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Species of Concern
Rangewide Status Assessment of Hill's Thistle
Michael R. Penskar
Michigan Natural Features Inventory
8th Floor Mason Bldg.
December 26, 1997
East Lansing Field Office
2651 Coolidge Rd.
East Lansing, Mich. 48823
Cirsium hillii (Canby) Fern., commonly known as Hill's thistle or occasionally called pasture or prairie thistle, is a relatively wide-ranging species principally of the Upper Midwest. This native thistle, currently ranked G3 by The Nature Conservancy, occurs in variety of open, fire-prone habitats, including such natural communities as gravel hill or bluff prairies, dry mesic to mesic sand prairies, oak savannas, oak barrens, pine barrens, and alvar grasslands, the latter frequently referred to as limestone barrens. The distribution ranges from southern Ontario, Canada, through Michigan, Wisconsin, Minnesota, Iowa, Illinois, and Indiana. If the treatments of several manuals are followed, especially the Ohio Asteraceae treatment by Fisher (1988), as well as standard regional manuals (Gleason and Cronquist 1991, Fernald 1950), the range includes Ohio, although in a recent state status assessment by Cusick (1995), this has been determined to be in error. The loss of habitat, primarily through fragmentation and outright destruction, but particularly through widespread fire suppression, has resulted in the elimination or severe diminishment of populations over most of the range of this species.
The U.S. Fish and Wildlife Service (USFWS) classified Hill's thistle as a C2 candidate species until this listing category was formally eliminated (Federal Register 1996). This rangewide assessment, initiated during the moratorium for consideration of C2 species, and prior to the formal loss of the category, was completed nonetheless to determine if there was any further merit in considering Cirsium hillii as a candidate under the formal definition of this term or category.
RESULTS AND DISCUSSION
Table 1 presents a summary of the occurrences known for all states and the sole Canadian province (Ontario) within the range of Cirsium hillii. For purposes of evaluation and discussion, an "occurrence" of C. hillii, as tracked by heritage programs, is defined as a geographic locality containing individuals of this species . Rangewide, a total of 356 occurrences were identified through examination of heritage program records from 1996 to 1997. Of the 356 records, 221, or 62%, have been observed within the last 25 years, with 8% observed from 1950-1971, 13% observed from 1900-1949, and 5% observed before 1900. The percentages do not total to 100%, as the number of Ontario occurrences was estimated, and a few records had unknown observation dates. Scrutiny of the occurrence record data by each state and province indicated that approximately 141 occurrences are viable, at least over the short term, based on the criterion of a C-rank or higher for an occurrence. In a few cases, a rank was estimated for purposes of this analysis, if sufficient data were available to make a tentative assessment. Potentially viable occurrences, such as those ranked CD or CD?, were not counted, and thus the total is likely somewhat conservative. Little additional time could be allocated to corroborating further the number of occurrences protected within preserves (whether public or private) or identifying those actively managed or otherwise protected on state or federal lands. However, study of occurrence record data indicates that approximately 50 occurrences are protected and/or managed for their perpetuation; which is probably a conservative accounting. Internet and literature searches revealed virtually no information or citations concerning research or monitoring of this species, resulting only in articles and lists in which Cirsium hillii is cited as a plant associate or noted as a state listed or former federal candidate species. These searches, though conducted several times, cannot be considered definitive, but do highlight the paucity of information on this Midwestern taxon.
The primary threats to Hill's thistle are clearly those related to the fragmentation of its once considerably more extensive habitat. The principal threat at present appears to be the continued decline of remaining habitat through plant succession, canopy closure, and shading, largely through the loss of ecological factors within landscapes, such as wildfires, that formerly functioned to maintain patches of suitable habitat. This has led to the highly increased vulnerability of colonies to stochastic (random) events as well as numerous human pressures, the latter including such activities as encroachment through development, herbiciding, grazing, impacts from recreational land use and development, certain agricultural and forest management practices, and maintenance activities related to the upkeep of railroad, pipeline, and road rights-of-way.
