COSEWIC Assessment and Status Report on the Eastern Flowering Dogwood in Canada
Table of Contents
Assessment and Status Report
Eastern Flowering Dogwood
Committee on the Status
of Endangered Wildlife
Comité sur la situation
des espèces en péril
COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:
COSEWIC 2007. COSEWIC assessment and status report on the eastern flowering dogwood Cornus florida in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 22 pp.
(Species at Risk Status Reports)
COSEWIC would like to acknowledge John Ambrose for writing the status report on the eastern flowering dogwood (Cornus florida) in Canada, prepared under contract with Environment Canada, overseen and edited by Dr. Erich Haber, Co-chair (Vascular Plants), COSEWIC Plants and Lichens Species Specialist Subcommittee .
For additional copies contact:
c/o Canadian Wildlife Service
Également disponible en français sous le titre Évaluation et Rapport de situation du COSEPAC sur le cornouiller fleuri (Cornus florida) au Canada.
Eastern flowering dogwood -- Illustration from Britton and Brown (1913).
© Her Majesty the Queen in Right of Canada 2007
Catalogue No.: CW69-14/533-2007E-PDF
Assessment Summary – April 2007
Eastern Flowering Dogwood
Reason for designation:
A small understory or forest-edge tree present only as small populations within the fragmented woodlots of southern Ontario's Carolinian forest. The spread of dogwood anthracnose disease has caused dramatic declines in the Canadian populations that reflect similar declines throughout the species' range in eastern North America. This assessment of risk applies only to wild populations and not to cultivated plants in nurseries, parks, and gardens.
Designated Endangered in April 2007. Assessment based on a new status report.
Eastern Flowering Dogwood
Eastern flowering dogwood (Cornus florida) is a small tree of the forest understory or edge. It has branches in whorls from the main trunk, giving a tiered appearance. Leaves are simple, opposite and deciduous. Flowers are in terminal clusters with four conspicuous white floral bracts. Fruit occurs in tight clusters; scarlet, shiny, one-seeded, ovoid and fleshy. It is similar to Cornus alternifolia, but the latter has alternate leaves and branching, lacks the floral bracts and fruit are dark green to blue in an open cluster.
It is a species of eastern North America, ranging in the north from southern Michigan, southern Ontario and southern Maine, south to eastern Texas to northern Florida; two disjunct populations occur in the mountains of eastern Mexico. Only a small percentage of its range is in Canada.
Cornus florida typically occurs as an understory or edge tree of open dry to slightly moist deciduous or mixed forests, on sand to sandy loam or occasionally on clay loam soils, typically in mid-age to mature forests.
This species is a long-lived, slow growing woody perennial. Flowers are perfect (having both male and female parts) and occur in mid-spring as the leaves are beginning to open. Individuals are pollinated by insects; cross-pollination produces better seed set. The mature red fruits are dispersed by numerous birds and a few mammals. Germination and seedling establishment occur in forest shade but intermediate light levels promote growth.
Population Sizes and Trends
Populations range from individual trees, to many that have one to a few dozen trees, to one with over 100. Since the arrival of dogwood anthracnose (a leaf disease causing mortality) in the mid-1990s many populations are in a steep decline, an average of 7-8% per year.
Limiting Factors and Threats
The greatest threat is the dogwood anthracnose disease that has decimated many populations and only a few seem to have so far avoided its impact. In addition to high mortality, the remaining individuals have reduced flowering and fruiting.
Special Significance of the Species
Cornus florida is a conspicuous understory tree in southern Ontario forests, providing nutrition for pollinating insects in the spring and frugivorous (fruit-eating) forest birds in late summer. It is also an important small, ornamental landscape tree. First Nations made use of Cornus florida for various medicinal treatments. This species, plus its western counterpart Cornus nuttallii, are threatened by dogwood anthracnose throughout their ranges.
Several jurisdictions are reviewing the status of this species due to its drastic decline after the arrival of dogwood anthracnose. Three states have it listed as S1 (critically imperiled); Ontario currently lists it as S2 (imperiled).
COSEWIC History, Mandate, Membership and Definitions
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.
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 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.
- 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.
