Willowleaf Aster (Symphyotrichum praealtum) : COSEWIC assessment and status report 2024

Official title: COSEWIC assessment and status report on the Willowleaf Aster (Symphyotrichum praealtum) in Canada

Committee on the status of Endangered Wildlife in Canada (COSEWIC)

Special Concern

2024

COSEWIC assessment summary

Assessment summary - May 2024

Common name

Willowleaf Aster

Scientific name

Symphyotrichum praealtum

Status

Special Concern

Reason for designation

This herbaceous perennial plant reaches the northern limit of its range in Canada, and is geographically a highly restricted species that has undergone range contraction. Nine subpopulations occur in extreme southern Ontario. Recent surveys have found additional subpopulations, with over 200,000 known flowering shoots in Canada, mostly within transplanted subpopulations. The change in status reflects this higher estimate of abundance, changes to the way that criteria are applied, and new discovery of subpopulations. However, the species is still under threat from habitat loss and degradation due to mowing and maintenance of roads, trails, powerline corridors, fire suppression, encroachment of woody vegetation due to natural ecological succession, and residential development.

Occurrence

Ontario

Status history

Designated Special Concern in April1999. Status re-examined and designated Threatened in May 2003. Status re-examined and designated Special Concern in May 2024.

COSEWIC executive summary

Willowleaf Aster

Symphyotrichum praealtum

Wildlife species description and significance

Willowleaf Aster is an herbaceous perennial plant in the aster family. It is characterized by composite heads of near-white to deep purple ray florets and yellow disc florets, narrow leaves with distinctive reticulate venation beneath, and rhizomatous growth habit.

Willowleaf Aster is at the northern fringe of its global range in Canada, which may be important for the future adaptive potential of the species.

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. There is no Aboriginal Traditional Knowledge specific to Willowleaf Aster in this report.

Distribution

Willowleaf Aster is found only in North America, where it occurs from northern Mexico to a small area in extreme southwestern Ontario. The majority of its range is in the central and eastern United States.

Habitat

In Canada, Willowleaf Aster occurs in tallgrass prairies and other open habitats, and occasionally in thickets and along roads and trails. Some open habitats of Willowleaf Aster in Canada are transitioning to thicket and woodland habitats through natural ecological succession, which could negatively affect the species.

Biology

Willowleaf Aster is a rhizomatous species that can form large colonies of up to 1,000 clonal stems. It is insect-pollinated and produces seeds inside an achene with a pappus of fine bristles to enable dispersal by wind. It requires cross-pollination by insects to successfully produce seeds, so individuals must be in close enough proximity that pollen can be transported between them by insects such as bees, wasps, and flies. It can successfully flower and produce fruit in the first year of growth following seed germination. The short lifespan of its seeds limits the potential for recovery from the seed bank.

Population sizes and trends

There are currently at least 210,530 mature individuals (flowering clonal stems) of Willowleaf Aster in Canada. Eighty-six percent of these are based on estimates of transplanted individuals that are established and reproducing. This number was inferred from a combination of fieldwork conducted by the report writers in 2022 and secondary sources, namely observation data from Ontario’s Natural Heritage Information Centre and monitoring by the Ontario Ministry of Transportation. Individuals are distributed among seven extant subpopulations, but 98.6% of mature individuals occur in just one subpopulation in Windsor and LaSalle, Ontario. The status of two additional subpopulations at Bkejwanong (Walpole Island) First Nation is unknown but the subpopulations are likely still extant, given the continued presence of suitable habitat.

The estimate of abundance of Willowleaf Aster in Canada has increased by several orders of magnitude over the past 10 years. However, this is primarily a reflection of more detailed surveys for the species in recent years, mostly within transplanted colonies. Determining trends since the previous status report is complicated by a lack of abundance information in secondary sources and by differences in how individuals were defined and counted.

Threats

The main threat to Willowleaf Aster in Canada appears to be natural succession of open habitats (for example, prairies and meadows) to thickets and woodlands that are not suitable for the species. It requires open habitats that are maintained through natural disturbance (for example, fires) or light anthropogenic disturbance (for example, brush clearing). Other threats include habitat loss and degradation due to mowing and maintenance of roads, trails, and powerline corridors, along with fire suppression, encroachment of woody vegetation due to natural ecological succession, and housing and trail development.

Protection, status, and recovery activities

Willowleaf Aster is currently listed as Threatened on Schedule 1 of the federal Species at Risk Act (2002) and under Ontario’s Endangered Species Act (2007). It has a global conservation rank of G5 (Secure), a national conservation rank of N2 (Imperilled) in Canada, and a provincial conservation rank of S2 (Imperilled) in Ontario. It is considered Secure (N5) in the United States but has a rank of S1 (Critically Imperilled) or S3 (Vulnerable) in seven states. It is unranked in the majority of states where it occurs and has no legal status in Mexico.

Technical summary

Symphyotrichum praealtum

Willowleaf Aster

Aster très élevé

Indigenous names: none known

Range of occurrence in Canada: Ontario

Demographic information:

Generation time (usually average age of parents in the population)

Approximately 3 years

Based on Zhang et al. 1999. Seeds remain viable in the seed bank for up to four years.

Is there an inferred continuing decline in number of mature individuals?

No

Although most of the increase in abundance since 1999 is due to more comprehensive information becoming available, monitoring data from the Ontario Ministry of Transportation (MTO) indicates that abundance, at least in Windsor-LaSalle, is increasing. Some subpopulations have become extirpated since 1999, but it is difficult to determine whether this coincided with a decrease in the number of individuals. If the manipulated portion of the Windsor-LaSalle subpopulation is excluded, there has been a decline in the number of mature individuals.

Estimated percent of continuing decline in total number of mature individuals within 3 years

n/a

Estimated percent of continuing decline in total number of mature individuals within 5 years

n/a

Percent reduction or increase in total number of mature individuals over the last 10 years

Unknown

Historical data are incomplete

Percent reduction or increase in total number of mature individuals over the next 10 years

Unknown

Unknown

Percent reduction or increase in total number of mature individuals over any period of 10 years, including both the past and future

Unknown

Unknown

Are the causes of the decline clearly reversible?

Yes

Maintenance of existing open habitats and creation of new open habitats could contribute to restoring extirpated subpopulations and reverse local declines in abundance.

Are the causes of the decline clearly understood?

Yes

Declines are the result of succession of open habitats to treed habitats, mechanical damage from road maintenance activities, and reproductive failure of subpopulations consisting of single clones.

Are the causes of the decline clearly ceased?

Unknown

It is unknown whether ongoing habitat maintenance and creation activities offset ongoing habitat loss.

Are there extreme fluctuations in number of mature individuals

No

Extreme fluctuations in the genet are unlikely, since they are long-lived perennials. The number of ramets (flowering stems) varies annually but it is not known to exceed an order of magnitude.

Extent and occupancy information:

Estimated extent of occurrence (EOO)

7,872 km²

Calculated based on minimum convex polygon around extant and presumed extant (unknown) occurrences.

Index of area of occupancy (IAO), reported as 2x2 km grid value

72 km²

Calculated using a 2 x 2 km grid drawn over extant and presumed extant (unknown) occurrences.

Is the population “severely fragmented,” that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. No
2. No

Most of the Canadian population is within Windsor-LaSalle subpopulation, which is anticipated to be large enough for this species to persist.

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

>10

There are potentially 100s of locations in the Windsor–LaSalle area based on land ownership.

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

Yes

Observed 36% decline in EOO from 12,358 km².since the last assessment and the decline is projected to continue.

Is there an [observed, inferred, or projected] continuing decline in area of occupancy?

Yes

There has been an observed loss of subpopulations since the last assessment, and discoveries of new subpopulations that may have become established since the previous report. If all known subpopulations that are extant now, were extant in 1999, this would be a decline of 33.3% from 108 km².since the last assessment; the decline is projected to continue.

Is there an inferred continuing decline in number of subpopulations?

Yes

Inferred / projected based on subpopulations with single clones that may not be viable

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

Yes

Inferred / projected based on some subpopulations / locations with single clones that may not be viable

Is there an observed continuing decline in quality of habitat?

No

Some open habitats of Willowleaf Aster are undergoing succession to thicket and woodland communities, but creation and maintenance of suitable open habitats is ongoing.

Are there extreme fluctuations in number of subpopulations?

