White Wood Aster (Eurybia divaricate): COSEWIC assessment and status report 2023

Official title: COSEWIC assessment and status report on the White Wood Aster (Eurybia divaricata) in Canada

Special Concern

2023

COSEWIC assessment summary

Assessment summary – December 2023

Common name

White Wood Aster

Scientific name

Eurybia divaricata

Status

Special Concern

Reason for designation

This herbaceous perennial plant is found in the temperate forests of eastern North America, reaching the northern limit of its range in Canada. Thirty geographically-restricted and isolated subpopulations occur in patches of deciduous forest in extreme southern Ontario and southwestern Quebec. Recent surveys have found additional subpopulations, with over 100,000 known mature plants in Canada. The change in status reflects this higher estimate of abundance, and changes to the way that criteria are applied. However, the species is still threatened by habitat loss and degradation due to housing, recreational activities, agriculture, and forestry, as well as invasive species and deer browsing.

Occurrence

Ontario, Quebec

Status history

Designated Threatened in April 1995. Status re-examined and confirmed in November 2002. Status re-examined and designated Special Concern in December 2023.

COSEWIC executive summary

White Wood Aster

Eurybia divaricata

Wildlife species description and significance

White Wood Aster (Eurybia divaricata) is a fall-flowering herbaceous perennial. Its upper leaves are lanceolate and deeply toothed, while its lower leaves are narrowly cordate and toothed. The inflorescence is a head of yellow or purple disk flowers surrounded by white ray flowers. The flower heads are united in a corymb.

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. There is no species-specific ATK in the report.

Distribution

White Wood Aster is present in Canada and the United States. The species is common across most of its main range in the Appalachians, from New England south to Georgia and Alabama. In Canada, the species is at the northern limit of its range. It is found in isolated subpopulations in southern Ontario (Niagara, Norfolk, and City of Quinte West regions), as well as in a few wooded areas in southwestern Quebec (Montérégie and Estrie).

Habitat

The species grows in dry to moist deciduous forests with well-drained soils. It prefers habitats with openings in the tree canopy.

Biology

White Wood Aster spreads mainly asexually (rhizomes), but sexual reproduction is possible when conditions are favourable. Flowering occurs from the beginning of August to September, and fruiting, between the middle and the end of September.

Population sizes and trends

The documented abundance of the Canadian population has increased since the last COSEWIC status report. This is mainly due to an increase in search effort and reporting of sites, rather than real population growth. The most recent data indicate 68 subpopulations in Canada. In Quebec, there are 15 subpopulations: 14 extant and one extirpated. In Ontario, there are 53 subpopulations: 45 extant, two historical, and six extirpated. The previous report documented 25 subpopulations.

The most recent count of White Wood Aster in Canada estimates more than 101,000 flowering stems, including approximately 4,300 in Quebec and 97,150 in Ontario. The previous report documented just over 10,000 stems.

Threats

The species is mainly threatened by the loss of habitats linked to development, agriculture, forestry, invasive species, browsing by White-tailed Deer, and recreational activities. The subpopulations are mostly isolated, which can limit genetic exchanges.

Protection, status, and recovery activities

The species was previously designated as Threatened by COSEWIC in 1995 and 2002. The species has been listed as Threatened on Schedule 1 of the Species at Risk Act (SARA) since 2005. In Ontario, the species has been listed as a Threatened species under the provincial Endangered Species Act since 2007. In Quebec, it has been listed as a "Threatened" species under Quebec's Act Respecting Threatened or Vulnerable Species since 2005, which provides general habitat protection.

Globally, the species is considered "Secure" (G5), while it is "Vulnerable" (N3) in Canada, "Imperilled" (S2) in Quebec and "Vulnerable" (S3) in Ontario.

Technical summary

Scientific name: Eurybia divaricata

English name: White Wood Aster

French name: Aster à rameaux étalés

Range of occurrence in Canada: Ontario, Quebec

Demographic information:

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

10+ years

Long-lived perennial species with individual ramets capable of flowering over several years. Seeds can probably germinate and reach maturity within 3 years.

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

yes

Estimated based on repeated inventories of subpopulations in Quebec (2021-2022) and in Ontario (2018-2022). Where trends are known, subpopulations show declines and extirpations. The non-genuine population increase reflects greater survey effort.

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

unknown

Suspected decline based on the 2 generation decline below, with insufficient data to estimate trend.

Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

unknown

Among subpopulations where trends are known, there was an observed decline of 5,022 stems (-31%).

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

unknown

Suspected decline based on the 2 generation decline above, with insufficient data to estimate trend.

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

unknown

Suspected decline based on the 2 generation decline above, with insufficient data to estimate trend.

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

unknown

Suspected decline based on the 2 generation decline above, with insufficient data to estimate trend.

Are the causes of the decline clearly reversible?

No

The main decline of habitat loss to residential development is not easily reversible.

Are the causes of the decline clearly understood?

Yes

Documented

Are the causes of the decline clearly ceased?

No

Based on observations

Are there extreme fluctuations in number of mature individuals

No

Based on biology

Extent and occupancy information:

Estimated extent of occurrence (EOO)

42,920 km²

Encompassing data from 2001-2022.

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

304 km²

Quebec = 88 km²,

Ontario = 216 km² the actual IAO is likely to be larger.