The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.
Lists of Figures and Tables
List of Figures
- Figure 1: Leafy Branchlet of Cornus florida with a Terminal Flower Cluster of Tiny Flowers Surrounded by Four, Large, White, Petal-like Bracts and a Longitudinal Section of a Single, Enlarged, Flower
- Figure 2: Global Distribution of Cornus florida
- Figure 3: Total Native Distribution of Cornus florida in Canada
- Figure 4: Comparison of Proportion of Live and Dead Cornus floridain Backus Woods EMAN Plot 1995-2005
- Figure 5: Comparison of Proportion of Live and Dead Cornus florida in Wilson Tract EMAN Plot 1995-2005
List of Tables
- Table 1: Summary of Ontario Recorded Sites of Cornus florida
- Table 2: Summary of Extant Populations of Cornus florida in Ontario, Inventoried 2004-05
COSEWIC Status Report
Eastern Flowering Dogwood
Cornus florida L., commonly known as eastern flowering dogwood or flowering dogwood (French: Cornouiller fleuri), has only one synonym and it is not in current use: Cynoxylonfloridum (L.) Raf. ex B.D. Jackson. The genus is in the dogwood family (Cornaceae).
A small tree of the forest understory or edge 3-10 m high; branches are in whorls from the main trunk, giving a tiered appearance. Bark on larger specimens is grey-brown and separates into quadrangular plates, forming an alligator skin pattern. Each branch is composed of a sequence of annual growth segments originating from a sub-terminal position, resulting in a conspicuous scalloped appearance. Leaves are simple, opposite and deciduous. Blades are ovate, elliptic to broadly oval, 5-15 cm long and 2.5-8 cm wide. Flowers are in terminal clusters with four conspicuous white floral bracts, appearing in mid-spring as the leaves are just beginning to develop (Figure 1). The floral buds are stalked, large and have a flattened dome shape. Fruits are scarlet, shiny, one-seeded, ovoid and fleshy and occur in tight clusters. The species is similar to alternate-leaved dogwood (Cornus alternifolia), but the latter has alternate leaves and branching and lacks the floral bracts; the fruits are dark green to blue in an open cluster.
There are no conspicuous barriers to gene flow within populations; flowers are perfect and self incompatibility occurs with some but not all self-pollinations (Reed, 2004). Craddock et al. (1997) found that 3 of 7 cultivars of this species would not set fruit with self-pollination but did with most cross-pollinations. Sork et al. (2002) estimated 8-11 effective pollen donors for Cornus florida trees in Missouri forests. Fruit commonly is present on trees that have flowered. Habitat fragmentation has resulted in moderate geographic barriers between some populations. Little is known of population differences other than some observations on geographic differences, for example, fruit weights tend to decrease with decreasing latitude and increasing length of growing season (Winstead et al., 1977); however, the trend reverses in Mexico, where Cornus florida var. urbaniana, a variety found in the mountains of Nuevo León and Veracruz, differs from the typical species by its greyer twigs and larger fruit (Vimmerstedt, 1965).
A single designatable unit is here recognized for this widespread eastern North American species.
Figure 1: Leafy Branchlet of Cornus florida with a Terminal Flower Cluster of Tiny Flowers Surrounded by Four, Large, White, Petal-like Bracts (centre) and a Longitudinal Section of a Single, Enlarged, Flower (left)
Illustration from Britton and Brown (1913).
This is a species of eastern North America, ranging from southern Michigan, southern Ontario and eastward to southern Maine in the north and south to eastern Texas and to northern Florida. It is common in the deciduous forests of the middle and southern states, occurring from the coast to high in the Allegheny Mountains in the south. Two well separated populations occur in the mountains of north-eastern and central-eastern Mexico (Figure 2).
After Little, 1971.