No

This is a long-lived perennial plant

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (by subpopulation):

Quantitative analysis:

Is the probability of extinction in the wild at least 20% within 20 years or 10% within 100 years

Unknown

Analysis not conducted

Threats:

Was a threats calculator completed for this species?

Yes, 21 April 2023

Overall assigned threat impact:

Medium to Low (2023)

Key threats were identified as:

1. Other Ecosystem Modifications (IUCN Threat 7.3, Medium-Low Impact)
2. Fire and Fire Suppression (IUCN Threat 7.1, Low Impact)
3. Housing and Urban Areas (IUCN Threat 1.1, Low Impact)
4. Tourism and Recreation Areas (IUCN Threat 1.3, Low Impact)
5. Problematic Native Species (IUCN Threat 8.2, Low Impact)

What limiting factors are relevant?

• Dependence on cross-pollination by insects for successful gamete production
• Short-lived seed bank
• Late-season flowering period
• Intolerance for growing in the shade

Rescue effect (from outside Canada):

Status of outside populations most likely to provide immigrants to Canada.

Vulnerable

Vulnerable (S3) in Michigan where the closest subpopulations occur; Apparently Secure (S4) in Pennsylvania; Not Ranked in all other states adjacent to Canada.

Is immigration known or possible?

Possible

The likelihood of seeds from U.S. population arriving in Canada by wind or water is unknown but this is possible.

Would immigrants be adapted to survive in Canada?

Yes

The same variety of Willowleaf Aster occurs in the United States, in similar habitats in close proximity to the Canadian border.

Is there sufficient habitat for immigrants in Canada?

Yes

There are still abundant suitable habitats in the species’ Canadian range.

Are conditions deteriorating in Canada?

Yes

Natural succession of open habitats to thickets and woodlands has been observed, but activities are also occurring which create/maintain open habitats.

Are conditions for the source (that is, outside) population deteriorating?

Unknown

The United States and Mexico populations are not well studied.

Is the Canadian population considered to be a sink?

Unknown

There is no evidence that the Canadian population is a sink.

Is rescue from outside Canada likely, such that it could lead to a change in status?

Unknown

The likelihood of seeds from Michigan subpopulations arriving in suitable habitat in Canada via wind or water is unknown.

Wildlife species with sensitive occurrence data (general caution for consideration):

Could release of certain occurrence data result in increased harm to the Wildlife Species or its habitat?

No

n/a

Status history:

COSEWIC status history

Designated Special Concern in April1999. Status re-examined and designated Threatened in May 2003. Status re-examined and designated Special Concern in May 2024.

Status and reasons for designation:

Status

Special Concern

Alpha-numeric codes

Not applicable

Reason for change in status

1(i) due to increases in manipulated subpopulations; 4(i,vi); 5(ii)

Reasons for designation

This herbaceous perennial plant reaches the northern limit of its range in Canada, and is geographically a highly restricted species that has undergone range contraction. Nine subpopulations occur in extreme southern Ontario. Recent surveys have found additional subpopulations, with over 200,000 known flowering shoots in Canada, mostly within transplanted subpopulations. The change in status reflects this higher estimate of abundance, changes to the way that criteria are applied, and new discovery of subpopulations. However, the species is still under threat from habitat loss and degradation due to mowing and maintenance of roads, trails, powerline corridors, fire suppression, encroachment of woody vegetation due to natural ecological succession, and residential development.

Applicability of criteria:

A: Decline in total number of mature individuals

Not Applicable.

Insufficient data to reliably infer, project, or suspect population trends. Declines in EOO and IAO > 30% beginning over 3 generations ago but uncertain if corresponding decline > 30% of mature individuals co-occurred.

B: Small range and decline or fluctuation

Not Applicable.

EOO of 7,872 km² is below the threshold for Threatened and IAO of 72 km² is below the threshold for Endangered. The population is not severely fragmented, occurs at > 10 locations, and does not experience extreme fluctuations. However, there is a decline in EOO, and possibly a continuing decline in IAO, number of subpopulations, and number of locations exposing the species to continued risk of extirpation.

C: Small and declining number of mature individuals

Not Applicable.

Number of mature individuals is 210,530, exceeding threshold for Threatened.

D: Very small or restricted population

Not Applicable

Estimate of 210,530 mature individuals exceeds thresholds for D1, and population is not vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not Applicable

Analysis not conducted.

Willowleaf Aster may become Threatened if factors suspected of negatively influencing the persistence of the species are neither reversed nor managed with demonstrable effectiveness. Several subpopulations have been extirpated since the last assessment in 2003, and the species is near to qualifying, under criterion B, for Threatened status.

Preface

Willowleaf Aster was assessed as Threatened by COSEWIC in 2003 based on the Zhang et al. (1999) status report. Since that assessment, new data have been collected on the species and reported in provincial and federal recovery strategies (Jones 2013; Environment and Climate Change Canada 2017) and submitted to Ontario’s Natural Heritage Information Centre (NHIC) by experts and community scientists. Additional data, observations, and search effort have resulted in the identification of new subpopulations that were not included in the 2003 assessment. Consequentially, a clearer picture of the number of mature individuals in Canada has emerged.

The 1999 status report for Willowleaf Aster in Canada did not define subpopulations according to the standards currently used by COSEWIC. Using current standards and based on observation data from a variety of sources as of 2022, this updated status report lists 15 subpopulations in Canada (Table 1). To the extent possible, the same criteria have been applied to the sites listed in 1999 to enable comparisons between current and past conditions. However, it is important to understand that the 15 subpopulations listed in this update status report do not match those listed in the 1999 status report one-to-one. Furthermore, the 1999 status report did not provide site-level abundance information for Willowleaf Aster because field investigations were not conducted. In addition, abundance information reported in herbarium collections was generally qualitative and inconsistent with COSEWIC’s definition of an “individual.” Therefore, this update status report treats Willowleaf Aster as “present” at the majority of subpopulations in 1999 rather than inferring abundances (that is, number of mature individuals) based on assumptions.

Between 2010 and 2015, the Ontario Ministry of Transportation (MTO) relocated 130,849 Willowleaf Aster stems from the footprint of the Herb Gray Parkway to suitable habitat elsewhere in the Windsor-LaSalle subpopulation and to the Bear Creek unit of St. Clair National Wildlife Area (AMEC Foster Wheeler 2018; Wood 2020; Heather Barna Consulting and Dan Barcza, Sage Earth Consulting 2022; WSP 2022; Charlton pers. comm. 2023). Ongoing monitoring of these individuals has resulted in detailed quantitative estimates of abundance, which are reflected in the abundance data in this report where they satisfy the COSEWIC guidelines on the inclusion of manipulated populations. MTO data indicate that the abundance of Willowleaf Aster in Canada is substantially larger than previously estimated.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2024)

Wildlife Species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)^*

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)^**

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)^***

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

^* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
^** Formerly described as “Not In Any Category”, or “No Designation Required.”
^*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment and Climate Change Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Wildlife species description and significance

Name and classification

Current classification¹:

Clade: Eudicots

Order: Asterales

Family: Asteraceae

Genus: Symphyotrichum

Species: Symphyotrichum praealtum (Poir.) G.L. Nesom

Varieties in Canada: var. praealtum

Taxonomic changes since previous report (for reassessments): No taxonomic changes have occurred since the previous reassessment in 2003.

Common names^{Footnote 1}:

English: Willowleaf Aster, Willow-leaved Aster, Willow Aster, Veiny-lined Aster

French: aster très élevé

Indigenous (specify language): none known

Synonyms and notes¹:

Aster caerulescens DC.

Aster coerulescens DC.

Aster nebraskensis Britton

Aster novi-belgii var. litoreus A. Gray

Aster praealtus Poir.

Aster praealtus Poiret var. praealtus

Aster praealtus var. imbricatior Wiegand

Aster praealtus var. coerulescens (de Candolle) A.G. Jones

Aster salicifolius Aiton non Lam.

Aster subasper Lindl.