The following discussion provides a status summary by state and provincial distribution, with commentary regarding habitat, characteristic plant associates, population size and viability, management programs, and threats to populations.
Swink and Wilhelm (1994) note that Cirsium hillii in Illinois is a rare prairie plant, occurring with such typical species as little bluestem (Andropogon scoparius), side-oats gramma grass (Bouteloua curtipendula), downy gentian (Gentiana puberulenta), purple prairie clover (Petalostemum purpureum), and prairie dropseed (Sporobolus heterolepis), among several other associates. According to the Illinois Natural Heritage Database (1997), there are currently 39 occurrences tracked for the state. Habitat data indicate that Hill's thistle is frequently found inhabiting several prairie communities, especially gravel hill prairies, loess prairies, and dry mesic to mesic sand prairies. Of the 39 tracked occurrences, 38 have been observed within the last 25 years (Table 1). Although none of these occurrences are currently ranked by the heritage program and the available data are quite limited, it appears that a majority of the occurrences, approximately 25, may be considered viable, at least over the short term, based primarily on a perusal of population size. At least 17 occurrences are within a nature preserve and/or are actively managed to maintain populations and their prairie habitat. Five sites include populations persisting in degraded prairie remnants contained within cemeteries, where mowing discourages woody plant establishment and succession but also prevents Hill's thistle and associated prairie plants from flowering. Similar to conditions elsewhere within its range, this species is vulnerable due to its fragmented habitat and such threats as woody plant succession, fire suppression, grazing, artificial disturbance from recreational activities, invasive plants (e.g. black locust, Robinia pseudoacacia, and honeysuckle, Lonicera spp.), and such maintenance activities as mowing and herbiciding.
Cirsium hillii is restricted principally to the northern portion of the state, although extant localities are known only from two counties in the northeast region (Hedge et al. 1992). Habitat for this species includes such natural communities as dry mesic sand prairies and savannas, dry mesic clay loam prairie, gravel hill prairies, and mesic silt loam prairies (Hedge et al. 1992). Typical associates may include such species as prairie dropseed, purple prairie clover, Kalm's brome grass (Bromus kalmii), big bluestem, New Jersey tea (Ceanothus americanus), and the native thistle Cirsium discolor (also referred to as pasture or prairie thistle). Of the 16 recorded occurrences, only five are considered or known to be extant, with eight known or assumed extirpated, and three remaining sites classified as historical. For counties with historically known populations, no specific location data are known, thus further investigation will be required to direct field surveys, if needed, to determine or presume the status of these occurrences. Overall, the status of Hill's thistle is extremely poor. Of the five known extant populations, only one, Cook Prairie, supported a moderately large colony of 50-100 plants, and these were last observed in 1987. By 1990, only six plants were observed in this site, and no plants were subsequently observed in 1991, presumably due to drought. Three of the state's extant occurrences, including the aforementioned, lie within protected areas, but only Cook Prairie is known to have supported more than a few plants. The remaining two extant sites are comprised of colonies persisting in mowed cemeteries, where they were last observed in 1981. Hedge et al. note that that habitat destruction is the principal reason for the severe decline of Hill's thistle in Indiana, with additional threats including mowing, herbiciding, and plant succession leading to closure of prairie habitats. They also considered the introduction of exotic insects to control non-native, weedy thistles to be a potential threat to C. hillii.