In Canada this species is limited to the Carolinian Zone (Deciduous Forest Region) of southern Ontario. It ranges from just west of Toronto in Oakville, along or near the Niagara Escarpment through Halton and Hamilton regions to several sites in the Niagara Region and south-westward. It is especially abundant on the sand plain of Norfolk County with outlying populations in Brant (historical), Middlesex, Elgin, and Lambton counties, and another cluster of sites in Chatham-Kent and Essex County. This species continues to have some occurrence in all the historical regions (Fox & Soper, 1952), but not extending to Brant County. The extent of occurrence (EO) is estimated to approximate the area of the Carolinian Zone (22 500 km2), based on its distribution shown in Figure 3. The area of occupancy (AO) is estimated as 1 km2per site (based on personal observation). With 154 sites documented between 1975 and 2005 this totals about 150 km2, considering some overlap of grids. The dots of Figure 3 are based on a comprehensive list of precise locations compiled from herbarium records, Natural Heritage Information Centre records and recent field observations.
Cornus florida typically occurs as an understory or edge tree of open dry-mesic oak-hickory to mesic maple-beech deciduous or mixed forests. It often occurs in drier aspects of its habitats, such as the top of slopes or on a southern exposure. Occasionally it is found in moist or lowland sites within its forest habitat, but not areas that are periodically flooded. It occurs on sand to sandy loam or occasionally on clay loam soils. Forests where it occurs are commonly mid-age to mature. There may be a light to moderate amount of disturbance within occupied habitats, such as where sustainable forest harvesting practices have taken place. This tends to promote seedling establishment in habitats with closed canopies. Cornus florida is less tolerant of more severe practices resulting in major forest openings or clearings. It does not appear to be dependent on other specific species.
Suitable habitat often appears to occur through more of a site than is occupied. The species typically occurs in clusters of individuals within larger areas of habitat that are apparently equally suitable but unoccupied. On a landscape level, suitable forest habitat is highly fragmented in the Carolinian Zone; in Chatham-Kent and Essex County the forest cover reaches an extreme low; only a few percent of the landscape is forested.
Other woody Species at Risk that occur in the same habitats include: American chestnut (Castanea dentata) and butternut (Juglans cinerea). The following species with restricted distribution in the Carolinian Zone occur with Cornus florida: dwarf chinquapin oak (Quercus prinoides), tulip tree (Liriodendron tulipifera), winged sumac (Rhus copallina), black oak (Quercus velutina), black walnut (Juglans nigra), pignut hickory (Carya glabra), American witch-hazel (Hamamelis virginiana), black gum (Nyssa sylvatica) and pin oak (Quercus palustris).
Historically, this species occurred through much of the Carolinian Zone, an area that has been severely cleared for its productive agricultural lands and related settlements and industry. Thus, the historical trend is a significant reduction and fragmentation of forest cover and suitable habitat. In the last twenty-five years renewed efforts have been made to document and map significant areas and provide them with protection through designation, acquisition or other securement devices such as conservation easements and stewardship agreements, culminating in efforts to reverse the trend of habitat degradation and loss. Local habitat restoration projects have attempted to turn around the past loss in several areas of southern Ontario. Nevertheless, forest cover and important habitat continue to decline in some areas.
Some of the sites with Cornus florida habitat are protected as provincial parks (Wheatley, Short Hills), conservation areas, (Balls Falls, Rock Glen, Backus Woods), county forests (Norfolk County), or designated environmentally significant areas but not necessarily with strong local planning protection. However, many municipalities are adding natural heritage mapping to their planning tools; the future looks brighter for these.
Cornus florida is an understory and edge tree of mid-age to mature forests. Its flowers (made conspicuous by large floral bracts) are insect-pollinated and its clusters of red fruit are mostly bird-dispersed.
This species is a long-lived, slow growing, woody perennial (typically living to 125 years, Strobl & Bland, 2000). Before the arrival of dogwood anthracnose (Discula destructiva), the exotic fungal disease that is causing a major decline in the species, individuals with trunk diameters of 20-30 cm were occasionally encountered in southern Ontario. The smallest individuals observed flowering were in the range of 4-7 cm, diameter breast height (dbh). Flowers are perfect (hermaphrodite) and occur in mid-spring as the leaves are beginning to open. In 2004, flowering in Ontario occurred between May 12 (floral bracts open but flowers not quite at anthesis (flowers open and ready to release pollen)) and May 27 (the last of the floral bracts present, just past anthesis). Fruits mature and turn red by late summer (Soper & Heimburger, 1982).