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. yes
   1. When present, the subpopulations are not limited by space
   2. Subpopulations are separated by large areas

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

>10 estimated 70-80

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

no

observed

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

no

observed

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

no

observed

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

no

observed

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

yes

Because habitat loss is the most significant threat, continuing decline in quality and likely area can be both inferred and projected

Are there extreme fluctuations in number of subpopulations?

no

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):

54 extant, three historical, and seven extirpated subpopulations (see Table 1)

See Table 1

Total: 54 extant subpopulations

More than 101,458 stems

According to latest available data (between 2000 and 2022)

Quantitative analysis:

Is the probability of extinction in the wild at least 20% within 20 years [or 5 generations], or 10% within 100 years]

unknown

analysis not conducted

Threats:

Was a threats calculator completed for this species?

Yes (see Appendix 1)

Overall, Threat impact Medium (December 2022)

Key threats were identified as:

1. Housing and urban areas (IUCN 1.1) – low impact
2. Annual and perennial non-limber crops (IUCN 2.1) – low impact
3. Logging and wood harvesting (IUCN 5.3) – low impact
4. Recreational activities (IUCN 6.1) – low impact
5. Other ecosystem modifications (IUCN 7.3) – low impact
6. Problematic native species/diseases (IUCN 8.2) – low impact
7. Invasive non-native/alien species/diseases (IUCN 8.1) – unknown impact

What limiting factors are relevant?

• Isolated subpopulations limit genetic exchanges, potentially reducing the genetic diversity of the Canadian population

Rescue effect (from outside Canada):

Status of outside population(s) most likely to provide immigrants to Canada.

MA (S3); NY (S5); NH and VT (SNR)

Maine Vulnerable (S3); New York Secure (S5); New Hampshire and Vermont Not Ranked (SNR)

Is immigration known or possible?

Unknown but possible

Geographically very close

Would immigrants be adapted to survive in Canada?

yes

Similar climate

Is there sufficient habitat for immigrants in Canada?

yes

There is apparently suitable habitat that is not currently occupied.

Are conditions deteriorating in Canada?

Yes (Ontario)

No (Quebec)

Threats are lower in Quebec

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

unknown

Is the Canadian population considered to be a sink?

no

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

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

Status history:

COSEWIC status history

Designated Threatened in April 1995. Status re-examined and confirmed in November 2002. Status re-examined and designated Special Concern in December 2023.

Status and reasons for designation:

Status

Special Concern (b)

Alpha-numeric codes

Not applicable.

Reason for change in status

4(i,vi); 5(ii)

Reasons for designation

This herbaceous perennial plant is found in the temperate forests of eastern North America, reaching the northern limit of its range in Canada. Thirty geographically-restricted and isolated subpopulations occur in patches of deciduous forest in extreme southern Ontario and southwestern Quebec. Recent surveys have found additional subpopulations, with over 100,000 known mature plants in Canada. The change in status reflects this higher estimate of abundance, and changes to the way that criteria are applied. However, the species is still threatened by habitat loss and degradation due to housing, recreational activities, agriculture, and forestry, as well as invasive species and deer browsing.

Applicability of criteria:

A: Decline in total number of mature individuals:

Not applicable

Insufficient data to reliably infer, project, or suspect population trends.

B: Small range and decline or fluctuation

Not applicable

Although the index of area of occupancy of 304 km² is below the threshold for Endangered and there is a continuing decline in extent and quality of habitat, and number of mature individuals, the population is not severely fragmented, occurs at more than 10 locations, and there is no evidence of extreme fluctuations.

C: Small and declining number of mature individuals

Not applicable

The number of mature individuals (101,458 stems), exceeds the threshold for Threatened.

D: Very small or restricted population

Not applicable

Estimate of over 101,458 mature individuals exceeds thresholds for D1, and the population does not have a very restricted IAO nor does it have so few locations that it is vulnerable to rapid and substantial decline.

E: Quantitative analysis

Not applicable

Analysis not conducted.

White Wood 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 2002 and trends in others show declines.

Preface

An increase in extent of occurrence has been observed since the last assessment (2002). This increase is due to the discovery of a subpopulation in the municipality of Racine, Quebec, east of the previously identified area of occurrence, as well as the discovery of a subpopulation on a Long Point Basin Land Trust property, west of the previously identified extent of occurrence.

This report documents 64 subpopulations compared to 25 in the previous status report. Among these, 54 are extant, three are historical, and seven are extirpated. The most recent count of White Wood Aster in Canada estimates the abundance at over 101,458 flowering stems. This increase is due to greater search effort and reporting of sites, rather than to a real growth in the Canadian population. A federal recovery strategy was published in 2018 (ECCC 2018). An Ontario strategy was published in 2019 (Ministry of the Environment Conservation and Parks. 2019), followed by an Ontario government response statement in 2020 (Ministry of the Environment Conservation and Parks. 2020).

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 (2023)

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 is 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 classification:

Class: Magnoliopsida

Order: Asterales

Family: Asteraceae

Genus: Eurybia

Species: Eurybia divaricata

Common names: VASCAN

English: White Wood Aster

French: aster à rameaux étalés; aster divariqué

Synonyms and notes:

White Wood Aster is a member of the Asteraceae family. In 1994, the taxonomy of the genus Aster was revised (Nesom 1994) and the species has since been classified in the genus Eurybia (COSEWIC 2002). Synonyms include Aster boykinii; A. castaneus; A. corymbosus; A. corymbosus var. alatus; A. divaricatus; A. excavatus; A. flexilis; A. stilettiformis; A. tenebrosus; Biotia corymbosa; B. corymbosa var. alata; B. macrophylla var. divaricata; Eurybia corymbosa (Brouillet 2006); Aster argillarius; A. fragrans (Brouillet et al. 2010+).