Symphyotrichum novi-belgii var. litoreum (A. Gray) G.L. Nesom

Willowleaf Aster was commonly treated as Aster praealtus until 1994, when it was revised to Symphyotrichum praealtum by G.L. Nesom. Two varieties are formally recognized: the nominate S. p. var. praealtum, which occurs in the midwestern United States and extreme southern Canada, and S. p. var. angustior, which occurs in the Appalachian Mountains and southern New England. Three additional varieties may be recognized pending phylogenetic studies: S. p. var. nebraskense, S. p. var. subasperum, and S. p. var. texicola (Nesom 1994, 1997; Semple et al. 2002; Brouillet et al. 2006; Semple 2021). The only variety found in Canada is S. p. var. praealtum. Unless specified, all references to Willowleaf Aster in this report refer to the S. p. var. praealtum.

Description of wildlife species

Willowleaf Aster (Figure 1) is an herbaceous, rhizomatous, perennial angiosperm that grows 1 to 1.5 m tall. The “flowers” of Willowleaf Aster are composed of near-white to deep purple ray florets and yellow disc florets. The fruits are achenes with a pappus of fine bristles that aid in dispersal. Detailed morphological descriptions of Willowleaf Aster can be found in Semple et al. (2002) and Brouillet et al. (2006).

Figure 1. Identifying features of Willowleaf Aster: A) capitulum with purple rays and orange disc florets; B) involucres with spreading phyllaries; C) leaf undersides with conspicuous reticulate venation; D) entire plant. Photos by William Van Hemessen.

Willowleaf Aster is most easily recognized when the plants are in flower but can be identified by experienced botanists at other times of year using leaf characteristics, including narrow leaves with distinctive reticulate venation beneath, and rhizomatous growth habit. Accurate identification of Willowleaf Aster requires careful examination of multiple morphological features because it can be easily confused with similar species such as Shining Aster (Symphyotrichum firmum), Panicled Aster (S. lanceolatum), New England Aster (S. novae-angliae), New York Aster (S. novi-belgii), and Swamp Aster (S. puniceum). The hybrid Amethyst Aster (S. x amethystinum) is also superficially similar. There is no single feature that immediately distinguishes Willowleaf Aster from similar species in Canada, so a combination of features is required for observations to be verifiable.

Designatable units

Willowleaf Aster occurs entirely in one COSEWIC National Ecological Area (Great Lakes Plains) and constitutes a single Designatable Unit (DU). The entire Canadian population of Willowleaf Aster consists of the nominate variety praealtum. The 15 subpopulations reported here all occur in relatively close geographical proximity in southern Ontario. The largest separation distance between Canadian subpopulations (including extant, extirpated, and historical [presumed extant] status subpopulations) is approximately 65 km. There is no evidence of heritable traits or markers that clearly distinguish the putative DU from other Dus, and there is no evidence that sufficient time has passed that either natural selection or genetic drift are likely to have produced evolutionarily discrete units. There is no evidence of significance as the species is not known to harbour heritable adaptive traits or an evolutionary history not found elsewhere in Canada. Anthropogenic habitat fragmentation is discussed under Threats and Dispersal and migration.

Special significance

No special cultural or commercial values of Willowleaf Aster are known. Other asters have been used as components of medicines for fever (Moerman 1998), but this use is not attributed to Willowleaf Aster specifically. It is occasionally cultivated as an ornamental plant but is not widespread in the nursery trade.

No wildlife species are known to rely exclusively on Willowleaf Aster. In Canadian subpopulations, it is rarely a major component of the plant communities in which it occurs, nor is it abundant relative to other aster species. Its significance as a host plant for invertebrates that feed or reproduce on asters is probably low, and likewise its significance as a forage plant.

Willowleaf Aster subpopulations in Canada are on the northern periphery of their global range. Populations at the edge of a species’ range may be important for the future adaptive potential of the species (Lesica and Allendorf 1995).

Aboriginal (Indigenous) knowledge

Aboriginal Traditional Knowledge (ATK) is relationship-based. It involves information on ecological relationships between humans and their environment, including characteristics of species, habitats, and locations. Laws and protocols for human relationships with the environment are passed on through teachings and stories, and Indigenous languages, and can be based on long-term observations. Place names provide information about harvesting areas, ecological processes, spiritual significance or the products of harvest. ATK can identify life history characteristics of a species or distinct differences between similar species.

Cultural significance to Indigenous peoples

There is no species-specific ATK in the report. However, Willowleaf Aster is important to Indigenous Peoples who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

Willowleaf Aster is native to North America where it ranges from northern Mexico in the southwest to New Hampshire in the northeast (see Figure 2). The two accepted varieties (S. p. var. praealtum and S. p. var. angustior) occur in geographically distinct regions (Figure 2; Weakley 2020; Semple 2021). Willowleaf Aster has been introduced outside its native range and has been reported in cultivation throughout North America and in northern Europe. Supposedly naturalized occurrences of Willowleaf Aster outside its native range originated as deliberate plantings or escapes from cultivation, but some are of unknown origin (Hinds 2000; Semple et al. 2002; Brouillet et al. 2006).

Figure 2. Distribution of Willowleaf Aster Symphyotrichum praealtum in North America (Smith 2016)

Canadian range

Less than 1% of the global range of Willowleaf Aster occurs in Canada where it is naturally found only in extreme southwestern Ontario (Figure 3). The northernmost and easternmost subpopulation in Canada was in Perth County, Ontario at 43.2ºN, but this subpopulation was not found during fieldwork in 2022 and is believed to be extirpated. The northernmost extant subpopulation is in Point Edward, Ontario, at 43.0°N, and the easternmost extant subpopulation is in Port Stanley, Ontario, at 81.2°W. The southernmost extant subpopulation is in Harrow, Ontario, at 42.0°N, and the westernmost extant subpopulation is in Windsor and LaSalle, Ontario, at 83.1°W. A historical occurrence in New Brunswick is believed to have been an ephemeral introduction that did not persist (Hinds 2000).

Figure 3. Extent of occurrence (EOO) and Index of Area of Occupancy (IAO) based on extant and historical (presumed extant) sites (2022), created by A. Filion (COSEWIC Secretariat March 2024).

Based on the best available information, including fieldwork and secondary sources described herein, 15 subpopulations of Willowleaf Aster in Canada are reported (Table 1). As of 2022, seven subpopulations are extant, six are presumed to be extirpated, and two are historical. The subpopulation located on Walpole Island is of unknown status, but is believed to still be extant because it was last observed in 2008 and abundant suitable habitat remains available. Cedar Creek, Marshfield Woods, and Port Stanley only have one colony and therefore may not be sexually viable. The subpopulation at Rondeau Provincial Park has not been reported since 1956 and may be extirpated.

Willowleaf Aster is occasionally planted outside its natural range in Canada in gardens and as a component of naturalization projects. Cultivated occurrences are not included in this report. Individuals that were transplanted as part of recovery actions and permit requirements associated with construction of the Herb Gray Parkway in Windsor and LaSalle, Ontario, are established and reproducing at their transplant sites within the Windsor-LaSalle subpopulation (Wood 2020; WSP 2022; Charlton pers. comm. 2023). In accordance with the COSEWIC Guidelines on Manipulated Wildlife Species (COSEWIC 2018), these plants have been included in this status report. Individuals that were transplanted to the St. Clair National Wildlife Area (NWA) Bear Creek Unit do not appear to be reproducing (AMEC Foster Wheeler 2018; Heather Barna Consulting and Dan Barcza, Sage Earth Consulting 2022) and have been excluded from the quantitative criteria in this report based on the guidelines.

The Windsor-LaSalle subpopulation is the largest subpopulation in Canada in terms of geographic size and abundance. It includes plants that were internally transplanted within the subpopulation as part of construction of the Herb Gray Parkway. Subpopulations at Port Stanley and Point Edward represent areas of importance as they occur on the fringes of the species’ range in Canada, which may increase or decrease depending on the persistence of these subpopulations. In 2022, the Port Stanley and Point Edward subpopulations were noted to have low abundance and may be at risk of extirpation.

The distribution of Willowleaf Aster has been documented in detail in the Windsor and LaSalle area. However, other subpopulations are less well documented and additional individuals may be present on lands not visited during fieldwork for this report. Although locating additional individuals in these areas might fill gaps within the existing range, this is not expected to expand the species’ range in Ontario. Because this species is not restricted to a narrow range of habitats, surveying the entirety of potentially suitable habitat in Ontario is not possible.