Individuals are cross-pollinated by insects (Mayor et al., 1999). There is evidence of partial self-incompatibility (Craddock et al., 1997). Hybridization has not been reported in natural stands of Cornus florida in Ontario or elsewhere; however, in cultivation it has been hybridized with the western Cornus nuttallii and the oriental C. kousa and there are hybrids of these species in the horticultural trade.
The mature fruit are dispersed by numerous birds and a few mammals. Germination and seedling establishment occur in forest shade but intermediate light levels promote growth (Strobl & Bland, 2000; McLemore, 1990). Although most of the individuals are well separated, suggesting an origin from dispersed seeds, some reproduction occurs by vegetative means, primarily layering of low lying branches. This seems to be most prominent in smaller plants where large clusters of apparently once connected stems can be found. On examination, some near-surface connections can be found between upright stems. In some cases of clustered young stems, the stems may also have arisen from stump sprouts of trees that have died. Prior to the arrival of dogwood anthracnose, occurrences of clonal growth were not commonly observed by the writer. From personal observation in the field, there was some suggestion that dogwood anthracnose may be stimulating limited clonal growth in the form of multiple trunks slightly beyond the common base plus the spread of low clonal growth at ground level.
Seeds germinate in the spring a year or two after dispersal (Strobl & Bland, 2000) after having experienced a winter on the forest floor. The timing for germination can be accelerated by first providing warm stratification for 60 days followed by a cold stratification (just above freezing) for 90-120 days (Kock, 1998).
Deer and rabbits are known to browse on this species (Strobl & Bland, 2000). From field observations, this appears to be a problem only for seedlings and saplings, possibly promoting the proliferation of low vegetative shoots.
No special physiological requirements or adaptations are known for this species, other than the previously mentioned pre-treatments for seed germination.
Numerous birds and mammal species have been reported to feed on the fruit (Tirmenstein, 1991); no doubt many of these are effective dispersal agents. Birds may take the intact fruits beyond the immediate area of the parent tree and later separate the seed from the flesh and drop the seed, consume the fruit whole and later pass the seed through their digestive system at a greater distance, or cache them for later consumption. Gravity also plays a role, but these seeds would move little beyond the parent tree unless picked up by mammals. Populations tend to be clusters of individuals within a larger forest habitat, suggesting that most of the effective dispersal is over a short distance. Seeds carried longer distances are less likely to fall in a suitable habitat.
Dispersal across the Canada-US boundary is possible, for example, by migratory birds. With the original occurrence of dogwood anthracnose in the US, populations there are also diminished and likely the remaining trees are producing little fruit, as is the condition in Canada.
Forest fragmentation in the Carolinian Zone reduces the possibility of effective long distance dispersal, such as by birds from an occupied habitat to an unoccupied, suitable habitat.
Cornus florida depends on pollinating insects for seed set and frugivores for dispersal.
Browsing animals, such as deer and rabbits, negatively impact populations but are only likely a serious problem during times of high population density of the browsers. Animals that feed on flowering dogwood fruit are impacted by the loss of mature trees and reduction of fruiting in survivors (Rossell et al., 2001).
The recent arrival of the dogwood anthracnose fungusin Ontario (NRC, 2004) represents the most serious interspecific interaction, putting the survival of Cornus florida in Canada in jeopardy. Symptoms are first apparent as leaf spots and later some trees show cankers on twigs and branches. In the final stages of the disease, branch death occurs and clusters of epicormic shoots develop on the trunk; these are shoots that grow from dormant buds on a stem after a wound or other stress. Infection is thought to occur during cool wet conditions (NRC, 2004).
This species can survive periodic openings in its forest habitat and such openings provide increased light that can promote seedling/sapling growth. However, excessive light also promotes open site species that would out-compete flowering dogwood. Although it tends to grow on well-drained sites, periods of extremes in drought conditions likely limit recruitment.
Cornus florida is commercially cultivated and widely available, though most large landscape nurseries likely bring in stock from the United States which would not be suitable for re-introduction into natural habitats. Stock propagated from known local Ontario sources would be better adapted and more likely to do well; local stock is available from some native plant nurseries.