Description of wildlife species

White Wood Aster is a colony-forming perennial herb. It spreads mainly through rhizomes (horizontal underground stems), but sexual reproduction is possible. The species reaches a height of 30 to 90 cm (Brouillet 2006). The upper leaves are lanceolate and toothed, while the lower leaves are heart-shaped, toothed (Sharp et al. 1995), and deciduous at the time of flowering. The inflorescences are flower heads united in a corymb, from which the species takes its French common name (spreading branches). Each flower head has five to ten white petal-like rays, surrounding a disc of yellow flowers that turn purple with age (Cover photo; Figure 1). Flowering begins in early August and fruiting occurs from mid-September to late September (COSEWIC 2002). The fruits are achenes surmounted by a ring of bristles.

Big-leaved Aster (Eurybia macrophylla) and Heart-leaved Aster (Symphyotrichum cordifolium) are related species and can be distinguished, as the peduncles of the White Wood Aster are hairy but not glandular, its bracts are not glandular, or only sparsely so, and its flower heads are white with a yellow centre (MELCC 2017).

Figure 1. Illustration of the White Wood Aster. Drawing by Réjean Roy, Centre de données sur le patrimoine naturel du Québec.

Designatable units

There are no recognized subspecies/varieties or discrete/evolutionarily significant differences among subpopulations in Canada to warrant recognition as designatable units (DU). There is no known evidence of heritable traits or markers that clearly distinguish DU structure or natural geographic disjunctions. The occurrence of White Wood Aster in Canada is therefore considered one DU.

Special significance

White Wood Aster reaches the northern limit of its global range in southern Ontario and Quebec, where it often co-occurs with numerous other nationally rare plant species and is representative of a community type that is highly fragmented and increasingly degraded. 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, White Wood Aster is important to Indigenous Peoples who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

White Wood Aster is endemic to eastern North America and is present in Canada and the United States. The species is common in most of its main range, located in the Appalachians, from New England to Georgia and Alabama. In Canada, the species is at the northern limit of its range (Figure 2).

Figure 2. Worldwide distribution of the White Wood Aster (Eurybia divaricata). Compiled by Émilie Beaulieu (BEA) from data obtained from the Biota of North America Program (BONAP), the Centre de données sur le patrimoine naturel du Québec (CDPNQ), and the Ontario National Heritage Information Center (NHIC).

Canadian range

The Canadian range of White Wood Aster includes the Niagara, Norfolk, and City of Quinte West regions of southern Ontario, as well as some wooded areas in southwestern Quebec (Montérégie and Estrie). Historically, White Wood Aster was also found in the intervening area around Kingston, but this subpopulation and potentially other subpopulations have been extirpated, and the species has not been reported in the area since 1901 (Table 1; Figure 3). The population is within the COSEWIC Great Lakes Plains National Ecological Area. It is estimated that less than 5% of the species' global range is in Canada (Environment and Climate Change Canada 2018).

A=Excellent viability; B=Good viability; C=Fair viability; D=Poor viability; E=Verified extant; H=Historical; X=Extirpated (NatureServe 2008).

Figure 3. Canadian distribution of White Wood Aster. [Alain Filion, ECCC]

In the 2002 assessment, there were 15 confirmed subpopulations in Ontario and 10 in Quebec. Surveys since then have uncovered new subpopulations within the known range, but also some outside the range identified in the previous assessment in 2002. The species’ distribution therefore extends a little further east (about 35 km) and a little further west (about 40 km). These findings are due to an increase in search effort rather than an increase in population (Environment and Climate Change Canada 2018).

The most recent data indicate that there are 68 subpopulations in Canada. In Quebec, there are 15 subpopulations: 14 extant and one extirpated. In Ontario, there are 53 subpopulations: 45 extant, two historical, and six extirpated (Table 1).

Population structure

In this report, the term “location” defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of White Wood Aster present; “site” refers to different patches of White Wood Aster that are closer than 1 km to one another and therefore have been grouped under a single subpopulation. “Subpopulation” refers to one or more patches separated by a minimum of” 1 km (unless linked by a waterway or other special cases as defined by NatureServe 2020); “population” refers to the total number of mature individuals found in Canada. This definition of subpopulation is consistent with the COSEWIC definition as gene flow through insect-vectored pollen transfer between patches separated by more than 1 km is unlikely. The term “subpopulation” is consistent and used interchangeably with the NatureServe term “element occurrences” (EOs) (NatureServe 2020), as used by regional conservation data centres for this species in Canada.

The extant Ontario subpopulations are located several hundred kilometres (about 500) from the Quebec subpopulations, suggesting the absence of genetic exchange between the two provinces (Figure 3). No morphological or genetic differences between individuals from Quebec and those from Ontario are known. Most subpopulations (about 45 out of 52), both in Ontario and Quebec, are isolated by human developments (residential, road, agricultural) where the typical habitat of the species is absent in intervening areas, which considerably limits the possibilities of genetic exchange between subpopulations.

Extent of occurrence and area of occupancy

The estimated extent of occurrence (EOO) in Canada is 42,920 km², calculated using the least convex polygon method encompassing data from 2001 to 2022. This calculation includes land and water area in the United States but does not include records from the United States range (Figure 3). It is plausible that the actual EOO could be somewhat larger.

The index of area of occupancy (IAO) in Canada is 304 km² (QC = 88 km², ON = 216 km²), calculated using a 2 km × 2 km grid of squares drawn from known data from the period 2001-2021. The actual IAO is likely to be larger as additional sites are likely to be found in Canada.

