to increase the density of populations in Illinois.
In the eastern portion of its range, Hill's thistle may have survived along the buffalo traces where trampling and grazing provided adequate habitat disturbance (Betz pers. comm., Wilhelm pers. comm.). Thin-soil prairie ridges and sand prairie/savanna sites (which provide habitat for the bulk of extant occurrences) appear to provide appropriate germination and development sites without significant animal-induced disturbance regimes. Simple prescribed burning management practices appear to be successful and should be considered a primary need (Schwegman pers. comm.). At mesic, loamy sites, managed grazing may eventually be needed to provide necessary soil disturbance and openings for seed germination and seedling growth.
Hill's thistle is responding well to the fire management for jack pines in Michigan's Kirtland's Warbler habitat (Crispin pers. comm.). The prescribed burns reduce shade and litter and seedlings are able to establish themselves. Prescribed burning in the jack pine and oak savanna regions is a necessity in order to maintain appropriate habitat and existing C. hillii populations.
Spring (May) burns do not appear to harm the rosettes of C. hillii plants (Martin pers. comm.). After a spring prairie burn, however, grasses respond vigorously and according to Betz (pers. comm.) and Nuzzo (pers. comm.), may crowd out C. hillii plants. In general, late winter/early spring or fall burning is less beneficial to warm-season grasses than mid to late spring burning, in the range of C. hillii (Heitlinger pers. comm.).
Management Programs: The Illinois Department of Conservation is managing C. hillii habitat through prescribed burning, as is typical of all prairies in their possession. Contact: John Schwegman, Botany Program Manager, Illinois Department of Conservation, Lincoln Tower Plaza, 524 South Second Street, Springfield, IL 62701-1787. Telephone No. (217) 785-8774.
The Minnesota Field Office of The Nature Conservancy manages three prairie preserves that possess populations of C. hillii: Schaeffer, Roscoe and Ordway Prairies. Prairie habitat is managed with a prescribed burning regime, with an approximate four-year rotation. Outside of prescribed burning, no special management treatment is being given C. hillii populations. Contact: Rick Johnson, Director of Land Stewardship, Minnesota Field Office, The Nature Conservancy, 1313 5th Street SE, Box 110, Minneapolis, MN 55414. Telephone No. (612) 379-2134.
Two small 35-acre tracts in the Shakey Lakes Savanna portion of the Escanaba State Forest in Michigan are being managed through prescribed burns (Henson pers. comm.). One burn unit contains a small population of C. hillii. It is hoped that the use of prescribed burning within the Savanna will be enhanced in the future. Burning is done in conjunction with the state Department of Natural Resources. Contact: Don Henson, Artist and Botanical Consultant, Tamarack Studios, P.O. Box 453, Manistique, MI 49854. Telephone No. (906) 341-6309.
Monitoring Programs: > The Illinois Department of Conservation conducts annual demographic monitoring at one central and one northern Illinois site (Schwegman pers. comm.). Apparently, other researchers are monitoring at two additional northern Illinois sites. Protractor plots are being used by the Department of Conservation as developed by Schwegman in 1986 (Schwegman pers. comm.). Contact: John Schwegman, Botany Program Manager, Natural Heritage Division, Illinois Department of Conservation, 524 S. Second Street, Springfield, IL 62706. Telephone No. (217) 785-8774.
The Minnesota Field Office of The Nature Conservancy is mapping occurrences and estimating numbers within C. hillii populations on its preserves on a periodic basis. At present, Hill's thistle is found on three TNC preserves within the state. Contact: Rick Johnson, Director of Land Stewardship, Minnesota Field Office, The Nature Conservancy, 1313 5th Street SE, Box 110, Minneapolis, MN 55414. Telephone No. (612) 379-2134.
Rich Henderson and June Dobberpuhl of The Wisconsin Department of Natural Resources have established a monitoring program within 10 C. hillii colonies at five sites in south-central Wisconsin. At each monitoring location, 8m x 8m plots have been permanently marked. Additional markers have been set along the periphery of the plot at 2-meter intervals. While surveying the population within the plot, string is stretched between markers, creating 16, two-meter square cells. Individuals plants are mapped, marked with wire tags, and recorded with respect to size, number of leaves, diameter of basal rosette, and basic flowering information. Monitoring at one site has been on-going for the past six years, while the remaining four sites have been monitored for two or three years. Contact: June Dobberpuhl, Natural Heritage Program, Endangered Resources/4, Wisconsin Department of Natural Resources, 101 S. Webster Street, Box 7921, Madison, WI 53707. Telephone No. (608) 266-0924; OR, Rich Henderson, Department of Natural Resources - Research, 3911 Fish Hatchery Road, Fitchburg, WI 53711. Telephone No. (608) 275-3214.
Don Henson, Botanical Consultant from Manistique, Michigan, and Bob Doepker, Michigan Department of Natural Resources Biologist, have monitored C. hillii populations within the Shakey Lakes Savanna region for the past 6-7 years (since 1983-84). A scientific approach to monitoring using plots is being developed for the site, an upgrade from sheer number counts in the past. Monitoring will continue indefinitely. Contact: Don Henson, Tamarack Studios, P.O. Box 453, Manistique, MI 49854. Telephone No. (906) 341-6309.