Population structure

In this document, “population” refers to all individuals of Willowleaf Aster in Canada as a single unit. “Subpopulations” are geographically distinct groups within the Canadian population between which there is believed to be little migration or genetic exchange (typically one successful migrant gamete per year or less). “Site” refers to a single property or properties under continuous ownership and management. Subpopulation corresponds to the habitat-based plant element occurrence delimitation standards (NatureServe 2020) where a subpopulation is defined as a group of occurrences that are separated by less than 1 km; or if separated by 1 to 3 km, with no break in suitable habitat between them exceeding 1 km; or if separated by 3 to 10 km but connected by linear water flow and having no break in suitable habitat between them exceeding 3 km.

Willowleaf Aster is rhizomatous and forms colonies of multiple flowering and non-flowering clonal stems. Studies of Panicled Aster, a closely related rhizomatous species, indicate that clonal stems are spaced up to 15 cm apart (Chmielewski and Semple 2001). Although stems within a colony are genetically identical (that is, clonal), this report treats each flowering stem as a “mature individual,” which is consistent with the currently accepted IUCN and COSEWIC definitions.

Extent of occurrence and area of occupancy

Current EOO:

Extent of occurrence (EOO) within Canada is 7,872 km², calculated using a minimum convex polygon that encompasses subpopulations that are extant or presumed extant (historical; Figure 3). This is a decline of 36.3% from 12,358 km² in the previous assessment (Zhang et al. 1999; Figure 4). The historical EOO was 13,319 km² based on all known occurrences (Figure 4).

Figure 4. Extent of occurrence (EOO) including all subpopulations (extant or unknown in 1999); created by A. Filion (COSEWIC Secretariat March 2024).

Current IAO:

The index of area of occupancy (IAO) within Canada is 72 km², calculated using a 2 x 2 km grid drawn over extant and presumed extant (unknown) occurrences (Figures 3). This represents 48 km² in the Windsor area and 24 km² in the remainder of the province. This is an increase from 56 km² in the previous assessment (Zhang et al. 1999). However, if the subpopulations were extant but undiscovered in 1999, the total Canadian IAO would have been 108 km², which would be a 33.3% overall decline in Canada.

Fluctuations and trends in distribution

The current EOO is 36.3% smaller than in 1999 (12,358 km²) and represents a decline of 40.94% (13,319 km²), which includes all known sites (Table 1; Figure 5). The current IAO is 35.7% smaller than historically (112 km²). This figure encompasses all known sites, including those extirpated since the last assessment (Table 1; Figure 3). Below is a summary of changes in the status of subpopulations since Zhang et al. (1999).

• Two subpopulations (Cedar Creek #11 and Windsor-LaSalle #15) that were extant in 1999 are still extant as of 2022
• Three subpopulations (4th Concession Ellice #1, Mount Brydges #2, and Muirkirk #9) that were extant in 1999 could not be relocated during fieldwork in 2022 and are believed to be extirpated
• One subpopulation (Point Edward #4) believed to be extirpated in 1999 was rediscovered during fieldwork in 2022
• The subpopulation at Walpole Island (#7) was listed as extant in 1999 but was not visited in 2022. The subpopulation on Squirrel Island (#6) was considered unknown or extirpated in 1999. These subpopulations are assumed to still be extant because there appears to be abundant suitable habitat, and no obvious land cover changes are visible in historical imagery of the sites
• Four new subpopulations (Port Stanley #8, Big Creek #12, Marshfield Woods #13, and 6th Concession Colchester #14) have been discovered since 1999. They are believed to have been extant in 1999 but had not yet been detected and therefore were not included in the 1999 status report. These subpopulations were still extant in 2022
• The subpopulation at Rondeau Provincial Park (#10) was considered unknown or extirpated in 1999 and was not visited in 2022. However, the subpopulation has not been reported since 1956 and it is believed to be extirpated
• The subpopulation at Cairngorm (#3) that was unknown or extirpated in 1999 could not be relocated during fieldwork in 2022 and is believed to be extirpated
• The subpopulation at Oil Springs (#5) was discovered after the previous report and is now considered extirpated

There are no available quantitative analyses that estimate extinction probability based on known life history, habitat requirements, threats, and management options. The age of individuals is unknown, so no trends related to changes in mortality or recruitment rates over time have been calculated.

Biology and habitat use

There has been little research into the life history and reproduction of Willowleaf Aster compared with closely related species such as Panicled Aster (Chmielewski and Semple 2001; Nesic et al. 2022). The information in this section is based on the best available knowledge about Willowleaf Aster but has largely been inferred from knowledge of Panicled Aster and other species.

Life cycle and reproduction

Willowleaf Aster is a rhizomatous, perennial herb that forms large clonal colonies, a situation that makes it difficult to derive abundance estimates (see Population structure). Willowleaf Aster flowers in late fall between late September and October, but may occasionally flower into November (Jones 2013). Clonal individuals spread readily in appropriate habitat^{Footnote 2}. Panicled Aster, a closely related species, produces clonal stems up to 15 cm apart and this morphology is assumed to be similar in Willowleaf Aster (Chmielewski and Semple 2001).

It is unknown how long a seed-derived individual Willowleaf Aster takes to reach sexual maturity (that is, the length of time from germination to flowering). Closely related species typically produce flowers in their first year of growth but this may take up to two years (Chmielewski and Semple 2001).

Willowleaf Aster is monoecious, but self-pollination does not typically produce viable fruits. Outbreeding (that is, pollination by genetically different individuals) is generally required for viable seed set (Jones 1978). Large colonies with multiple flowering stems can produce very few seeds (Zhang et al. 1999). Pollen is primarily dispersed by insects, such as Hymenoptera (bees and wasps), Diptera (flies), and Lepidoptera (butterflies and moths). The achenes of Willowleaf Aster are wind-dispersed with the aid of a pappus of fine bristles.

It is unknown how long Willowleaf Aster seeds remain viable in the seed bank, but most asters have relatively short-lived seeds, typically remaining viable for no more than four years (Jones 1978; Cullina 2000; Chmielewski and Semple 2001). Willowleaf Aster seeds are therefore assumed to survive for no more than four years in the seed bank. Seed germination rates and environmental conditions that promote the germination of Willowleaf Aster seeds represent another knowledge gap (Environment and Climate Change Canada 2017). Studies on Panicled Aster observed a maximum germination rate of 88% which is optimized at diurnal temperature regimes of 15 to 30°C and 20 to 35°C (Nesic et al. 2022). This may be similar for Willowleaf Aster.

The average and maximum age of individual Willowleaf Asters is unknown and is difficult to determine, because the parent stems may die even as a colony persists. Generation time (that is, the average age of individuals within the population) is two to three years (Zhang et al. 1999). For the purposes of this status report, generation time is being treated as three years for stems within a colony. Genets live much longer.

Habitat requirements

Throughout its natural range, Willowleaf Aster is found in a variety of moist to mesic, open to partially treed habitats, including roadsides, meadows, prairies, savannahs, woodlands, thickets, and streambanks (Deam 2002; Brouillet et al., 2006; Yatskievych 2006; Reznicek et al. 2011; Semple 2021; Michigan Natural Features Inventory 2022). In the southeastern United States, it is associated with calcareous sites (Yatskievych 2006; Weakley 2020). In Canada, it is most frequently found in moist tallgrass prairie habitats, but it is also found on roadsides, in old fields and other previously disturbed sites (Zhang et al. 1999). It has a wetness coefficient of -3 and is considered a facultative wetland species (Oldham et al. 1995). The soil pH of Canadian subpopulations has not been tested.

Based on field observations by the report writers in 2022, Canadian subpopulations of Willowleaf Aster appear to be strongly associated with sandy soils. Of the extant subpopulations, all but two are located on soils mapped as sand or sandy loam in county soil surveys (Richards et al. 1949; Matthews et al. 1957; Schut 1992). The two exceptions are on soils mapped as clay. However, the Willowleaf Aster occurrences in these subpopulations were found in sandy and gravelly microsites (that is, on sandy-gravelly mounds overlying clay substrate and growing in the gravel bed of a rail trail).

The most frequently observed associate species in Canadian subpopulations of Willowleaf Aster include Flat-stemmed Bluegrass (Poa compressa), Gray Dogwood (Cornus racemosa), Frost Aster (Symphyotrichum pilosum), Heath Aster (S. ericoides), New England Aster, Panicled Aster, Staghorn Sumac (Rhus typhina), and Tall Goldenrod (Solidago altissima).

