Hill’s thistle (Cirsium hillii) COSEWIC assessment and status report: chapter 6

Biology

General

The Nature Conservancy noted in 1990 that “Information is lacking on all phases of the life cycle of Hill’s Thistle. More is needed to be known about the biology and life history of C. hillii in order to manage it appropriately.” In his 2001 “Rangewide Assessment of Hill’s Thistle” for the US Fish & Wildlife Service, Penskar noted that “Internet and literature searches revealed virtually no information or citations concerning research or monitoring of this species. These searches, though conducted several times, cannot be considered definitive, but do highlight the paucity of information on this Midwestern taxon.” Penskar concluded by proposing that “Natural history studies of virtually any aspect of the biology and ecology of the species are recommended to augment and guide experimental management programs.” For the present study, the author found the situation with our basic knowledge of C. hillii unchanged.

Reproduction

Cirsium hillii is a relatively short-lived perennial, generally persisting two or three years but usually no more than four or five (Ostlie and Bender, 1990). Flowers are produced one or two seasons after the establishment of the basal rosette (see Figure 4), most typically in three-year-old plants (Higman & Penskar 1999). Reproduction also occurs vegetatively by adventitous buds that form along the lateral roots and give rise to basal rosettes. Several lateral shoots may be produced by a single plant (The Nature Conservancy 1990, Higman & Penskar 1999). The primary taproots die along with the remainder of the plant after flowering, and in some instances lateral shoots are produced prior to death and these grow in the following years (The Nature Conservancy 1990, Higman & Penskar 1999).

Seed germination in natural and controlled environments is reported as low (Ostlie and Bender, 1990). This has been cited as the primary cause for the rarity of C. hillii by field researchers, suggesting that, “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” (Ostlie and Bender, 1990). In greenhouse trials, Henderson reported a 10 to 20 percent germination rate with sowing seeds in flats, while Wade found that seeds sown into ¼ inch of soil in a greenhouse environment also germinated poorly. However, he found that naturally-dispersed seeds produced a significantly higher level of germination. Henderson suggests that the low rate of germination in C. hillii may be an artifact of low seed viability, while Wade adds that low light levels may also be responsible (Ostlie and Bender, 1990). A prescription for germination of seeds is to: “Sow at 68˚. If no germination occurs, move to 39˚ for four weeks, recycle, 14 to 30 days. Resents root disturbance” (Anonymous 2001).

Roberts and Chancellor (1979) conducted some interesting germination experiments on seven species of Cirsium and Carduus, including Cirsium eriophorum, C. palustre, C. vulgare, and C. arvense. They concluded that, “More than 90% of all the seedlings of Carduus and Cirsium species emerged within a year after sowing and the survival of achenes in cultivated soil was relatively short. Chepil (1946) in Canada found that achenes of Cirsium arvense persisted for 1 to 2 years in 7.5 cm of cultivated soil, and, when achenes were placed 1 cm deep in soil in the Netherlands, none was viable after 10 months (Bakker 1960). The survival of achenes in this species is greatly prolonged, however, when buried deeper than this in undisturbed soil. Bakker found that achenes buried some 40 cm deep showed no change in the capacity for germination during 4 years, while some buried achenes remained viable for 21 years in the US (Toole & Brown 1946) and 26 years in Denmark (Madsen 1962). These results suggest that achenes of C. arvense possess only short-lived innate dormancy, but that, when dormancy is enforced (Harper 1957), the period of survival can be considerable. This may also be true for other species of Carduus and Cirsium.”From their studies Roberts and Chancellor (1979) were also able to conclude that, “All six species of Carduus and Cirsium showed a similar pattern: a variable, usually small, percentage of achenes germinated in the first autumn but the main emergence occurred in the following spring. Although long-term survival may be possible in undisturbed soil, at least with C. arvense, persistence in a surface layer of cultivated soil was relatively short.”  (While these results may not be transferrable to Cirsium hillii, they are included here because of the paucity of information on the species, as noted earlier.).

Cirsium hillii is known to be pollinated by four species of long-tongued bees: Bombus pensylvanica and Psithyrus variabilis in the Apidae, and Megachile montivaga and Megachile pugnatus in the Megachilidae (Hilty 2003).

Seed production generally is abundant (TNC 1990, Higman & Penskar 1999).

From notes by field researchers and personal observations by Allen in 2002, the main apparent factor affecting the ability of the species to reproduce is the closing in of the surrounding forest and resulting decrease in available light to plants of C. hillii. This stress is exemplified at sub-populations at Sites 44 and 60, with basal rosettes in evidence in the shaded environments, but no evidence of reproducing individuals.

Survival

Both flowers and seeds are vulnerable to insects and perhaps fungi (TNC 1990).

Physiology

Moore & Frankton (1966) noted that across its range Cirsium hillii flowers from the second week of June through to the second week of September, with the peak occurring from mid-June to the end of July. In Ontario, flowering occurs from mid-July through August (Moore & Frankton 1974) (see Figure 8). From his work on the species in the Chicago area, Hill considered the season for the species as lasting six weeks, being virtually out of flower by July 25. By this date most plants were noted as already having shed their seed, and by the first of August the stems were generally withered and dry (Hill 1910).