Fluctuations and trends in distribution

The EOO and IAO were not calculated during the previous assessment. The increased number of extant subpopulations reflects the acquisition of new knowledge rather than a change in distribution.

Biology and habitat use

The information in the following section comes in part from the previous status report (COSEWIC 2002).

Life cycle and reproduction

The species reproduces sexually and asexually (rhizomes). New stems emerge at the end of the rhizomes and result in sometimes dense colonies. These are therefore often composed of cloned (genetically identical) stems of the same individual. Reproduction by rhizomes is a characteristic that is often mentioned for the species, although sexual reproduction is also possible. The self-incompatibility mechanisms existing in Asteraceae could reduce the production of fertile seeds within genetically poorly diversified subpopulations (Boisjoli 2010). White Wood Aster is a long-lived perennial species, with individual ramets being capable of repeated flowering. Seeds can probably germinate and reach maturity within three years. Although the actual generation time is unknown, an estimation of 10+ years is being used for this assessment.

Several groups of insects can pollinate White Wood Aster flowers (MacPhail 2013). The seeds are very small (2.5 mm by 0.7 mm) and have a pappus for wind dispersal (Boisjoli 2010). Blooming begins in early August, flowering continues until the end of August, and full bloom extends throughout the month of September. Fruiting begins in mid-September (Sharp et al. 1995).

It has been suggested that the flower turns from yellow to purple when pollinated (Britton and Brown 1970), but the colour change is sometimes attributed only to aging of the flower (MELCC 2017).

Habitat requirements

The Canadian range of White Wood Aster is located within the cold temperate and temperate ecoclimatic regions of Canada (Strong et al. 1989). The species grows in dry deciduous forests, although it has a fairly good tolerance to rainy conditions. It grows mainly on mesic substrate and has no affinity for a particular range of pH (Tardif et al. 2016).

In Ontario, the species is found in drainages ranging from very poorly drained to moderately well drained (Environment and Climate Change Canada 2018) and on slopes between 10% and 57% (Imrie et al. 2005). In Ontario, it is found in regions that offer some of the warmest temperatures and longest growing seasons in the province. Summers are humid and hot and winters are mild.

In Quebec, the species is found in maple-linden stands, as well as in maple-bitternut hickory stands. It also occurs in stands of Eastern Hemlock (Tsuga canadensis) or American Beech (Fagus grandifolia), particularly in forests that have been logged (COSEWIC 2002). Quebec subpopulations occur on well-drained soils or on rocky slopes (Boisjoli 2010). Southern Quebec is characterized by hot summers and relatively mild winters.

Thinning, whether of natural or anthropogenic origin, benefits the species by promoting sexual and asexual reproduction (Boisjoli 2010). The amount of light and the openness of the canopy are the environmental factors that have the greatest effect on the dynamics of Quebec subpopulations (Boisjoli 2010). White Wood Aster is frequently found along trails. According to other authors, the species prefers shady places, but it tolerates sunshine (Tardif et al. 2016), and the slope of the ground is not be a constraint (Boisjoli 2010).

Deep litter seems to have a strong positive effect on the species’ abundance, probably because it protects seeds and seedlings against frost and serves as a source of nutrients (Boisjoli 2010). Stem density is generally greater in old-growth forests, as the species is slow to repopulate regenerating forests (Hough 2008). White Wood Aster was significantly more frequent in old-woods than in post-agricultural forests, and stem density of White Wood Aster declined with increasing distance from old-woods (Singleton et al. 2001). It seems that the species does not always colonize neighbouring woods, which nevertheless meet the ecological requirements of the species (COSEWIC 2002).

The rarity of White Wood Aster in southern Quebec could be explained by a beneficial disturbance regime (gaps) that has become less frequent, given the young age of the forests, or because of habitat fragmentation (Boisjoli 2010).

Movements, migration, and dispersal

The clonal dispersal distance of White Wood Aster is very low, around 0.2 to 0.3 m per year, which represents annual clonal expansion up to a horizontal distance equivalent to the plant’s height (Singleton et al. 2001). The seeds are equipped with a pappus, which aids in dispersal, but prevents the seeds from sinking into the ground (Brouillet 2006), which can make them vulnerable to frost (Boisjoli 2010), depending on the thickness of the litter (see Habitat). Singleton (et al. 2001) considered White Wood Aster to be a species with low colonization distances compared to the other forest species in the study such as Southern Ground-cedar (Diphasiastrum digitatum) and Northern Starflower (Lysimachia borealis).

In Ontario, the sites where the species grows are located within patches of forest surrounded by residential and agricultural development, which severely limits the potential for the species to spread. With Ontario subpopulations often within 5 km of each other, pollination between genetically compatible individuals and the production of viable seed is not impossible.

In Quebec, the forests where the species has been found often extend well beyond the area occupied by the subpopulation, which may suggest a greater potential for future spread into unoccupied habitat. An exception would be the Notre-Dame-de-l'Île-Perrot (19830) subpopulation, which is on an island, wedged between farms and nearly 50 km from the nearest subpopulation. The two Mont Rougemont subpopulations (21901 and 3865) and the subpopulation at Saint-Blaise-sur-Richelieu (3872), although closer to other Quebec subpopulations to the east, are also found in areas isolated by agricultural development.

The minimum area of suitable habitat required to maintain viable subpopulations of White Wood Aster is unknown (Environment and Climate Change Canada 2018).