Population Sizes and Trends
Initially, all known information was brought together, including the records at Ontario’s Natural Heritage Information Centre, field notes from botanists working in the area (Mike Oldham, Jane Bowles, Allen Woodliffe, Gerry Waldron), current herbarium records where available and earlier records the author and associates compiled at the University of Guelph Arboretum. Norfolk County has several recent reports that record the presence of this species (Gartshore et al., 1987; Draper, 2002; Ambrose & Waldron, 2004). Knowledgeable individuals were also queried for specific regions; sometimes they also participated in the field survey.
Populations were sampled across the range of distribution in southern Ontario. An effort was made to visit all populations at the extremities of distribution and sample representative populations in the mid-regions. Where populations were small or in a limited area, an attempt was made to document each individual. In larger areas, such as Skunks’ Misery, St. Williams and Backus Woods, only enough of the site was sampled to get a sense of the impact of dogwood anthracnose on the population.
In the spring and summer of 2004, 17.5 person-days (approximately 140 person-hours) were spent inventorying populations, collecting samples and assessing dogwood anthracnose (DA); two days in the field were spent with Dr. Richard Wilson, provincial forest pathologist. Symptoms for DA were not visible until mid-summer. Samples of 25 populations were sent to Chuck Davis (Fungal Taxonomy and Ecology Technologist) at the Great Lakes Forestry Centre, Sault Ste. Marie, for determination of DA presence. In early May 2005, the two EMAN (Environment Canada’s Ecological Monitoring and Assessment Network) plots in Norfolk County were re-inventoried, with the help of about a dozen students taking a week-long monitoring workshop (8 person-days, approximately 64 person-hours). This early summer 2005 data set was used to give one more year of observational data for the charts on Figures 4 and 5. One more day was spent in the summer of 2005 to check additional sites. In total, 18.5 person days were spent in the field, in 2004 and 2005, not counting the class inventorying the two EMAN plots.
Recent records (1975-2005) indicate the presence of 154 populations in southern Ontario within 12 counties and regions.
Two historical records for Brant County (Fox & Soper, 1952) had imprecise geographic coordinates and could not be found; nor were they known by a local authority who also searched for this species (Larry Lamb, pers. com.).
Of a total of 180 recorded sites (Table 1), 32 were visited and surveyed; 468 living individual trees were recorded at these sites. Another 15 sites with precise counts were recorded in the last five years by Allen Woodliffe and Jane Bowles, for an additional 134 trees. Ten sites were recorded in Norfolk County in 2003 (Ambrose & Waldron, 2004) but not with precise counts; estimating an average from memory of 6 trees per site gives an additional 60 trees. A few sites were visited where the trees were known in the past and they are now absent, due to development or loss. Several more were records with imprecise locations and were not found. In all, of the 180 sites, 57 were observed in the past five years with a total of about 662 trees.
Of the 88 records from 1975-1994 plus an additional 9 recorded from 1995-2001 but not included in the previous total, it is likely that many of these 97 sites are still extant, but how many one can only roughly speculate on. Of the 50 sites cited in Soper & Fox (1952), 76% are still extant (Table 1). If we use 75 as the percent of sites being extant of the above 97, then 73 likely remain. If we assume the same average number of trees per site (12, based on the 662 trees at 57 sites), then another 873 (12 X 73) trees likely exist in Ontario, for an estimated total of 1535 for all 154 sites known between 1975 to present.
|County/region||up to 1952*||1953-1974||1975-1994||1995-present||Total|
* Fox and Soper, 1952; number listed (numbers of sites not seen since); each subsequent time period lists new records only (but includes the extant Fox & Soper records). This is a summary of the confidential appendix.
All of the small populations and most of the large populations in small areas (e.g., Cedar Creek, Decew Falls, Fireman’s Park) were sampled thoroughly, but others where trees were scattered over large areas (e.g, St. Williams, Queenston) were sampled only in part. For this latter group, there very likely are additional uncounted trees, perhaps adding another 50 or 100, so that the total living Ontario population is estimated to have been in the realm of 1600 +/- 50 a few years ago.