Hill's thistle is restricted to the eastern portion of the state, where it is usually associated with dry, sandy soils and calcareous limestone substrates, although it is not restricted to these habitats (Watson 1993). Typical associates include such prairie species as lead plant (Amorpha canescens), big bluestem, tall green milkweed (Aclepias hirtella), Indian plantain (Cacalia tuberosa), prairie dropseed, purple prairie clover, rattlesnake master (Eryngium yuccifolium), side-oats gramma grass, little bluestem, and shooting star (Dodecatheon media). A status survey was conducted by Watson (1993), who reviewed numerous herbarium records and then conducted field inventories of all historical collection sites that could be identified or deduced from label information. Field inventories were also conducted in additional sites where potential habitat existed for possible new localities. Although it is not clear from the status survey as to the number of historical occurrences that were represented by collection records, Watson's survey resulted in the confirmation of one historical occurrence and the verification of four new occurrences. The majority of sites consisted of relatively small populations within local patches of remnant habitat, with a collective total of 350 estimated plants. Most of these occurrences were found to be threatened by vegetative succession, exotic weeds, herbiciding, mowing, grazing, and siltation due to agricultural run-off. Some activities, such as mowing, were found to convey a degree of protection by inhibiting woody plant succession, yet at the same time were found to impede flowering and fruiting, a typical scenario for colonies persisting in cemetery prairie remnants. In general, these occurrences are quite vulnerable to extirpation due to their small size, limited habitat extent, and landscape context. No occurrences reside within nature preserves or are deliberately managed for their perpetuation. Because only portions of six counties in the eastern region of the state were targeted for the inventory, Watson noted that additional populations should be expected on sand and limestone substrates elsewhere within eastern Iowa. Lastly, Watson (1993) notes that Hill's thistle resilience and perhaps somewhat positive response to mowing suggests that it may have survived grazing pressure better than other prairie species, and thus should be especially sought in the uncultivated pasture lands of southeastern Iowa.
Michigan supports the largest known number of C. hillii occurrences throughout the states and sole Canadian province within the global range. Within the state, this species has been recorded from a number of habitats, but occurs primarily within three areas of concentration: 1) the jack pine barrens of large outwash plains in the northern Lower Peninsula; 2) the limestone or bedrock grassland habitat, known as alvar, throughout Drummond Island in the eastern Upper Peninsula, and 3) the oak-pine barrens of the Shakey Lakes savanna in the central Upper Peninsula, along the Menominee River and Wisconsin border. Hill's thistle is also known from several occurrences in the southern Lower Peninsula, where it apparently occurred in various types of southern Michigan oak barrens and savannas, and was also recorded from sand dunes near the Lake Michigan shore.
Within the extensive jack pine barrens of the Lower Peninsula, where the vast majority of occurrences have been documented, Hill's thistle thrives best in the most open, fire-prone areas, where it occurs in prairie-like communities with such species as big bluestem, little bluestem, birdfoot violet (Viola pedata), jack pine (Pinus banksiana), blazing star (Liatris spp.), and such notable state rarities as pale agoseris (Agoseris glauca), rough fescue (Festuca scabrella), and Alleghany plum (Prunus alleghaniensis var. davisii), the latter taxon also a former Federal candidate (C2) as a putative Michigan endemic. On Drummond Island, Hill's thistle is a well known species of alvar habitat, a natural community containing plants with boreal as well as prairie grassland affinities. Common associates includes such plants as prairie dropseed, big and little bluestem, Cooper's milk vetch (Astragalus neglectus), prairie smoke (Geum triflorum), bulrush sedge (Carex scirpoidea), Richardson's sedge (Carex richardsonii), ground juniper (Juniperus communis), and flattened spikerush (Eleocharis compressa). In the oak-pine barrens of the Shakey Lakes savanna in the central Upper Peninsula, Hill's thistle occurs in relatively small colonies within a relatively large savanna remnant, occupying openings with species such as big and little bluestem, Indian grass (Sorghastrum nutans), blazing star, puccoon (Lithospermum spp.), birdfoot violet, dwarf milkweed (Asclepias ovalifolia), Vasey's rush (Juncus vaseyi), Pennsylvania sedge (Carex pensylvanica), black oak (Quercus velutina), and aspen (Populus tremuloides and P. grandidentata). As of the preparation of this report, there are 130 recorded occurrences of Hill's thistle in Michigan. This number is conservative, owing to significant backlogged data that remain to be processed, as well as other reports requiring additional scrutiny before transcription and entry into the statewide data base. Numerous records have been derived from inventories over the last several years, primarily from pine barrens studies and state and U.S. Forest Service (USFS) compartment reviews, and this is reflected in the occurrence breakdown in Table 1. The majority of Michigan occurrences have been recorded within the last 25 years, nearly 50% of these occurring since 1990 (Michigan Natural Features Inventory 1997). Examination of occurrence ranks, which have been assigned for most sites, indicate that approximately 50% would be considered viable, although considerable data are lacking, especially information concerning population extent, condition, and landscape context or habitat quality, that would provide a more reliable assessment of populations. The majority of the southern Lower Michigan records are extremely dated, most ranging from 50 to more than 100 years old, and thus their status is highly doubtful.