Interspecific interactions

Several groups of insects can pollinate White Wood Aster flowers, such as hoverflies (especially Syrphus spp. and Toxomerus geminatus), ants (Superfamily Formicoidea), Common Eastern Bumble Bee (Bombus impatiens), and sweat bees (Lasioglossum sp., and Augochlora pura or Augochlorella aurata) (MacPhail 2013).

Physiological, behavioural, and other adaptations

The preferred habitat of White Wood Aster is dry, sparse deciduous forest with well-drained soils. However, it is also found in poorly drained soils, particularly in Ontario.

White Wood Aster is available at garden centres in the United States as a fast-spreading, hardy plant suitable for shady areas. The production of rhizomes and new stems is a favourable trait for transplanting.

Limiting factors

Isolated subpopulations limit genetic exchanges, potentially reducing the genetic diversity of the Canadian population.

Population sizes and trends

Subpopulation sizes are reported using flowering stem counts. When the number of stems is large, an estimate is made by calculating the number of stems on a given surface unit (for example, 1 m²), and then extrapolating this number to the measured surface of the colony.

Data sources, methodologies, and uncertainties

The results of several activities made it possible to gather the data needed to update the population information for this species (Table 1). For Quebec subpopulations, data on known subpopulations were first obtained from the Centre de données sur le patrimoine naturel du Québec. Surveys focusing on White Wood Aster were conducted in 2021 by the report writers for six of the known subpopulations in Quebec. Among these, the visit to one of the Sutton subpopulations (20860) resulted in no detected individuals, but 15 flowering stems were later confirmed by the Nature Conservancy of Canada (NCC) 700 m northwest of the previously reported locality. The subpopulation at Frelighsburg (3,873), last sighted in 1997, was confirmed to be extant. Based on track files, produced during surveys, it is believed that areas within the Eccles Hill subpopulation (22348) were not thoroughly searched. Therefore, the tally of 445 is a minimum number.

Abundance

The most recent count of White Wood Aster in Canada estimates that there are more than 101,458 flowering stems, including about 4,308 in Quebec and 97,150 in Ontario (Table 1). The real number is larger given that many counts represent a minimum number of stems (for example, Fonthill-Sandhill Valley ANSI (2018): >2,000 stems). The number of genetic individuals is difficult to estimate without invasive procedures, given the species’ rhizomatous habit. The number of flowering stems appears to be an appropriate parameter for monitoring the abundance of subpopulations (COSEWIC 2002). Subpopulation size varies from a few stems in some subpopulations to several thousand in others (Table 1).

Fluctuations and trends

The known abundance of the Canadian population has increased since the last COSEWIC status report (COSEWIC 2002). This increase is mainly due to a greater search effort and reporting of sites, rather than increased population size (Environment and Climate Change Canada 2018).

This report documents 31 new subpopulations that did not appear in the CDPNQ (Quebec) and NHIC (Ontario) databases at the time of writing (Table 2).

*Darkened subpopulations are extirpated

Trends range from increasing to declining depending on the subpopulation (Table 1). However, few historical data are available to confidently assess subpopulation trends. The few subpopulations for which more regular monitoring has been done show trends ranging from stable to possible decline. Among the subpopulations where available data was sufficient to suggest a trend provides an observed decline of 5,022 (previously reported as 4,430 in earlier versions of this report), suggesting a decline in number of mature individuals of 31.34%, with a loss of 5,022 individuals out of 16,026 (Table 1).

Severe fragmentation

The previous status report considered the total population severely fragmented, and indicated that most individuals are found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, that is, < 1 successful migrant/year (COSEWIC 2002). As mentioned under Population structure, the Canadian population of White Wood Aster occurs in two distinct regions in Canada. The species as a whole in Canada is not considered severely fragmented as defined by COSEWIC because most individuals are in large occurrences that, although isolated, are presumed to have good viability. The Ontario subpopulations are generally more highly exposed to disturbances because they occur in more densely urbanized areas than those in Quebec.

Rescue effect

White Wood Aster is present in a significant portion of the eastern United States (see Global range). In the event of the extirpation of the Canadian population, it is possible that individuals from adjacent states could recolonize suitable habitats on the Canadian side, in particular in southern Ontario, because the province is adjacent to the counties in the state of New York where the species is present and secure (S5 NatureServe 2022). However, this potential rescue would be greatly limited by the species’ relatively low dispersal potential (see Movements, migration, and dispersal) and the availability of suitable habitat along the border between Canada and the United States.

Threats

Historical, long-term, and continuing habitat trends

Ontario subpopulations are mostly found in small forest patches, fragmented by human activities and surrounded by residential and agricultural development. The presence of a dense local human population near suitable habitat suggests a significant potential for disturbance of natural environments, indicating that habitat for White Wood Aster in Ontario may continue to decline or at best be maintained if appropriate conservation measures are undertaken.

The Quebec subpopulations exist in a slightly different context. Although these are located near areas dedicated to residential development and agriculture, the forests that shelter them are generally larger and less fragmented than those in Ontario. Although the habitat of White Wood Aster in Quebec is not protected from harmful disturbances, the pressures appear to be less significant and the habitat less fragmented.

Anthropogenic disturbances, depending on their nature, can in some circumstances, be beneficial to the species. The presence of the species is often associated with canopy openings (Sharp et al. 1995), such as those created in trail corridors.