Management Research Programs: John Schwegman of the Illinois Natural Heritage Division is currently studying the species biology (demographics, recruitment, etc.) of a small population of C. hillii. Contact: John Schwegman, Natural Heritage Division, Illinois Department of Conservation, 524 S. Second Street, Springfield, IL 62706. Telephone No. (217) 785-8774.
Victoria Nuzzo is currently studying the demographics of large C. hillii populations located in the Nachusa Grasslands in Illinois. Four permanent plots (with a radius of 30 meters) have been established where individuals have been mapped, and diameter of the rosettes, leaf number/plant, flower production and evidence of herbivory are recorded. Propagation of plants has also been initiated from seeds collected in the field to determine if there are any differences in growth rate between greenhouse plants and those growing in the wild. The project was initiated in 1988 with funding through the Illinois Field Office of The Nature Conservancy. Continued research is now dependent on funding. Contact: Victoria Nuzzo, Consultant for Natural Area Management, Native landscapes, 124 Dawson Avenue, Rockford, IL 61107. Telephone No. (815) 399-5475.
Richard Henderson and June Dobberpuhl of the Wisconsin Department of Natural Resources are currently studying the demographics of C. hillii at two privately-owned sites in the state. Some management practices have been conducted within test plots. Contact: Rich Henderson, Department of Natural Resources - Research, 3911 Fish Hatchery Road, Fitchburg, WI 53711. Telephone No. (608) 275-3214; OR, June Dobberpuhl, Natural Heritage Program, Endangered Resources/4, Wisconsin Department of Natural Resources, 101 S. Webster Street, Box 7921, Madison, WI 53707. Telephone No. (608) 266-0924.
Tracy Tophooven, an intern with the Wisconsin Bureau of Endangered Resources, Natural Areas Program, collected C. hillii seeds this past summer (1990) from four sites in order to determine the number of seeds produced per head and percent viability of those seeds. Contact: June Dobberpuhl, Natural Heritage Program, Endangered Resources/4, Wisconsin Department of Natural Resources, 101 S. Webster Street, Box 7921, Madison, WI 53707. Telephone No. (608) 266-0924.
Allen Wade of Prairie Moon Nursery has been working on the germination and transplantation of C. hillii for prairie restoration projects. Contact: Allen Wade, Prairie Moon Nursery, RR#3, Box 163, Winona, MN 55987. Telephone No. (507) 452-4990.
Management Research Needs: Information is lacking on all phases of the life cycle of Hill's thistle. Requirements for seed germination and seedling establishment should be a priority for research. Potential genetic diversity in isolated populations should be determined. Possible asexual reproduction in the species should be studied. What is the percentage of viable seeds produced in an isolated population? More is needed to be known about the biology and life history of C. hillii in order to manage it appropriately (Schwegman pers. comm.).
Inventories to determine the true status of this species should be conducted in states where it has not already been done. In Iowa, casual collecting has turned up at least seven sites in the last 20 years, five of these in the last 12 years (Loeschke pers. comm.). In Michigan, most of the 47 recorded occurrences are based on herbarium sheets, and the status of these populations is not currently known (Penskar pers. comm.). There has never been an intensive search for the species in a number of states including Iowa (Loeschke pers. comm.) and Michigan (Penskar pers. comm.). Michigan appears to be the species' stronghold and a complete inventory of habitat in that state would yield a better understanding of the true range-wide status of the species.
Cusick (pers. comm.) has stated that a significant taxonomic problem may exist for this entity. Moore and Frankton (1966) treated this taxon as Cirsium pumilum (Nutt.) Spreng. ssp. hillii (Canby) Moore and Frankton.
Biological Research Needs: Natural history studies of many aspects of the biology and ecology of this species are needed to effectively guide management (Penskar 1997). Research is needed on the requirements for seed germination and seedling establishment, the percentage of viable seeds produced in isolated populations, and the possibility of asexual reproduction (Fant 2007, Fant et al 2007). The genetic diversity of isolated populations should also be assessed. Fant (2007) examined genetic diversity of several populations in the Chicago area and indicated that genetic diversity varied between populations. He suggested that there are significant gaps in knowledge regarding life history, especially reproduction (see also Fant et al. 2007). Monitoring is also needed to gather data on the condition and viability of populations, and to increase knowledge concerning population dynamics and the conditions necessary to maintain habitat, stimulate sexual reproduction, and perpetuate healthy colonies (Penskar 1997). Finally, taxonomic research would be helpful.
Relevance to Humans and Ecosystems
Stewardship Overview: Management options (depending on habitat type and circumstance) may include mowing (at an appropriate time), prescribed fire and/or light grazing. Monitoring of population stability (e.g., age structure, flower and seed production, recruitment) with and without various forms of management would be highly informative.
Names and Taxonomy
Comments: Cirsium hillii is generally recognized as a valid species in recent treatments (e.g. Gleason and Cronquist 1991, Voss 1996), but has also been treated as a subspecies or variety of the more eastern C. pumilum (e.g. Moore and Frankton 1966, Scoggan 1978). Although generally accepted as a full species, there is still some uncertainty concerning its taxonomic status (Oldham 2004). Flora of North America (2006) treats it as Cirsium pumilum var. hillii, stating that this taxon "has often been treated as a species distinct from Cirsium pumilum... [but] the differences separating these taxa are, for the most part, metric characters that show considerable overlap."
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