Despite the large number of occurrences, fewer than 10 occur within nature preserves or other natural areas where dedicated management is taking place to perpetuate populations and maintain viable habitat. Notable protected occurrences, or areas where active management is taking place, include portions of the Shakey Lakes savanna, where prescribed burns are being implemented within a nominated natural area on state forest land, a large Nature Conservancy preserve on Drummond Island (Maxton Plains), and significant areas in the Lower Peninsula within the Huron-Manistee National Forest (e.g. Valley Road Prairie) and portions of extensive state forest lands (e.g. Big Frost Pocket). It is not clear if Hill's thistle appears to benefit or not from some of the activities related to land management for the federal and state endangered Kirtland's warbler (Dendroica kirtlandii), and the effects of furrow planting in jack pine management areas with regard to rare plant species remain unclear.
As noted by USFS, Michigan Department of Natural Resources (MDNR), and Heritage Program specialists (botanists, ecologists, field biologists and foresters), Cirsium hillii has been observed to be relatively common over much of the pine barrens landscape. This species persists in a variety of disturbed pine plains and oak-pine barrens, along pipeline and railroad corridors and road rights-of-way, within red pine (Pinus resinosa) plantations, and as noted above, is resilient to (or possibly benefits from) jack pine plantation management and other timber management activities. Despite the relative frequency of colonies, particularly within the northern Lower Peninsula, there are few monitoring programs, and thus the true condition and viability of populations is not known, owing to a wide gap of knowledge concerning population dynamics and the conditions necessary to maintain habitat, stimulate sexual reproduction, and perpetuate healthy colonies. The principal threats include herbiciding, canopy closure, fire suppression, and direct impacts from some management activities, such as compaction and destruction by heavy equipment, and log skidding. Furrow planting followed by a 50-year rotation of jack pine in some areas is likely a threat to Hill's thistle (P. Comer, pers. comm. 1997).
Hill's thistle occurs in the southern portion of the state, where it is found from central Minnesota to the southeast corner, and is primarily restricted to habitats occupying the transition zone between the major forest and prairie biomes (Coffin & Pfannmuller 1988). According to Coffin & Pfannmuller, this is a somewhat unusual distribution, with no apparent reason for the pattern. Typical natural community types include dry bluff to gravel hill prairies, mesic sand prairies, limestone prairies, and various types of savannas and open woodlands (Minnesota Natural Heritage Program 1996; Coffin & Pfannmuller 1988). Associated species include such plants as Indian grass, big and little bluestem, needle grass (Stipa spartea), lead plant, Kalm's brome grass, Bicknell's sedge (Carex bicknellii), tickseed (Coreopsis palmata), blazing star (Liatris spp.), prairie dropseed, side oats gramma grass, prairie smoke, Missouri goldenrod (Solidago missouriensis), and western wormwood (Artemisia ludoviciana), in addition to numerous other prairie species and such common woody plants as Eastern red cedar (Juniperus virginiana), hazel nut (Corylus spp.), American wild plum (Prunus americana), oaks (Quercus spp.), and sumac (Rhus spp.). Occurrence records also indicate the relatively disturbed nature of several sites, as exemplified in the recording of many non-native, invasive species, such as Kentucky bluegrasses (Poa compressa and P. pratensis) and white sweet clover (Melilotus alba).