Current and projected future threats

White Wood Aster is vulnerable to the cumulative effects of various threats, in particular habitat loss caused by residential, agricultural, logging developments, invasive species, browsing by White-tailed Deer (Odocoileus virginianus), and recreational activities (Environment and Climate Change Canada 2018). 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 approximately 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 White Wood Aster is considered to be medium, corresponding to an anticipated further decline of between 3% and 30% with a median range of 12% to 19% over the next ten years (or three generations, whichever is longer). These values are to be interpreted with caution, as they 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. Site-specific threat information for plants in the Niagara region was gathered by O’Hara (2022).

The main threats are:

Residential and commercial development (IUCN 1) – low impact

Housing and urban areas (IUCN 1.1). In Ontario, as in Quebec, the main threat to the species’ survival comes from the destruction of its habitat. Several subpopulations, particularly in Ontario, are located in or around residential or agricultural areas (for example, Doan's Ridge-Cooks Mills 1, Ridgeway Battlefield Woods). Subpopulations are threatened in the short term by expansion of these activities.

Agriculture (IUCN 2) – low impact

Annual and perennial non-timber crops (IUCN 2.1). Expansion of agricultural development threatens subpopulations in adjacent and nearby woodlots. Half of the Culp's Woods subpopulation has recently been lost to agriculture land conversion. Many recently discovered subpopulations are situated adjacent to agricultural areas. Land in the Niagara Region, for example, is in high demand for vineyard expansion or tree fruit production.

Biological resource use (IUCN 5) – low impact

Logging and wood harvesting (IUCN 5.3). Logging activities may affect subpopulations through the crushing of stems by machinery and habitat modifications (for example, changes in canopy cover and understory vegetation, soil compaction, altered drainage). In most cases harvesting is selective. In Quebec, selective harvesting is not known to necessarily have direct detrimental effects.

Human intrusions and disturbance (IUCN 6) – low impact

Recreational activities (IUCN 6.1). White Wood Aster grows in forest clearings. Trail corridors may be partially beneficial in providing and maintaining openings but could also subject plants to trampling by hikers and all-terrain vehicles (Environment and Climate Change Canada 2018). Trampling occurs on both private and public lands (for example, municipal or provincial parks).

Natural system modifications (IUCN 7) – low impact

Other ecosystem modifications (IUCN 7.3). The exotic invasives Garlic Mustard (Alliaria petiolata) and European Common Reed (Phragmites australis subsp. australis) have been identified as significant threats to the integrity of the habitats occupied by White Wood Aster (Environment and Climate Change Canada 2018). Garlic Mustard is considered a highly invasive species across southern Canada (Catling et al. 2015). The subpopulation at Venise-en-Québec (3867) is threatened by encroachment of Japanese Knotweed (Reynoutria japonica).

Invasive and problematic species (IUCN 8) – low impact

Invasive non-native/alien species/diseases (IUCN 8.1) – unknown impact. In central Ohio, the introduced Hairy Broad-nosed Weevil (Barypeithes pellucidus), also found in Ontario, preferentially feeds on White Wood Aster (Galford 1987). Nearly 50% of the individuals observed in the Mont Pinacle subpopulation (3870) showed signs of damage caused by insect browsing in general (Désilets 2015).

Problematic native species/diseases (IUCN 8.2) – low impact. In Pennsylvania, White Wood Aster is browsed by White-tailed Deer so heavily that the impact on the plant is considered an indicator of the browsing intensity of this herbivore (Williams et al. 2000). Given the abundance of White-tailed Deer in southern Ontario and Quebec, this mammal likely poses a threat to the White Wood Aster (Environment and Climate Change Canada 2018). Protective measures against White-tailed Deer browsing have been put in place in some localities, including at the Saint-Blaise-sur-Richelieu subpopulation (3872).

Number of threat locations

Residential and agricultural development, and logging are threats that are dependent on decisions exercised by individual landowners. Therefore, the number of landowners appears to be an appropriate metric for determining the number of threat locations. Because the exact number of landowners is unknown, an estimated range of between 70 and 80 is proposed as the number of locations (Table 2).

Protection, status, and recovery activities

Legal protection and status

The Committee on the Status of Endangered Wildlife in Canada previously assessed White Wood Aster as Threatened in Canada (Sharp et al. 1995; COSEWIC 2002), and the species is listed as Threatened on Schedule 1 of the federal Species at Risk Act (SARA). A federal recovery strategy identifying critical habitat was prepared (ECCC 2018). In Quebec, White Wood Aster was designated as Threatened and is listed under the Act Respecting Threatened or Vulnerable Species. In addition, its habitat is governed by an authorization process pursuant to the Quebec Environment Quality Act (CQLR c. Q-2) and its regulations. In Ontario, it is listed as Threatened under the Endangered Species Act of 2007 and receives general habitat protection under this law.

Non-legal status and ranks

NatureServe (2022) has assigned this species a conservation rank of S3 (Vulnerable) for Ontario, S2 (Imperilled) for Quebec, N3 (Vulnerable, last assessed 2023) for Canada, and G5 (Secure, last assessed in 1984) global status. White Wood Aster is ranked S3S4 (Vulnerable to Apparently Secure) in Maine, S4 in Delaware (Apparently Secure), and S5 (Secure) or SNR (unranked) in the other 18 jurisdictions where it is found. COSSARO (Committee on the Status of Species at Risk in Ontario; Ministry of Natural Resources) has not assigned it any special status.

Land tenure and ownership

Two subpopulations occur on federal lands, including the St-Armand subpopulation (11275), which is located within the Philipsburg Migratory Bird Sanctuary, and the Paradise Grove subpopulation (92423), which is located on Parks Canada lands. The Four Mile Two Mile Ponds subpopulation (1708) is found on lands belonging to Parks Canada, the Department of National Defence, and the municipality of Niagara. In Quebec, the Mont-Orford subpopulation (80290) will soon be located inside a provincial park.