With an annual mortality rate of 7-8% per year (Wilson Tract and Backus Tract combined mortality of 86.5% over 11 years of monitoring), these numbers could have been reduced by about 20% over three years, with perhaps a more realistic figure of about 1260-1300, and declining (considering that some of the records are 2-4 years old and the recent inventory included trees that were dying).
With reduced flowering and fruiting (Table 2) and only rare sightings of seedlings, recruitment appears to be in decline as well.
|Co/Reg & site name||Mature||Live/|
|Individuals with flowers/fruit||Dogwood anthracnose||Observed mortality|
|Canard River KCT Woods||1||1/0||100%||(–)||0%|
|Cinnamon Fern Woods||35||26/9||32%||(+)||29%|
|Leamington Wh.Oak Wds||6||3/3||0%||(+)||50%|
|Halton: Sassafras Wds||10||6/4||30%||(+)||40%|
|Hamilton: Princess Point||1||1/0||+||(–)||0%|
|Chatham-Kent: Wheatley Prov. Park||27||22/5||19%||(+)||19%|
|Wheatley Prov. Park East||32||24/8||28%||(+)||25%|
|Middlesex: Ausable Valley||9||9/0||44%||–||0%|
|Niagara: Navy Island||4||3/1||0%||+||25|
|Queenston, E of quarry||1||1/0||+||+||0%|
|Firemen’s Park & N||75||52/23||45%||(+)||30%|
|3km E of Thorold||4||3/1||25%||(+)||25%|
|Decew Falls to Short Hills||37||32/5||48%||+||14%|
|N of Fonthill||3||3/0||100%||(–)||0%|
|De Cloet Woods||24||17/7||21%||+||29%|
|Wilson Tract EMAN plot*||8||4/4||0%||(+)||50%|
|Backus EMAN plot*||17||13/4||20%||(+)||24%|
|St. Williams, SW||29||25/4||59%||(+?)||14%|
(populations with more than 1 tree)
|8 confirmed, 12 likely||0-50%;|
mean: 17% average: 17.9%
* for EMAN plots, figures between 2003-2005 used, to be comparable with one-time observations for other sites in 2004-5
+Inventoried by J.D. Ambrose and others
Dogwood anthracnose: + = confirmed, (+) = conspicuous symptoms present but not confirmed,
– = no symptoms, (–) = no symptoms but collected early in summer when symptoms are not typically present
Since the arrival of dogwood anthracnose in Ontario, perhaps in the early 1990s, Cornus florida has been observed in decline, both through casual observation (Mary Gartshore, pers. comm.) and through monitoring of specific stands (Brian Craig, in NRC, 2004; pers. comm. and diagrams produced for Figures 4 & 5 below). The value of the EMAN plots is that they document the mortality of tagged and mapped trees over an eleven-year period. For the sites visited for this report, dead flowering dogwood trees were also recorded, but they can be determined with some certainty only up to about 2-3 years after death. Trees that died earlier may no longer be standing, or if they are, the diagnostic dogwood branching pattern and bark is likely missing. The percentage of identifiable dead trees in inventoried populations (Table 2) ranged from 0 to 50% (average about 18%; mean 17%); this agrees with the earlier calculated (based on the two EMAN plots) average of 7-8% decline per population per year.
Decline for the whole Ontario population is variable but dogwood anthracnose and tree mortality appear to occur in most areas. Its presence has been confirmed in the following counties/regions from submitted samples: Essex, Halton, Chatham-Kent, Middlesex, Niagara and Norfolk (Chuck Davis and Richard Wilson, pers. comm.). A few isolated areas so far seem to be free of dogwood anthracnose symptoms (e.g., Springwater, Elgin Co. and the Ausable Valley), but other isolated sites are infected, e.g., Bronte Creek and Navy Island.
Diagrams by Brian Craig, based on his EMAN monitoring study.