Movements, migration, and dispersal

Willowleaf Aster is insect-pollinated, and its seeds are wind dispersed via a pappus of plume-like bristles. Cross-pollination between colonies is required for successful seed production (Jones 1978; Chmielewski and Semple 2001). The frequency of insect cross-pollination has not been studied in Willowleaf Aster. In large colonies of Panicled Aster, cross-pollination was found to be rare because pollinators were satiated after visiting just one colony, negating the need to visit additional colonies (Chmielewski and Semple 2001). The area and stem density of Willowleaf Aster individuals may therefore affect pollination success due to pollinator satiation (for example, larger colonies may be less successful at cross-pollination).

The typical dispersal distance of Willowleaf Aster seeds is unknown. Long-distance wind dispersal of aster seeds is probably limited by obstacles such as forests and buildings, but could occasionally occur over long distances (for example, several kilometres) if high winds result in seeds reaching elevations that overtop physical barriers (Sheldon and Burrows 1973; Soons et al. 2004).

Interspecific interactions

Like other asters, Willowleaf Aster is pollinated by a diversity of generalist insect species, including bees, bumblebees, flies, wasps, butterflies, and moths (Chmielewski and Semple 2001). Honeybees (Apis mellifera) have been observed to visit asters more frequently than other wildflowers, but their importance compared with other pollinators is unknown (Blackwell and Powell 1981; Chmielewski and Semple 2001).

No wildlife species are known to rely exclusively on Willowleaf Aster for feeding or reproduction, but the genus Symphyotrichum is known to support a high diversity of insect fauna, including at least 6 bee species that feed exclusively on Symphyotrichum nectar and at least 20 gall-forming arthropods that reside exclusively on the genus (Fowler 2016; Gallformers Contributors 2022). Willowleaf Aster is not believed to be an important forage plant for mammals, birds, or other vertebrates. However, it is assumed to be opportunistically eaten by herbivores such as White-tailed Deer (Odocoileus virginianus). No evidence of vertebrate herbivory was observed on Willowleaf Aster during fieldwork in 2022.

Predators and competitors:

White-tailed Deer and other generalist herbivores may be considered predators.

Physiological, behavioural, and other adaptations

Willowleaf Aster is assumed to be similar to Panicled Aster in that it has a relatively rapid growth rate, with seed-derived individuals frequently producing flowers in the first year of growth and annual production of multiple clonal stems from rhizomes up to 15 cm away from the parent plant (Chmielewski and Semple 2001). Like other asters, Willowleaf Aster exhibits bolting (that is, the rapid production of aerial stems from rhizome buds) under optimal environmental conditions. It is therefore assumed that the growth rate is not closely tied to climate (for example, annual temperature or precipitation) because only a short period of optimal temperature and/or moisture conditions is required for an individual to successfully flower and reproduce.

The colonial nature of Willowleaf Aster makes individuals resilient to small-scale disturbances (for example, mowing) that destroy aerial stems or sever rhizomes (Chmielewski and Semple 2001). The effects of rhizome severing on plant survival or growth rate within colonies of other aster species are negligible and may in fact promote the survival of individual stems by reducing intracolonial competition (Schmid and Bazzaz 1987).

Willowleaf Aster seeds are shed with fully developed embryos and, like closely related species such as Panicled Aster, are assumed to be capable of germinating without stratification (Chmielewski and Semple 2001).

Willowleaf Aster occurs primarily in open habitats and is assumed to be shade intolerant. Effects of shading on Willowleaf Aster colonies have not been studied, but fieldwork in 2022 observed that colonies growing in thickets had fewer flowering stems relative to nearby colonies in open habitats. This is consistent with the findings of studies on similar species, which found that individuals in shady habitats exhibited fewer flowering stems, reduced rhizome size, and more frequent winter destruction of rhizomes (Chmielewski and Semple 2001).

Willowleaf Aster is rarely found in dry sites and is therefore assumed to be somewhat drought intolerant.

Limiting factors

Limiting factors are generally not human-induced and include intrinsic characteristics that make the species less likely to respond to conservation efforts. Limiting factors may become threats if they result in population decline. The main limiting factors for Willowleaf Aster have been described in previous sections and include:

• shade intolerance (natural succession of open habitats to thickets and woodlands may affect survival and reproductive success) (see Physiological, behavioural, and other adaptations)
• late-season flowering period (reproduction may be hindered by early frosts) (see Life cycle and reproduction)
• limited time seeds can survive in the seed bank (no more than four years) (see Life cycle and reproduction)
• dependence on cross-pollination by insects for successful gamete production (genetic exchange via pollination may be limited by individual size and distance between individuals) (see Movements, migration, and dispersal)

Population sizes and trends

Data sources, methodologies, and uncertainties

This report defines subpopulations as all occurrences of Willowleaf Aster that fall within a 3 km separation distance, without areas of unsuitable habitat at least 1 km wide between the two features (see Population structure), which is consistent with the guidelines prepared by NatureServe (2020).

Determination of the subpopulation occurrence status and abundance of Willowleaf Aster in Canada was done using data collected by the report writers during fieldwork in 2022, supplemented with data from the sources detailed below. The use of secondary data sources was necessary because the large number of Willowleaf Aster occurrences in Canada made it impractical to visit every occurrence in the field in 2022.

Ontario nhic

Data from Ontario’s NHIC (2022) included 18 element occurrences (EOs) and numerous additional candidate EOs. Note that EOs and candidate EOs identified by NHIC in 2022 do not match the subpopulations defined in this report one-to-one. The majority of observation data provided by NHIC did not include abundance information or consisted of rough verbatim abundance estimates (“thousands,” “a few plants,” “one stem,” etc.). Observations in NHIC’s database made since 2019 (that is, within the last three years) were assumed to be extant in 2022 unless fieldwork by the report writers proved otherwise.

MTO monitoring data

MTO has been monitoring Willowleaf Aster individuals that were transplanted from the footprint of the Herb Gray Parkway between 2010 and 2015 (Wood 2020; WSP 2022; Charlton pers. comm. 2023). A total of 130,849 Willowleaf Aster stems were transplanted from the Parkway construction footprint into restoration sites. The various methods for transplantation included rhizome cuttings (40,068), stems in sod clusters (73,165), and bare roots (17,616) (Wood 2020). MTO did not provide geographic location data for this status report due to data sensitivity; however, it is understood that the majority of transplants were taken from and relocated within the Windsor-LaSalle subpopulation and did not result in the creation of any new subpopulations (Charlton pers. comm. 2023). Some individuals were transplanted to the Bear Creek Unit of St. Clair NWA, but they are not persisting there and have therefore been excluded from this report in accordance with the COSEWIC Guidelines on Manipulated Wildlife Species (AMEC Foster Wheeler 2018; COSEWIC 2018; Heather Barna Consulting and Dan Barcza, Sage Earth Consulting 2022). Abundance estimates reported in a monitoring summary by MTO (2023) were used to inform the abundance of Willowleaf Aster at the Windsor-LaSalle subpopulation, because these plants are established and reproducing at their new sites within the subpopulation and therefore satisfy the above-mentioned guidelines (COSEWIC 2018). The total population size at the transplanted sites was estimated based on the number of stems found in quadrats and on transects (Wood 2020).

iNaturalist

The community science website iNaturalist was consulted to find verifiable observations of Willowleaf Aster (iNaturalist 2022). Observations from iNaturalist were independently reviewed and verified by the report writers and only “research grade” (that is, positively identifiable) observations were used.

Herbarium specimens

Voucher specimens from a variety of herbaria were examined both in person and via scans (see Collections Examined). The Global Biodiversity Information Facility (GBIF) (2022) was consulted to find reports of Willowleaf Aster based on herbarium specimens.

Field investigations

Because of the large number of occurrences of Willowleaf Aster, it was not possible to visit every occurrence in the field, so field work sites were prioritized based on three objectives:

1. visit occurrences not seen since 1999 (the publication date of the previous status report) to determine whether the species was persisting
2. visit other extant occurrences to record more accurate information on the abundance and extent of occurrence of Willowleaf Aster
3. find new occurrences of Willowleaf Aster by conducting additional surveys in areas of suitable habitat. This was a lower priority objective

A large number of recent observations (that is, 2019 to present) were not visited by the report writers in 2022 because they were presumed to still be extant. The majority of these were located within the Windsor-LaSalle subpopulation in publicly owned parks and natural areas.