Of the 58 occurrences tracked by the Minnesota Natural Heritage Program (1996), the majority have been observed within the last ten years, and thus most occurrences can be presumed to be extant. Based on occurrence ranks, about 30 of the states occurrences are essentially viable, but as elsewhere within the range, Hill's thistle persists in relatively isolated habitat fragments, where populations are fairly stable (Coffin & Pfannmuller 1988) but may face further decline without burn management to prevent canopy development and closure. At least nine sites are protected and managed within preserves and other types of natural areas. Additional occurrences may be at least incidentally protected on other state managed lands. Woody plant succession, resulting from fire suppression, is cited repeatedly in occurrence records as a principal threat to populations. Additional threats include grazing, herbiciding, and agricultural activities that result in the modification or direct destruction of habitat. Drought has also had an impact on populations
Hill's thistle has long been reported to occur in Ohio, cited as a plant of a wide range of open habitats with well-drained, sandy soils, including fields and various dry sand prairies (Cusick 1995). Although Fisher (1988) as well as Moore and Frankton (1966) considered Cirsium hillii a component of the Ohio flora, an exhaustive field search and comparative herbarium study by Cusick (1995) resulted in the strong conclusion that all Ohio specimens of Hill's thistle could be referred to the related, similar-looking C. pumilum (Nuttall) Sprengel. This is an opinion shared by other taxonomic botanists (A. Reznicek, pers. comm.), and thus for purposes of this status survey, Hill's thistle is not considered to be known for Ohio. It should be emphasized that Cusick also conducted detailed field inventories based on Ohio's putative Hill's thistle specimens such that additional morphological characters could be studied and better discerned in live plants. However, no documentation could be established for any presumed extant or historical occurrences, and thus Cusick recommended that Hill's thistle be deleted from the Ohio flora. In addition, Cusick cited preliminary evidence from a systematist (M. Hay) studying this group of thistles at the University of Georgia, noting that Hill's thistle is questionably distinct from C. pumilum, and at best may only be a variety or subspecies of that taxon. Further studies by Hay or others may ultimately clarify the relationship of C. hillii to C. pumilum and suggest the most appropriate taxonomic placement.
Hill's thistle ranges widely (or once ranged widely) through Wisconsin, where the number of recorded occurrences comprises the largest data base -- and likely the largest population -- outside Michigan. As of 1996, 68 occurrences were tracked for the state (Wisconsin Natural Heritage Program), where Hill's thistle occurs primarily in a variety of prairie communities, including dry mesic to mesic prairies, limestone to sandstone bluff prairies, quartzite outcrops, and oak savannas and barrens. Many occurrences within bluff and hill prairies are found in communities with south-facing aspects. Common associates of Hill's thistle include such species as big and little bluestem, Indian grass, Kalm's brome grass, alum root (Heuchera richardsonii), skullcap (Scutellaria leonardii), fringed gentian (Gentian crinita), prairie dropseed, lead plant, cream wild indigo (Baptisia leucophaea), prairie smoke, Pasque flower (Anemone patens), sedges (Carex abdita, C. richardsonii), and flowering spurge (Euphorbia corollata), among many other species. Of Wisconsin's 68 tracked occurrences, 22 are classified as extirpated and 11 are ranked as historical. Of the remaining 35 occurrences, at least 20 can be considered to be viable at the present time, and at least 11 occurrences lie within nature preserves and/or are actively managed to maintain populations and their habitat. The primary threats to populations, as noted in occurrence records, are grazing, herbicide use, certain agricultural activities (e.g. expansion of cultivation areas), park and related recreational development activities, lack of fire (or fire suppression), woody plant encroachment, exotic plant invasion, and drought.
