Plants of Concern

Citizen Science Rare Plant Monitoring Program

Cirsium hillii

Hill’s Thistle

Cirsium hillii

Illustration by Jeremie Fant

A Case Study

Hill’s thistle is restricted to the Great Lakes region and is declining across its range. With its large pink flowering heads, this attractive thistle is a characteristic member of dry prairie habitats.

Family: Asteraceae
Life History: Perennial
Illinois Status: Rare (not listed)

Populations monitored: 15
Subpopulations monitored: 49
Counties monitored: 4

Introduction

In North America the term Thistle is commonly associated with exotic weeds, but there are over sixty species of thistles in the genus Cirsium that are native to North America and many of these are considered rare. Hill’s thistle (Cirsium hillii) is an example of a rare thistle that is currently declining throughout its range1. It is native to drier prairies of the Great Lakes region, including the northern United States and southern Canada2. The plant was first described from a collection made in Lake County, Indiana, by Reverend E.J. Hill in 1890; hence, its name is a reference to its original collector rather than to a preferred topography. This thistle is often overlooked as it spends most of its life as a ground-hugging basal rosette, flowering infrequently. When it does flower, the stems are short, usually less than 60 centimeters tall3. Nonetheless, the large pink flowers of the Hill’s thistle are a favorite of bees and other pollinating insects4.

Habitat And Range

Populations of Hill’s thistle are found in southern Ontario, Michigan, Wisconsin, Minnesota, Iowa, Illinois, and Indiana. Despite this wide distribution, populations are restricted to dry-mesic prairie with gravelly to sandy soil, gravel hills, bluff prairies, oak savannas, oak barrens, pine barrens, and limestone alvars3, 5.

Species Biology

The flowering period for Hill’s thistle ranges from June through August, though a plant may persist as a basal rosette for a number of years. When a rosette initiates a flower, a stem is produced, which terminates in one or more large inflorescences. The flowers are fragrant and deep pink-purple in color, although occasionally white flowers have been reported. Originally it was thought that flowering occurred 1-3 years after the establishment of the rosette, and that plants died after flowering2. However, data collected by POC has shown that most plants in a population remain vegetative, with some rosettes persisting for up to 10 years. POC-collected data has also shown that many rosettes return after flowering and in rare cases bloom again in subsequent years. Typically only 2-10% of rosettes are reproductive in any given year.

Seeds are born on a fluffy pappus, which allows them to be wind-dispersed. However, Hill’s thistle seeds tend to be heavier than those of most other thistles, which limits the distance they can travel. Low seed production has been found in the Chicago region1, but in other cases seeds are noted as abundant3, 6, 7. Nonetheless, viability of seeds is uniformly considered poor for this species, representing a significant barrier to establishment of new plants2, 5, 7, 8.

Hill’s thistle populations are hypothesized to benefit from fire and summer mowing9, which helps maintain an open habitat and removes litter that would otherwise inhibit the growth of this short-statured species7 .

Threats

Primary threats to Hill’s thistle populations include the loss and fragmentation of their habitat as well as a lack of management8, 9, 10. With fire suppression or lack of summer mowing, previously open habitats can become densely shaded by native shrubs and trees as well as invasive exotic species, which exclude species like Hill’s thistle that require sunlight and space7, 11.

Another serious concern for this species is the extent of vegetative reproduction in many populations. In some populations the number of rosettes in a population does not reflect the true number of unique individuals. In populations that rely on clonal growth, there is potential for lower genetic diversity and increased chance that two flowers are genetically identical. This is important as many Cirsium species are known to have a sporophytic self-incompatibility, which means a flower will not accept its own pollen or pollen from any siblings. The combination of a low flowering rate and increased clonal growth can result in a limited number of suitable mates within a population, leading to poor seed set12.

Conservation Status

Hill’s thistle is only found in the broader Great Lakes Region and was once considered to be at high risk of extirpation from the Chicago region8, 11. The species is thought to have declined significantly across its historic range, and even in states where the species is not rare, most populations are declining1. Some populations are large and reproductively active, but most are small, fragmented, and at risk of local extinction.

Hill’s thistle was listed by the state of Illinois as threatened from 1994 to 2004, but was removed from that list because it was considered to be more common than previously thought6. Other Midwestern states have added protection for this species, including Indiana, where it is listed as endangered and Wisconsin, where it is listed as threatened. Official protection also extends to Canada, where it is protected by both the Canadian federal government as well as the province of Ontario8.  Currently listed as globally vulnerable (G3) by NatureServe1, POC considers Hill’s thistle to be rare in northeastern Illinois.

Monitoring & Research

In addition to ongoing annual monitoring of many populations, Hill’s thistle was part of an in-depth monitoring effort by POC where individual plants were tagged and tracked over 10 years. Monitoring and additional research showed that Hill’s thistle populations in northeast Illinois rarely flowered, had low seed viability, and primarily reproduced asexually. Researchers were concerned that populations would be negatively impacted by this lack of genetic outcrossing. Surprisingly, despite these reproductive obstacles, POC-monitored populations do not currently suffer from low genetic diversity. In fact, populations were found to have a similar degree of genetic diversity as healthy populations in Wisconsin, though the Chicago region’s genotypes are unique2. However, a continued lack of sexual reproduction could threaten the maintenance of this genetic diversity. Ongoing POC monitoring, which tracks flowering rate and population size, can help to detect if populations are experiencing negative consequences from low rates of outcrossing.

Impact

Data collected by POC monitors has contributed to an improved understanding of the biology of Hill’s thistle, revising our understanding of how and when plants reproduce, as well as providing an important baseline for partner researchers investigating the genetics of local populations. Further, the decision to remove this species from Illinois’ list of threatened species relied on POC data to understand northeastern Illinois populations. Although removal from Illinois’ list indicates an improved outlook, it’s clear that ongoing research and conservation are needed, specifically regarding low rates of flowering and viable seed production. POC volunteers and staff continue to regularly monitor populations and work with research partners towards the goal of promoting the long-term health of Hill’s thistle populations.

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