Entire areas of suitable habitat were searched where feasible (typically at occurrences with less than 100 ha of suitable habitat; Table 2). Other occurrences were only partially surveyed. The Windsor-LaSalle subpopulation contains an estimated 10,000 ha of suitable habitat, but only about 100 ha were surveyed over 24 hours of targeted survey effort (Table 2). For this reason, the abundance metrics provided herein are the minimum abundance of Willowleaf Aster in Canada. The actual abundance may be substantially higher.

The abundance of Willowleaf Aster was recorded by counting the number of mature individuals. A mature individual was defined as a single flowering stem (see Population structure), while genets (that is, colonies) were defined as groups of stems separated by a minimum distance of 15 cm. Any flowering or budding stem was counted as a sexually mature individual. Habitat details, associate species, and perceived threats were also documented.

At flowering time, Willowleaf Aster is readily identifiable using the combination of characters described under Morphological description. Fieldwork was therefore completed between September 30 and October 11, 2022, which was assumed to be the peak flowering period of this species. Excluding travel time, a total of 45.5 person-hours were spent surveying Canadian subpopulations of Willowleaf Aster for this update status report.

Surveyors found Willowleaf Aster in flower at all extant subpopulations, and non-flowering individuals suspected of being Willowleaf Aster were carefully checked. Photographs of diagnostic features were taken for representative individuals of Willowleaf Aster at every extant subpopulation visited during fieldwork. Photographs of diagnostic features of similar species were also taken at every occurrence visited, even if Willowleaf Aster was not detected. The report writers are confident in the detection and non-detection results for every occurrence visited.

The majority of known Willowleaf Aster occurrences in Canada are on publicly accessible land, so access constraints were not a significant challenge for this status update. Notable exceptions were Walpole Island #7 and Squirrel Island #6, which are located at Bkejwanong (Walpole Island First Nation), where permission to undertake surveys was not granted. The subpopulation at Rondeau Provincial Park has not had a targeted survey since it was observed in 1956, despite being located in a relatively well surveyed park. Some occurrences within the Windsor-LaSalle subpopulation are on private land and could not be accessed.

Willowleaf Aster appears to occur with some regularity in suitable habitats in Essex County. For example, the report writers surveyed a randomly selected 2 km stretch of the Chrysler Greenway (a public trail between Windsor and Leamington) and successfully found a new subpopulation containing 140 individuals of Willowleaf Aster. The writers suspect that a comprehensive inventory of Willowleaf Aster in Essex County, including the entire Windsor-LaSalle subpopulation, could more than double the number of known individuals.

Abundance

The Canadian population is estimated to be a minimum of 210,530 mature individuals. Eighty-six percent of these are based on estimates of transplanted individuals (Table 1). This value reflects the number of mature individuals at all subpopulations that are known to be extant and was generated using a combination of field data collected in 2022 and observation data from secondary sources (for example, NHIC and MTO). The actual abundance of Willowleaf Aster in Canada is expected to exceed this number because the largest subpopulation (Windsor-LaSalle, which contains 98.6% of mature individuals) has not been comprehensively surveyed. Additionally, the status and abundance of Willowleaf Aster at two subpopulations on Walpole and Squirrel islands are unknown; however, they could contain a large number of individuals given the large area of suitable habitat.

Only 100 ha of approximately 10,000 ha of suitable habitat in the Windsor-LaSalle subpopulation was surveyed in 2022, where 25,080 individuals were documented. There have been at least 108 verifiable observations by others within this subpopulation over the past three years (that is, 2019 or later) that are located outside of the area surveyed by the report writers in 2022.

Field investigations by the report writers in 2022 documented a total of 381 sexually mature genets (that is, colonies) within the following size classes:

• < 10 individuals – 313
• 10 to 100 individuals – 48
• > 100 individuals – 20

A total of 130,849 Willowleaf Aster stems were transplanted from the Parkway construction footprint into protected restoration sites (Wood 2020). The number of stems counted in quadrats varied from year to year and ranged from 7,853 (in 2012) to 31,036 (in 2018). The transplanted population estimate therefore ranged from approximately 162,559 to 900,470 stems over the course of the monitoring program (2011 to 2020; Wood 2020). The last transplanted population estimation in 2020, based on the quadrat and transect count, was 492,383. Compared to 2019, this estimate represents a decrease of approximately 71,940 stems (that is, the population estimate in 2019 was 564,323), bringing the 2020 population closer in value to that reported in 2016. Of the stems estimated in 2020, only 46% were flowering; therefore it is estimated that 226,496 mature individuals were present in 2020. However, although a smaller number of plots were sampled in 2022, it appears that there may have been about a 20% decrease in the total number of stems. Therefore, it is estimated that there may have been about 181,196 flowering stems (mature individuals) in 2022. Despite the annual and site-specific population fluctuations in the restoration sites, the Willowleaf Aster transplanted population is still robust and healthy, and the current population remains higher than the baseline transplanted population (Wood 2020).

Fluctuations and trends

Recent fluctuations and trends in the Canadian population of Willowleaf Aster are difficult to quantify, chiefly because previous assessments did not provide detailed abundance information and used different definitions for what are here referred to as subpopulations and individuals. Using the best available information from primary and secondary sources, the report writers have quantified the number of individuals present in each subpopulation in 2019 (that is, three years ago), 2017 (that is, five years ago), 2012 (that is, 10 years ago), and 1999 (see Table 1).

Available data suggest there are currently 210,530 mature individuals in Canada (see Abundance), which is several orders of magnitude larger than the number of individuals inferred in 1999 (Zhang et al. 1999; COSEWIC 2003a). The apparent increase since 1999 is primarily a reflection of more comprehensive surveys within the species’ Canadian range in recent years and does not necessarily represent an actual increase in abundance. There is evidence of an increase in abundance in the Windsor-LaSalle subpopulation since the 1970s, and more recently at the MTO transplant sites (Wood 2020; Pratt pers. comm. 2022). If we consider the Windsor-LaSalle subpopulation to be representative, apparent increases in the observed number of individuals are largely due to more comprehensive information, but there have also been natural increases in abundance.

Continuing decline^{Footnote 3} in number of mature individuals:

Although habitat loss and ecological succession have resulted in the probable extirpation of six subpopulations, there is insufficient evidence to clearly demonstrate a decline in the number of individuals in Canada. The abundance of Willowleaf Aster in Canada appears to have increased relative to the values given in previous reports and to observation data. Although some of this increase is attributable to more extensive targeted survey effort, monitoring data from MTO indicate that the number of individuals at transplanted occurrences in Windsor-LaSalle has increased. Because 86% of the known Canadian population occurs at transplanted sites, the future of the Canadian population is dependent on these sites.

Evidence for continuing decline (1 generation or 3 years, whichever is longer, usually up to 100 years):

There is no evidence for a decline in the abundance of Willowleaf Aster in Canada over the past three years (that is, since 2019). Available data and consultation with local experts indicate either no change in abundance or a slight increase in abundance within the last three years (Wood 2020; Deacon pers. comm. 2022; Jones pers. comm. 2022; Pratt pers. comm. 2022).

Evidence for continuing decline (2 generations or 5 years, whichever is longer, usually up to 100 years:

There is no evidence for a decline in the abundance of Willowleaf Aster in Canada over the past six years (that is, since 2017). Available data and consultation with local experts indicate either no change in abundance or a slight increase in abundance within the last five years (Wood 2020; Deacon pers. comm. 2022; Jones pers. comm. 2022; Pratt pers. comm. 2022).

Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify):

There is no evidence of a decline in the abundance of Willowleaf Aster over the past 10 years (that is, since 2012). Due to the inclusion of the manipulated sites. Three subpopulations that were extant during the last assessment, and two subpopulations that had unknown status are now believed to be extirpated.

Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years):

Natural succession of open habitats of Willowleaf Aster to thicket and forest communities may cause a decline in the species’ abundance over the next 10 years. However, this is difficult to predict with any certainty because maintenance and creation of open habitats is also ongoing. Most extant subpopulations are small and at risk of extirpation due to single disturbance events (for example, excavation of subpopulations along roads and trails).

Extinction risk based on quantitative analysis:

Analysis not completed.