Outside the United States, Hill's thistle only occurs in Canada, where it is restricted to the Lake Huron and Georgian Bay region of Ontario. The majority of locations are known from the southern shore of Manitoulin Island and the primarily along the western shoreline of the Bruce Peninsula, with one or two localities near the extreme southern shoreline of Georgian Bay. The principal habitats are alvars, which are occasionally referred to as limestone barrens or bedrock grasslands. Alvar communities are typified by the presence of limestone bedrock, which lies at or close to the surface and is overlain by discontinuous graminoid turfs, with scattered trees and shrubs such as Northern white cedar (Thuja occidentalis), balsam fir (Abies balsamea), and Eastern red cedar (Juniperus virginiana). Hill's thistle records also indicate the presence of colonies in the sandy soils of other community types, including dunes. Few data are available from herbarium label information; however, recent field data sheets from alvar inventories (Ontario Natural Heritage Information Centre 1996) provide detailed site and habitat data. On the alvars of Manitoulin Island and the Bruce Peninsula, Cirsium hillii occurs on shallow dolostone (a limestone formation) in openings and savannas, where it is commonly associated with such species as Northern white cedar, white spruce (Picea glauca), larch (Larix laricina), bearberry (Arctostaphylos uva-ursi), ebony sedge (Carex eburnea), little bluestem, ragwort (Senecio pauperculus), bastard toadflax (Comandra umbellata), dwarf lake iris (Iris lacustris), bulrush sedge, lobelia (Lobelia spicata), bellflower (Campanula rotundifolia), and smooth-leaved aster (Aster laevis). In several sites, Hill's thistle was found to occur within openings in encroaching aspen, cedar, and spruce thickets.
Occurrence data for Cirsium hillii in Ontario cannot be easily compared with information provided for the remainder of the rangewide distribution, owing to the fact that the present data base consists of an accounting of collection records. For the 162 collection records compiled by the heritage program (Ontario Natural Heritage Information Centre 1996), current point location maps clearly indicate that an eventual circumscription of sites will merge (i.e. subsume) many collections. It is estimated that fewer than 50 occurrences will ultimately be delineated. For purposes of the present rangewide assessment, it is assumed that there will be approximately 40 occurrences identified (M.J. Oldham, pers. comm. 1997). Beyond the habitat data and additional cursory collection information, there are few available data concerning protection and management status. Numerous historical sites where this species was collected will require inventory to determine if populations are extant. Although protection status could not be compiled, it should be noted that intensive inventories of Ontario alvars have taken place as a result of international alvar studies. Thus, many areas supporting Hill's thistle colonies will be highlighted for potential acquisition, natural area dedication, and site conservation plans to conserve alvar habitats and their biodiversity. Current threats to Hill's thistle in Ontario include development, fragmentation, likely the lack of fire in at least some alvar community types, which facilitates woody plant succession and canopy closure, and the use and increasing development of sites for recreational activities.
SUMMARY AND RECOMMENDATIONS
Although Cirsium hillii is an endemic Midwestern taxon far from being demonstrably secure, it is not sufficiently rare nor endangered throughout its range such as to merit consideration for federal listing at this time. Despite its extreme rarity in Iowa, Indiana, and to some extent Illinois, there are numerous records of viable populations within the remainder of its range, including Ontario, Michigan, Wisconsin, and Minnesota. Within states such as Michigan, the stronghold for Hill's thistle rangewide, this species persists well even though as a special concern or "watch-list" plant it has no legal protection, and can be expected to persist well into the future. Moreover, there are likely additional large to moderately large populations that will discovered through ongoing inventory in Michigan and elsewhere. Overall, the current G3 rank assigned by The Nature Conservancy is well justified, based on the current data. Even though there is insufficient merit for recommending federal listing, Hill's thistle remains vulnerable to continued decline in many portions of its range without determining the most appropriate management strategies. It is clear that the biology and ecology of this species is rather poorly known. Thus, management attempts will need to be coupled with long-term biological monitoring programs to ascertain which type and regime of management is best. Some monitoring programs have been initiated within the context of large-scale pine and oak-pine barrens management in Michigan, and should occur elsewhere within the range where possible, particularly in Illinois, Wisconsin, and Minnesota, where occurrence records have indicated that habitat closure threatens many populations. Without intervention through active management and a better understanding of the natural history of this species and its habitat, it is very possible that Hill's thistle could significantly decline over the next several decades
Lastly, though not emphasized nor fully explored for this rangewide status assessment, the taxonomic standing of Cirsium hillii remains of some question, and may require further investigation to clarify if this species is truly distinct from Cirsium pumilum.