Figure 7. Cirsium hillii habitat in large opening on dunes within red oak-white pine savannah at Wasaga Beach Provincial Park (July 1997).

Figure 7. Cirsium hillii habitat in large opening on dunes within red oak-white pine savannah at WasagaBeach Provincial Park(July 1997).

Figure 8. Cirsium hillii going to seed with previous year’s seed head in foreground. Wasaga Beach Provincial Park (July 1997).

Figure 8. Cirsium hillii going to seed with previous year’s seed head in foreground. WasagaBeach Provincial Park(July 1997).

All of the Ontario populations occur on calcareous sandy soil or on dolostone bedrock.

Movements/dispersal

In their 1967 study of seven species of Cirsium (including pumilum ssp. hillii and pumilum at that time), Moore and Frankton concluded that “Of this group of species, only the eastward migrant [C. pumilum] shows a change in chromosome number from the ancestral 34 and it may be that the reduction to 30 entailed genetic change that made eastward migration possible. The somatic number 30 has been reported for nine collections of C. pumilum s.l. from widely separated locations representative of the range and it appears that the new karyome has remained stable.”

At maturity, the seed head breaks off and is blown away (TNC 1990) with the result that the seeds of Cirsium hillii are wind-dispersed.

Behaviour/adaptability

Cirsium hillii is a species that is dependent on natural disturbance in order to maintain its critical habitat, primarily fire, but also drought. Natural wildfire has been suppressed for at least one hundred years throughout its range. With this altered disturbance regime, Penskar (2001) has concluded that the principal threat to the species in the US “…appears to be the continued decline of remaining habitat through plant succession, canopy closure, and shading. This has led to the highly increased vulnerability of colonies to stochastic 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 right-of-way.” In Ontario the scenarios are basically the same. The importance (and evidence or lack) of fire has been noted for the Ontario stations by Jones (2000), Morton (2002), and Johnson (2002). Much of the Bruce Peninsula was burned in catastrophic fire over one hundred years ago, and the importance of fire in today’s C. hillii habitat is reinforced by the statement by Johnson (2002), in referring to site 52, that, “[C. hillii is] Apparently always where the forest burned in the early 1900s.” Fernald (1930) was impressed with Great Cloche Island and notes excursions into “much burned Manitoulin Island.” (pers. comm. Morton 2002). Today there is “essentially no burning” on Manitoulin Island (pers. comm. Morton 2002). The other natural disturbance that has benefited C. hillii is the fluctuations in Great Lakes water levels, and this has served to keep habitat open along the shores of Lake Huron, particularly in the alvar habitats (pers. comm. Morton 2002).

The species is susceptible to severe drought, as presumed at Cook Prairie in Indiana as the primary cause of final extirpation of the largest state population (Penskar 2001); other factors included mowing, herbiciding, siltation due to agricultural runoff, compaction and destruction by heavy equipment, and skidding. At several of the Ontario stations the affinity of the species to human disturbance along trails and tote roads has been noted, and these are often either the only location where plants can be located, or where fruiting plants can be found. Brunton (1989) for example noted that at site 60, “The plants seen were all growing singly and all were on the stabilized edge of fire roads through the dry forests of the high dunes under red oak-pine forest cover.” Similarly, Jones (2001) noted at Site 22 that, “Road bisects the small patches of habitat where thistles occur, but this may actually be helping the thistles by providing an open, fern-free area.”

It has not yet been determined whether Hill’s thistle is simply resilient to, or actually benefits from, furrow planting in the jack pine plantations of Michigan directed at maintaining the endangered Kirtland’s Warbler, as well as other timber management practices in that state. Furrow planting followed by a 50-year rotation of jack pine is believed to be a threat to Hill’s thistle (Penskar 2001).

The species can tolerate environmental degradation, noted earlier with the references to its ability to re-establish in agricultural pastures and compete, apparently successfully with white and red clover, timothy, and Poa pratensis and P. compressa (Hill 1910), as well as its ability to survive in grazed areas (Cusick 1995). In Illinois C. hillii has demonstrated the ability to persist in degraded prairie remnants within cemetaries, where it is maintained by mowing which discourages woody plant establishment and succession, but also prevents the species from flowering (Penskar 2001).

C. hillii does not contend well with other invasive species, and has been noted as vulnerable to black locust (Robinia pseuodoacacia), honeysuckles (Lonicera spp.), and weedy thistles (Penskar 2001).

In a report by The Conservancy (Ostlie and Bender, 1990), it was stated that the recovery potential of Cirsium hillii was uncertain. At the time success with transplants of the species had been mixed, with many efforts ending in failure. Wade found transplants to be typically successful, provided they were moved while still in the rosette stage (Ostlie and Bender, 1990). He utilized both the bare-root and root ball methods. Henderson successfully moved plants from freshly ploughed prairie with good success.

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