The extent of legally protected White Wood Aster habitat is minimal, as the majority of the population is on private land (Table 2). However, the species benefits from other forms of protection at some sites. The newly discovered subpopulation on the Graymont property at Saint-Armand (11275) is located on land subject to a conservation easement (agreement with the Nature Conservancy of Canada). A similar agreement also covers part of the Sutton subpopulation (20860). The Venise-en-Québec subpopulation (3867) and part of the St-Armand subpopulation (3866) are also located on land owned by the Nature Conservancy of Canada.

In Ontario, Areas of Natural and Scientific Interest (ANSIs) include the following subpopulations: Short Hills Provincial Park (1711, public land), Marcy’s Woods (31886, private conservation authority), Fonthill-Sandhill Valley (31887, public land), and North Pelham Valley (31898, private land, to be confirmed). Subpopulation 31888 is located within the St. John’s Conservation Area (private-public conservation authority).

Recovery activities

The species is the subject of federal (Environment and Climate Change Canada 2018) and Ontario (Ministry of the Environment, Conservation and Parks 2019) recovery strategies and a government response statement for Ontario (Ministry of the Environment, Conservation and Parks 2020). The federal strategy aims to maintain the species’ current distribution and abundance (number of stems) and, where possible, to support the natural increase in abundance of existing subpopulations. Strategies to address threats include surveys and monitoring, research, habitat conservation and management, land use policy and planning, communication, awareness, and education. A monitoring program in Ontario spanning the years 2018 and 2021 made it possible to acquire a considerable amount of data and to confirm the existence of several new subpopulations (O'Hara 2022).

Information sources

Boisjoli, G. 2010. Dynamique des populations et étude du microhabitat d’un aster forestier rare et menacé (Eurybia divaricata). Université du Québec à Montréal.

Britton, N.L., and A. Brown. 1970. An Illustrated Flora of the Northern United States and Canada: Entian to Thistle. Gentianaceae to Compositae. Dover Publications.

Brouillet, L. 2006. Eurybia. In Flora of North America Editorial Committee (Ed.), Flora of North America North of Mexico, 20: 365-382.

Brouillet, L., F. Coursol, S.J. Meades, M. Favreau, M. Anions, P. Bélisle, and P. Desmet. 2010+. VASCAN, the Database of Vascular Plants of Canada.

Catling, P.M., G. Mitrow, and A. Ward. 2015. 12. Garlic Mustard, Alliaire officinale: Alliaria petiolata (M. Bieberstein) Cavara and Grande. In CBA/ABC Bulletin (Ed.), Major invasive alien plants of natural habitats in Canada pp 51 to 60.

COSEWIC 2002. COSEWIC assessment and update status report on the White Wood Aster Eurybia divaricata in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 23 pp.

Désilets, P. 2015. Inventaire d’aster à rameaux étalés (Eurybia divaricata) au Mont Petit Pinacle. Unpublished report.

Environment and Climate Change Canada (ECCC). 2018. Recovery Strategy for the White Wood Aster (Eurybia divaricata) in Canada, Species at Risk Act Recovery Strategy Series. Environment and Climate Change Canada, Ottawa, viii + 67 pp.

Galford, J. R. 1987. Feeding habits of the weevil Barypeithes pellucidus (Coleoptera: Curculionidae). Entomological News, 98(4):163164.

Hough, M. 2008. Possible limiting agents to the early establishment and growth of understory herbs in post-agricultural forests in central New York. Master of Science Thesis, State University of New York., College of Environmental Science and Forestry, Syracus, New York.

Imrie, A., R. Theisen, T. Staton, and P. Patel. 2005. Ecology of the White Wood Aster (Eurybia divaricata) in Short Hills Provincial Park. Report submitted in order to fulfill the requirements of the Field Project course of the Ecosystem Restoration program at Niagara College. Niagara College.

Lesica, P., and F.W. Allendorf. 1995. When are peripheral populations valuable for conservation? Conservation Biology 9: 753-760.

MacPhail, V.J. 2013. Investigating the pollination biology of species-at-risk plants in southern Ontario – Results from 2013. Wildlife Preservation Canada’s Pollinators Project: The Forgotten Pollinators - Restoring Insect Biodiversity to Maximize Ecosystem Services and Sustainability, November 19. Funding: Ontario Ministry of Natural Resources Species at Risk Stewardship Fund grant to Wildlife Preservation Canada.

Master, L.L., D. Faber-Langendoen, R. Bittman, G.A. Hammerson, B. Heidel, L. Ramsay, K. Snow, A. Teucher, and A. Tomaino. 2012. NatureServe conservation status assessments: factors for evaluating species and ecosystems risk. NatureServe, Arlington, Virginia.

MELCC. 2017. Espèce menacée au Québec. Aster à rameaux étalés.

Ministry of the Environment, Conservation and Parks. 2019. Recovery strategy for the White Wood Aster (Eurybia divaricata) in Ontario. Ontario Recovery Strategy Series. Prepared by the Ministry of the Environment, Conservation and Parks. Peterborough, Ontario. iv + 7 pp. + Appendix.

Ministry of the Environment, Conservation and Parks. 2020. White Wood Aster Government Response Statement.

NatureServe 2008. Biotics 5: Ranking species occurrences: a generic approach and decision key.