1. Cirsium hillii is not sufficiently rare, vulnerable, or in decline throughout its range to merit consideration for federal listing at this time.
2. Active management and long-term biological monitoring programs are recommended to prevent further decline and determine the most appropriate management strategies to perpetuate populations and sustain suitable habitat.
3. Natural history studies of virtually any aspect of the biology and ecology of the species are recommended to augment and guide experimental management programs.
4. Investigation of the systematic relationship of C. hillii, C. pumilum, and other related taxa are suggested to clarify the most appropriate taxonomic treatment of this group.
5. Continued inventory within the known range of C. hillii is recommended, and is likely to result in the identification of additional populations. In several areas within the known range, there is continued merit in attempting to ascertain the status of historical populations. In addition, the proximity of historical populations in southern Lower Michigan to Ohio suggests that inventory could be appropriate in the northern portion of the latter state, if potential habitat is present, for the possible future discovery of Hill's thistle there.
This status assessment was completed through funding provided by the U.S. Fish and Wildlife Service (Region 3), via the East Lansing Field Office (ELFO). First and foremost, I owe special thanks to Mike DeCapita, ELFO endangered species coordinator, for his usual blend of abundant patience, encouragement, knowledge, and guidance. I am especially indebted to several heritage botanists and information managers, who kindly took the time to promptly respond to my request for status data and other relevant information on Hill's thistle within their state or province. For sending element occurrence information and additional data, I would like to tender special thanks to Terry Campos (Illinois Natural Heritage Program), June Dobberpuhl (Wisconsin Natural Heritage Program), Cloyce Hedge (Indiana Natural Heritage Program), Mike Oldham (Ontario Natural Heritage Program), John Pearson (Iowa Natural Areas Inventory), and Nancy Sather (Minnesota Natural Heritage Program). The Ohio status report was provided by Allison Cusick (Div. of Natural Areas and Preserves, ODNR). I would also like to thank Dr. Tony Reznicek, Curator of Vascular Plants at the University of Michigan Herbarium, for reviewing specimens with me and providing comments, opinions, and as always, the astute botanical instruction that so few are able to give or find the time to do! Lastly, and by no means least, I would like to thank Sue Ridge, MNFI Director of Administration, for assisting me in keeping a small budget on track and helping as usual in a myriad of small but deceptively important ways.
Table 1. Summary of Cirsium hillii occurrences by state and province, with a breakdown by last observation date. (#=footnotes)
State or Province
Element Rank (#3)
Number of Occurrences Recorded
* Historical occurrences documented through several collection records include this category but were not provided in the state status report and thus cannot be detailed here. The number of occurrences recorded for Iowa only reference confirmed extant populations .
3 The sub-global rank for Hill's thistle within the states and province in its range.
4 Reported falsely within the state.
5 Because the number of Ontario sites are estimated from numerous collection records, a breakdown by collection period cannot be provided.
1) Heritage programs track must track species (i.e. "elements") on an arbitrarily determined locality basis for their databases. Thus, an occurrence may not necessarily constitute what one might consider a population, and in many cases may represent a series of colonies or patches that can be collectively be described as metapopulations.
Argus, G.W. and K.M. Pryer. 1990. Rare vascular plants in Canada, our Natural Heritage. Botany Division, Canadian Museum of Nature, Ottawa, Ontario. 191 pp. + maps.
Coffin, B. and L. Pfannmuller, eds. 1988. Page 175 in: Minnesota's endangered flora and fauna. Univ. of Minn. Press, Minneapolis. 473 pp.
Cusick, A.W. 1995. Report on the status of Cirsium hillii (Hill's pasture thistle) in Ohio. Status report submitted to U.S. Fish and Wildlife Service, Region 3, Minneapolis. Ohio Dept. of Natural Resources, Div. of Natural Areas and Preserves, Columbus, OH. 6 pp.