For the two sites monitored between 1995 and 2005, mortality counts are cumulative, i.e., in Figure 4, the 38 dead trees for 2003 include the 9 counted in 1995, and the 42 in 2005 include the previous 38 dead trees. Because each tree was tagged and mapped, its condition was tracked during each inventory and compared with its previous condition. The percentage of mortality increased from 14 to 91% for Backus and 19 to 84% for the Wilson Tract. Together, an average 86.5% decline over 11 years was documented (when combining the number of trees vs. averaging the two mortality rates). While this at first seems more drastic than other populations, if dead trees can only be identified to species for 2-3 years after death, then the mean value of 17% dead trees of all inventoried sites agrees with the individual year inventory values for the EMAN plots of new deaths plus living trees.
The continuing loss of trees between 2003 and 2005 at the two EMAN plots does not show an optimistic levelling-off of mortality. Furthermore, among the few survivors in these two plots almost no flowering was observed.
Only one population can be confirmed to be lost, due to residential development in the Windsor area. Other historical records with imprecise locations could not be relocated.
Because dogwood anthracnose-the single factor that seems to be the cause of significant decline in this species-is thought to have originated in the United States, bringing stock from there is not a viable option. Bordering populations as well as those further south are heavily impacted by this disease (NRC, 2004; Schwegman et al., 1998; Anderson et al., 1994). In addition, stock from the south would be less well adapted to conditions in Ontario.
Migratory forest birds likely carry seeds of Cornus florida on occasion, but in the autumn they are more likely going from Ontario to south than vice versa, so only in a very rare event, as in periods of extreme fluctuations in temperature, is recruitment from long distance dispersal likely. However, even though the Carolinian forest habitat is highly fragmented, there is still considerable habitat in which seeds arriving from a long- distance dispersal event could become established.
One could hope to find dogwood anthracnose resistant individuals as part of a strategy to assist recovery, but so far there is little apparent evidence of resistant individuals in stands of infected trees observed in Ontario.
Limiting Factors and Threats
The primary threat is dogwood anthracnose, a fungal disease that is causing a high level of die-back (typically 25-75% in infected populations; but 97% in one Illinois population (Schwegman et al., 1998)) and mortality throughout the range of Cornus florida in eastern North America. The symptoms were observed in the United States since the mid-1970s and the causal agent was identified and described as a new species in 1988 (Redlin, 1991). There is a suspicion that it originated in the orient since Chinese dogwood (Cornus kousa) demonstrates resistance to dogwood anthracnose (Ranney et al., 1994) similar to other geographic patterns of resistance, such as the various oriental chestnuts having resistance to chestnut blight. Symptoms were beginning to appear in Ontario populations in the mid-1990s and the disease was confirmed in 1998. The disease can kill flowering dogwoods of all sizes and ages but especially understory trees and seedlings. Infection appears to occur during cool, wet weather in spring and fall, as masses of white to salmon-coloured conidia ooze from infected twigs and leaves, but can happen any time during the growing season. Host susceptibility appears to increase with drought and winter injury (NRC, 2004). There have been reports of less severe incidence of dogwood anthracnose in areas of higher light or wind exposure (e.g., Chellemi & Britton, 1992); however, in Ontario there was not a noticeable difference: severely affected trees were observed at the edge of forest sites as well as within them.
The University of Guelph Arboretum (Henry Kock, pers. comm.) attempted to limit the movement of nursery stock from the U.S. when dogwood anthracnose was well recognized there but not yet in Canada. Currently there are still no special restrictions on the importation of nursery stock of this species from the U.S., other than routine certification from source and border inspections (Ken Marchant, Forestry Specialist CFIA, pers. com.).
Apparent dogwood anthracnose resistance, while rarely documented, was observed in a few individuals at the Catoctin Mountain Park, Maryland, after the disease swept through killing most of the trees (79% mortality in 1991). One of these trees was cloned and named ‘Appalachian Spring’ (Windham et al., 1998).
The steep decline in the two populations monitored over 11 years, as shown in Figures 3 and 4, suggests that we will likely begin to see extirpation of populations within a few years. From the experience of Dutch elm disease and chestnut blight, it is unlikely that extirpation will be 100%.