NatureServe. 2022. Eurybia divaricata, White Wood-aster. NatureServe Explorer.

Nesom, G.L. 1994. Review of the taxonomy of Aster sensu lato (Asteraceae: Astereae), emphasizing the New World species. Phytologia, 77(3):141-297.

O’Hara, P. 2022. 2018 / 2021 White Wood Aster (Eurybia divaricata) Survey Final Report. Ministry of Natural Resources and Forestry Species at Risk Stewardship Fund (SARSF) _8_18_BONL Final Report, February 28, 2022. Blue Oak Native Landscapes, 113 Locke Street North, Hamilton, ON.

Salafsky, N., D. Salzer, A.J. Stattersfield, C. Hilton-Taylor, R. Neugarten, S.H.M. Butchart, B. Collen, N. Cox, L.L. Master, S. O’Connor, and D. Wilkie. 2008. A standard lexicon for biodiversity conservation: unified classifications of threats and actions. Conservation Biology 22:897-911.

Sharp, M.J., C.J. Manderson, and W.J. Crins. 1995. COSEWIC status report on the White Wood Aster Eurybia divaricata in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 33 pp.

Singleton, R., S. Gardescu, P. Marks, and M.A. Geber. 2001. Forest herb colonization of postagricultural forests in central New York State, USA. Journal of Ecology, 89(3): 325-338.

Strong, W.L., S.C. Zoltai, and G.R. Ironside. 1989. Régions écoclimatiques du Canada. Service canadien de la faune, 127.

Tardif, B., N.D.U. Québec, and B. Tardif. 2016. Les plantes vasculaires en situation précaire au Québec.

Williams, C.E., E.V. Mosbacher, and W.J. Moriarity. 2000. Use of Turtlehead (Chelone glabra L.) and other herbaceous plants to assess intensity of White-tailed Deer browsing on Allegheny Plateau riparian forests, USA. Biological Conservation, 92(2): 207-215.

Collections examined

No collections were examined for the preparation of this report.

Authorities contacted

• De Forest, Leah. March 2023. Species Conservation Specialist, Parks Canada
• Labrecque, Jacques. February 2021. Botanist, Natural Heritage Data Centre, Ministry of Environment and the Fight Against Climate Change, Department for the Conservation of Flora and Natural Environments, Ecological Heritage and Sustainable Development Branch, Quebec City, Quebec
• Natural Heritage Information Centre (NHIC). February 2021. Eurybia divaricata Element Occurrence and Observation data in Ontario Tracked Species database as provided to COSEWIC Secretariat, October 2021. Natural Heritage Information Centre, Ministry of Natural Resources and Forestry, Peterborough, Ontario
• McDonald, Rachel. March 2023. Senior Environmental Advisor, Environment and Sustainable Management, ADM (Infastructure and Environment), National Defence

Acknowledgements

This report was funded by the Canadian Wildlife Service of Environment and Climate Change Canada. The experts listed above provided valuable data and/or advice. Bruce Bennett, Co-chair of the Vascular Plants Specialist Subcommittee (VPSSC), was most helpful in providing technical guidance and editorial comments throughout the drafting of this report. The report writer would like to thank all those who shared their expertise and personal observations, especially Benoit Tremblay, Caroline Tanguay, Lauriane Monette, Victor Grivegnée-Dumoulin, William van Hemessen, Patrick Deacon and Burke Korol. Many reviewers contributed to improving this report including Gina Schalk, Holly Bickerton, Kevin Yang, Marie-Claude Archambault, Karolyne Pickett, and Patricia Désilets of Environment and Climate Change Canada; and Vivian Brownell, David Mazerolle, and Danna Leaman of the VPSSC.

Biographical summary of report writer(s)

Dominic Desjardins holds a doctorate in biological sciences from the University of Montreal with a specialty in ecology and plant biology. He is the author of several scientific and technical publications in the plant field. He is currently a project manager on mandates for the ecological characterization of natural environments, with a particular interest in flora.

Audrey Lachance has acquired expertise in characterizing natural environments and carrying out inventories and monitoring related to rare plants. She obtained a diploma in natural environment techniques and wildlife management in 2005. She has worked for several years carrying out inventories, demographic monitoring, and writing various documents on several endangered plant species. She also works on rare species in Quebec, on threat assessment, on the development of appropriate mitigation measures and on the vulnerability of rare plants to climate change.

Appendix 1. Threats assessment for the white wood aster

Threats assessment worksheet

Species or ecosystem scientific name:

White Wood Aster; aster à rameaux étalés; Eurybia divaricata

Element ID :

1058512

Elcode :

PDASTEB0Q0

Date :

12/14/2022

Assessor(s):

Bruce Bennett, Del Meidinger, Vivian Brownell, Dana Leaman, Sean Blaney, Lauriane Monette, Jacques Labrecque, Benoit Tremblay, Dominic Desjardins, David Mazzerole, Caroline Tanguay

Assigned overall threat impact:

C = Medium

Impact adjustment reasons:

Medium may be high based on past declines; although 6 Low impact threats, most are likely at low end of Low impact; considerable uncertainty at this point; low is 1 to 10% medium is 3 to 30%

Overall, threat comments:

Generation time of 3 plus years; three generations of 10 years (plus?); 101,000 stems in Canada (as a proxy for mature individuals); two new subpopulations on iNaturalist (one with 10,000 stems); second in Norfolk; Frelighsburg Eccles Hill may be around 10,000 stems are included; about 16% in parks or national lands; legislation protects other subpopulations but development could occur.