Long-term trends:

Willowleaf Aster has been known to occur in Canada since at least the early 20thcentury (Kutner 2019). Given the extent of land cover change experienced within its Canadian range over the past two centuries, namely the near-total conversion of tallgrass prairie communities to agricultural and urban land cover, it is reasonable to assume that this species was more abundant prior to the arrival of European settlers. However, there is not enough information to infer long-term trends in abundance. Open habitats that currently support Willowleaf Aster may transition to thicket or woodland habitats due to natural ecological succession, which could negatively affect some individuals or subpopulations. However, there is not enough evidence to determine whether this may result in a long-term decline in abundance. Ongoing management of prairie habitats to prevent natural succession and control invasive species may ensure that the long-term trend for Willowleaf Aster is stable. However, there is no guarantee that these programs will continue over the long term.

Population fluctuations, including extreme fluctuations:

The number of subpopulations known to be extant has decreased by two since 1999 (see Fluctuations and trends in Distribution), but there are insufficient data to conclude whether subpopulations are indeed extirpated. There is also insufficient information to conclude whether abundance has declined since 1999.

Severe fragmentation

Willowleaf Aster is not considered severely fragmented because the majority of the extant individuals occur within one subpopulation (Windsor-LaSalle). However, throughout the remainder of its range in Canada, Willowleaf Aster is severely fragmented. Based on fieldwork done in 2022, three subpopulations are suspected of having just one genet (that is, colony). These subpopulations with single clones are unsustainable in the long term because successful reproduction is highly reduced without outcrossing (see Life cycle and reproduction).

Rescue effect

Willowleaf Aster occurs in southeastern Michigan within 5 km of the Windsor-LaSalle subpopulation. Natural, unassisted movement of Willowleaf Aster from the United States to Canada would require wind dispersal of seeds over this distance, which is considered possible. However, the maximum dispersal distance of Willowleaf Aster seeds remains a significant knowledge gap.

Threats

Historical, long-term, and continuing habitat trends

Tallgrass prairies, one of the preferred habitats of Willowleaf Aster in Canada, have declined significantly in North America since the arrival of European settlers (Faber-Langendoen and Maycock 1994; Robertson et al. 1997; Rodger and Woodliffe 2001). Existing tallgrass prairie habitat in Ontario represents less than 3% of its pre-European settlement extent (Bakowsky and Riley 1994; Rodger 1998; Rodger and Woodliffe 2001). However, fieldwork at extant subpopulations in 2022 determined that Willowleaf Aster is not restricted to tallgrass prairies and can be found in a variety of other open country habitats as well as along roadsides and trails. Habitat loss due to natural ecological succession of open habitats to thickets was observed at three subpopulations in 2022. This may be a threat to all subpopulations in the absence of fire and other periodic disturbances which maintain open habitats (see Fire and Fire Suppression under Current and Projected Future Threats).

Current and projected future threats

Willowleaf Aster is vulnerable to the cumulative effects of various threats, especially ecosystem modifications (mowing and maintenance of roads, trails, powerline corridors, fire suppression, encroachment of woody vegetation due to natural ecological succession, etc.) and residential development. The nature, scope, and severity of these threats have been described in Appendix 1, following the IUCN-CMP (International Union for the Conservation of Nature – Conservation Measures Partnership) unified threats classification system (see Salafsky et al. 2008 for definitions and Master et al. 2012 for guidelines). The threat assessment process consists of assessing impacts for each of 11 main categories of threats and their subcategories, based on the scope (proportion of population exposed to the threat over the next 10-year period), severity (predicted population decline within the scope during the next 10 years or 3 generations, whichever is longer, up to ~100 years), and timing of each threat. The overall threat impact is calculated by taking into account the separate impacts of all threat categories and can be adjusted by the species experts participating in the threats evaluation.

The overall threat impact for Willowleaf Aster is considered to be medium to low (Appendix 1). This value is to be interpreted with caution, as it may be based on subjective information, such as expert opinion, although efforts have been made to corroborate the scores with available studies and quantitative data.

Other ecosystem modifications (IUCN threat 7.3, medium-low impact)

The most important threat to Willowleaf Aster in Canada is believed to be natural succession of grasslands and other open habitats to thickets, forests, and other shaded habitats. In the absence of natural disturbances or management activities that maintain openness, suitable habitat for Willowleaf Aster can be expected to decline.

Encroachment of woody vegetation into habitats of Willowleaf Aster was observed at five of the seven extant subpopulations, and the succeeding vegetation consisted mostly of native woody species, namely Staghorn Sumac (Rhus typhina) and dogwoods (Cornus spp.). Succession of open habitats to other vegetation communities consisting of native species is considered a form of ecosystem modification.

Seven subpopulations of Willowleaf Aster include occurrences along roads, managed trails, laneways, powerline rights-of-way, and other areas that are periodically mowed or disturbed. At three of these subpopulations, Willowleaf Aster was not found despite extensive searches in 2022. It may have been extirpated as a result of road maintenance activities (mowing, construction, snow plowing, etc.). The Port Stanley subpopulation, while still extant as of 2022, is located entirely in a roadside ditch and may be particularly vulnerable to road maintenance activities.

Some landowners in Windsor and LaSalle may mow all suitable habitats on their properties, and encroachment onto public land is frequent. However, depending on timing, mowing and other activities that maintain habitat openness may be directly beneficial to Willowleaf Aster because it requires open habitats to persist.

Fire and fire suppression (IUCN threat 7.1, low impact)

Fire was historically a natural disturbance that aided in maintaining the openness of tallgrass prairie and meadow habitats in Ontario. Fire suppression is a factor in the natural succession of open habitats to habitats dominated by woody vegetation, which has been identified as a threat to other species at risk in Canada (COSEWIC 2003b, 2010 a,b, 2015). See also Problematic Native Species (IUCN Threat 8.2).

Housing and urban areas (IUCN threat 1.1, low impact)

In 1999, the Windsor-LaSalle area had been heavily disturbed by industrial and housing development (COSEWIC 2003a). Since that time, lands with habitats that support Willowleaf Aster have been acquired for the purposes of conservation by the City of Windsor, the City of LaSalle, Ontario Parks, and the Nature Conservancy of Canada. At present, the majority of extant occurrences of Willowleaf Aster are on lands managed for conservation and recreation. However, the threat from urbanization still exists for Willowleaf Aster occurrences on private land. Ongoing residential development was observed adjacent to an occurrence of Willowleaf Aster in LaSalle during fieldwork in 2022. While development controls may protect Willowleaf Aster and its habitats to some degree (see Legal protection and status), sites on private land may still be threatened by ongoing, proposed, or future housing developments.

Tourism and recreation areas (IUCN threat 1.3, low impact)

Trail improvements, including paving, are occurring in some areas where plants are adjacent to existing trails.

Problematic native species (IUCN threat 8.2, low impact)

Succession of open habitats to shrub and tree-covered habitats was observed to varying degrees at all seven extant subpopulations of Willowleaf Aster. Four subpopulations are located entirely within or adjacent to thickets of Staghorn Sumac, Gray Dogwood, and other native shrubs. At Point Edward, colonies growing in Staghorn Sumac thickets were observed to be depauperate (that is, lower stem density and fewer heads per stem) relative to those growing in open habitats.

Number of threat locations

The number of locations is estimated to be in the order of several hundred, which exceeds the threshold of 10 locations required to meet criteria under B and D. The number of threat locations was estimated using a combination of threats observed at each subpopulation, number of occurrences, and number of ownership types (see Table 1). Other ecosystem modifications, especially natural succession of open habitats to shaded habitats, are a threat at all seven extant subpopulations. Fire suppression may be a factor in succession of open habitats to shrubby habitats in at least three subpopulations (Point Edward, Big Creek, and Windsor-LaSalle). Residential development is only believed to be a threat at the Windsor-LaSalle subpopulation. Note that the number of threat locations does not include threats to subpopulations whose status is unknown.

Protection, status, and recovery activities

Legal protection and status

Willowleaf Aster is currently listed as Threatened in Canada on Schedule 1 of the federal Species at Risk Act (2002). It is also listed as Threatened under Ontario’s Endangered Species Act (2007). Some suitable habitats for Willowleaf Aster receive limited protection through the natural heritage policies of Ontario’s Provincial Policy Statement (PPS) (2020), which affords conditional protection to tallgrass prairie communities deemed to be provincially significant. Other suitable habitats (meadows, roadsides, rail trails, etc.) receive no protection other than the legislative protection afforded to Willowleaf Aster itself, if it is present.