Federal Register. 1996. Review of plant and animal taxa that are candidates for listing as endangered or threatened. February 28, Vol. 61. No. 40: 7596-7613.
Fernald, M. L. 1950. Gray's Manual of Botany; eighth edition, illustrated. D. Van Nostrand Company. lxiv + 1632 pp.
Fisher, T.R. 1988. The Dicotyledoneae of Ohio. Part 3. Asteraceae. Ohio State Univ. Press, Columbus. x + 280 pp.
Gleason, H. A., and A. Cronquist. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. Second edition. The New York Botanical Garden. Bronx, New York. lxxv + 910 pp.
Hedge, C. M. Homoya, & C. Baker. Report on the status of Cirsium hillii (prairie thistle) in Indiana. Department of Natural Resources, Division of Nature Preserves. Indianapolis, IN. 13 pp.
Illinois Natural Heritage Program. 1997. Element Occurrence Records for Cirsium hillii. Springfield, IL.
Michigan Natural Features Inventory. 1997. Element Occurrence Records for Cirsium hillii. Lansing, MI.
Minnesota Natural Heritage Program. 1996. Element Occurrence Records for Cirsium hillii.
Moore, R.J. and C. Frankton. 1966. An evaluation of the status of Cirsium pumilum and C. hillii. Can. J. Bot. 44:581-595.
Natural Heritage Information Centre. 1996. Collection records for Cirsium hillii. Ontario Heritage Program. Peterborough, Ontario.
Swink, F. & G. Wilhelm. 1994. Plants of the Chicago Region, 4th ed. Indiana Acad. Sci., Indianapolis. 921 pp.
Watson, W.C. 1993. Inventory of selected counties in northeastern Iowa for Cirsium hillii (Canby) Fern. The Nature Conservancy, Iowa Field Office. Status report submitted to Iowa Dept. of Natural Resources, Des Moines, IA. 14 pp.
Wisconsin Natural Heritage Program. 1996. Element Occurrence Records for Cirsium hillii. Bureau of Endangered Resources, Dept. of Natural Resources. Madison, WI.
MNFI/TNC ELEMENT OCCURRENCE RANKING CRITERIA
A Excellent Occurrence. Protection of A-ranked occurrences is essential to conservation of the maximum diversity and viability of an element in the state. A-ranked communities are essentially undisturbed by humans or have nearly recovered from early human disturbance. Species composition shows little departure from original structure and composition (except in seral or disturbancedependent communities). A-ranked populations of a sensitive species are large in number of individuals, stable or growing, show good reproduction, and exist in a natural, sustainable habitat.
B Good Occurrence. Protection of these occurrences is important to the survival of an element in the state, especially if very few or no A-ranked occurrences exist or in natural regions of the state where there are few or no A-ranked occurrences. A B-ranked community is still recovering from early disturbance or recent light disturbance but eventually will reach a B-rank. Presence of exotic species (if only localized and/or a minor component of the flora), a recoverable departure from original structure and composition for the site (except in seral and disturbance-dependent communities), result in a B-rank. B-ranked populations of a sensitive species are at least stable, occur in minimally disturbed habitat, and are of moderate population size.
C Fair Occurrence. Protection of these occurrences helps conserve the biotic diversity on a regional or local level and is important to statewide conservation only if no higher-ranked occurrences exist. A C-ranked community is in an early stage of recovery from disturbance or its structure and composition have been altered such that the original vegetation of the site will never rejuvenate, yet with management and time, partial restoration of the community is possible. C-ranked populations of sensitive species are in clearly disturbed habitats, small in size and/or number, and possibly declining.
D Poor Occurrence. Protection of these occurrences is seldom worthwhile except for historical reasons or only if no better occurrences exist. D-ranked communities are severely disturbed, their structure and composition have been greatly altered, and recovery to original conditions, despite management and time, essentially will not take place. D-ranked populations of sensitive species are very small with a high likelihood of dying out or being destroyed and exist in highly disturbed and vulnerable habitats.
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