In the United States, Willowleaf Aster receives no protection under federal or state level legislation. The species has no legal status in Mexico under the General Wildlife Law (Ley General de Vida Silvestre) (2000).

Non-legal status and ranks

Willowleaf Aster has a global conservation rank of Secure (G5, status last reviewed in 2024), a national conservation rank of Imperilled (N2 last reviewed in 2023) in Canada, and a national conservation rank of Secure (N5) in the United States. In Ontario, Willowleaf Aster has a subnational conservation rank of Imperilled (S2). In New Brunswick, Willowleaf Aster is considered ephemeral and has a subnational rank of Status Not Applicable (SNA).

In the United States, Willowleaf Aster has the following subnational ranks: Extirpated (SX) in the District of Columbia; Critically Imperilled (S1) in Georgia, Maryland, Massachusetts, Tennessee, and Virginia; Vulnerable (S3) in Michigan and West Virginia; Apparently Secure (S4) in Iowa and Pennsylvania; Unrankable (SU) in Connecticut; and Not Ranked (SNR) in the 24 remaining states where the species occurs (NatureServe 2024).

Willowleaf Aster is listed as Least Concern on the IUCN Red List of Threatened Species (Smith 2016).

Land tenure and ownership

The remaining subpopulations (excluding those presumed extirpated) include those located on a First Nations Reserve (2), conservation authority land (2), municipal land (3), and one case with municipal, provincial, and private landholders. The Windsor-LaSalle subpopulation occurs on lands with a mix of provincial, municipal, and private ownership. One of the two occurrences on private land (6th Concession Colchester) is owned by the Nature Conservancy of Canada and managed for the purposes of conservation.

Recovery activities

The following recovery actions have occurred in Ontario since 2003:

• a total of nine projects that included Willowleaf Aster as a target species have received funding from the Species at Risk Stewardship Program (Snyder, pers. comm. 2022)
• controlled burns are periodically undertaken on the lands managed for conservation within Windsor and LaSalle (City of Windsor 2021). The frequency of controlled burns at these sites is unknown
• the Ministry of Transportation Ontario has managed European Common Reed at sites in Windsor according to best management practices for control of European Common Reed (Jones 2013; COSEWIC 2015)
• Walpole Island Land Trust has secured over 300 acres of prairie and wetland habitat on Walpole Island for conservation. Controlled burns are used to manage prairie and savanna habitat on Walpole Island (Jones 2013)

Information sources

References cited

Collections examined

All Willowleaf Aster specimens at the Canadian Museum of Nature (CAN) and the Western University Herbarium (UWO) were examined. Information on specimens at the NHIC herbarium was obtained but the specimens were not examined. Using the Global Biodiversity Information Facility (GBIF), additional collections from Canada were found at the Bruce Bennett Herbarium (BABY), Université de Montréal (MT), University of North Carolina at Chapel Hill (NCU), and Utah State University (USU). However, these specimens were not examined.

Authorities contacted

• Barcza, D. President. Sage Earth Consulting. Loretto, Ontario
• Brinker, S. Botanist. NHIC, Ontario Ministry of Natural Resources and Forestry. Peterborough, Ontario
• Buchanan, L. Rural Lambton Stewardship Network. Wallaceburg, Ontario
• Burke, P.S. Senior Ecologist. GEI Consultants Inc. London, Ontario
• Charlton, J. Senior Environmental Planner. Ontario Ministry of Transportation. London, Ontario
• Cornelis, L. Independent naturalist. Chatham, Ontario
• Deacon, P. Ecologist. Natural Resources Solutions Inc. Kitchener, Ontario
• Huys, W. Arborist. MTE Consultants. West Elgin, Ontario
• Jones, R. Local naturalist. Amherstburg, Ontario
• Kamstra, J. Senior Terrestrial Ecologist. AECOM. Port Perry, Ontario
• Naday, B.M. Protected Areas Technician. Canadian Wildlife Service. Brighton, Ontario
• McDougall, B. St. Clair Region Conservation Authority. Strathroy, Ontario
• Miller, B. Botanist. Stantec Consulting Limited. Peterborough, Ontario
• Oldham. M. Retired (formerly Botanist at NHIC, Ontario Ministry of Natural Resources and Forestry). Peterborough, Ontario
• Pratt, P.D. Retired (formerly at City of Windsor Department of Parks and Recreation). Windsor, Ontario
• Preney, T. Ojibway Prairie. Windsor, Ontario
• Semple. J. Professor Emeritus. University of Waterloo. Waterloo, Ontario
• Snyder, E. Plant Species at Risk Specialist. Ontario Ministry of the Environment, Conservation and Parks, Peterborough, Ontario
• van Amelsvoort, I. Senior Ecologist. North-South Environmental Inc. Cambridge, Ontario
• Waldron, G. Retired (former independent consultant). Amherstburg, Ontario
• Woodliffe, A. Retired (former Aylmer District Ecologist at Ontario Ministry of Natural Resources). Chatham, Ontario

Acknowledgements

Funding for the preparation of this report was provided by Environment and Climate Change Canada. The authorities listed above provided valuable data and/or advice. A large number of community scientists deserve acknowledgement for submitting observations of Willowleaf Aster to iNaturalist, which included observations used to prepare this status report. The writers would also like to acknowledge Jay J. Zhang, David E. Stephenson, and Dr. John C. Semple for their efforts in preparing the original status report in 1999. We would also like to thank COSEWIC reviewers who provided help and comments, including Bruce Bennett as Co-chair overseeing the report; Del Meidinger, Danna Leaman, Sean Blaney, and Cary Hamel of the Vascular Plant SSC; Holly Bickerton, Deng Aruei, Kevin Yang, and Gina Schalk of Environment and Climate Change Canada.

Biographical summary of report writers

William Van Hemessen is an independent consulting ecologist in Cambridge, Ontario. William has over 12 years of professional experience in ecology and has authored peer- reviewed articles pertaining to botany and natural area management. He has a bachelor’s degree in environmental studies from the University of Waterloo and is currently a master’s candidate in ecology and evolution at Western University. He sits on the Board of Directors of the Field Botanists of Ontario and previously sat on the Board of Directors of the Thames Talbot Land Trust in London, Ontario. He wrote the COSEWIC status report for Pumpkin Ash (Fraxinus profunda) and the recovery strategies for Black Ash (Fraxinus nigra) and Gillman’s Goldenrod (Solidago gillmanii) in Ontario.

Pauline Catling has an undergraduate degree in Wildlife Biology from the University of Guelph and a master’s degree in biology from the University of Manitoba where she studied the ecological classification of alvar vegetation in the Interlake region of Manitoba. From 2009 to 2013 she worked on various projects concerning the conservation and monitoring of species at risk in Ontario. She currently works as a Senior Ecologist for North-South Environmental Inc. She has over 15 years of experience working on various ecological research, monitoring and conservation projects in Ontario and Manitoba. She wrote COSEWIC status reports for Western Prairie Fringed Orchid (Platanthera praeclara) and Pumpkin Ash as well as the recovery strategies for Gillman’s Goldenrod, Black Ash, Hudsonian Godwit (Limosa haemastica), and Lesser Yellowlegs (Tringa flavipes) in Ontario. She has developed standardized survey protocols for Small White Lady’s-slipper (Cypripedium candidum), False Hop Sedge (Carex lupuliformis), Purple Twayblade (Liparis liliifolia), and Branched Bartonia (Bartonia paniculata).

Appendix 1. Threats assessment on the Willowleaf Aster (Symphyotrichum praealtum)

Threats assessment worksheet

Species scientific name

Symphyotrichum praealtum

Element ID

1055345

Elcode

PDASTE8200

Date:

4/21/2023

Assessor(s):

William D. Van Hemessen, Pauline K. Catling (report writers), Bruce Bennett, Del Meidinger (co-chairs VPSSC), Cary Hamel, Danna Leaman (VPSSC), Holly Bickerton (ECCC)

Assigned overall threat impact:

CD = Medium - Low

Overall threat comments

Mature flowering stem generation time 3 years; genet generation time is decades.