For additional consideration, Strobl & Bland (2000) list root knot nematode, defoliators and borers as other pests, but certainly of a minor level of significance compared to dogwood anthracnose. Habitat loss and fragmentation throughout the Carolinian Zone, but especially in the extreme southwest of Essex County and Chatham-Kent, reduce the amount of available habitat and hence its continuity, which is important for re-colonization. While it can not be concluded that habitat is limiting, habitat loss and fragmentation are additional stresses that may exacerbate the impact of the disease, mainly by not allowing as much opportunity for the species to adapt a natural resistance to the disease. This particular threat is reversible through region-wide restoration programs such as Essex County’s Natural Heritage Restoration Program.
Special Significance of the Species
This species, plus its western counterpart Cornus nuttallii, are threatened by dogwood anthracnose throughout their ranges. Both are important as small, ornamental landscape trees, with graceful pagoda-form of tiered branches, flowers with striking floral bracts and brilliant fall colour. Ecologically, Cornus florida is a conspicuous understory tree in southern Ontario forests at flowering time, providing nutrition for pollinating insects in the spring and frugivorous forest birds in late summer.
First Nations used Cornus florida mainly for medicinal purposes. The wood was also used for carving and tools (Moerman, accessed July 2004).
The dense, fine-grained wood was well suited for early industrial applications such as shuttles in mills (Hosie, 1979); no doubt early settlers found similar uses for the wood of this species on the farm and local industry.
Existing Protection or Other Status Designations
NatureServe (accessed May 2006) lists three states that consider this species as critically imperilled (S1): Maine, Vermont and Kansas. Two states have it ranked as S4 (apparently secure): New York and North Carolina. It is listed in 22 states as under review/not ranked, plus six as S5 (secure); North Carolina, recently changed rank from S5 to S4 when an inquiry was made about its condition (Misty Franklin, pers. comm.). In Ontario it is currently listed as S2 (imperiled).
Globally it is listed as:G5 - SecureG5 - Secure G5, secure, “Widespread, still common, but being depleted by a fungus disease.Widespread, still common, but being depleted by a fungus disease.widespread, still common, but being depleted by a fungus disease”, and N5 in the United States. In Canada it is listed as NNR, not ranked (NatureServe, 2007).
Cornus florida L.
Range of Occurrence in Canada: Carolinian Zone/ Deciduous Forest Region of extreme southern Ontario.
Extent and Area Information
approximately the area of the Carolinian Zone
Specify trend in EO
no significant change
Are there extreme fluctuations in EO?
Area of occupancy (AO) (km2)
(1 km2 per site;154 sites, 1975-2005)
about 150 km2
Specify trend in AO
(one site lost to development, several imprecise historical records not found)
no significant change
Are there extreme fluctuations in AO?
Number of known or inferred current locations
About 154 (66 1995-present plus 88, 11-30 yr. old records)
Specify trend in #
Are there extreme fluctuations in number of locations?
Specify trend in area, extent or quality of habitat
Continuing minor decline of area and quality of habitat
Number of mature individuals
Based on estimated 7-8% per annum mortality in the last 3 years of diseased trees out of a total of about 1600 trees estimated as occurring at 154 sites recorded from 1975 to 2005 [about 660 from 57 recently observed sites, 940 estimated from others].
Total population trend:
% decline over the last/next 10 years.
87% decline recorded over 11 years at two sites;
17% mean mortality (~3 years) observed in 32 sites
Are there extreme fluctuations in number of mature individuals?
no, steady decline
Is the total population severely fragmented?
Specify trend in number of populations
Loss of populations imminent with trends in population sizes
only slight decline at this time
Are there extreme fluctuations in number of populations?
List populations with number of mature individuals in each: See Table 2 for detailed summary of 32 sites and 468 live trees. Similar details are not available for 122 sites except for total counts or estimates of numbers.
Threats (actual or imminent threats to populations or habitats)
Dogwood anthracnose is the major threat; first confirmed in Canada in 1998.
Habitat loss, degradation and fragmentation, slow decline as woodlots are cleared for agriculture or converted to housing.
Rescue Effect (immigration from an outside source)
Is immigration known or possible?
Would immigrants be adapted to survive in Canada?
Is there sufficient habitat for immigrants in Canada?
Is rescue from outside populations likely?
No; infrequent dispersal of seeds from the south, but populations there also affected by dogwood anthracnose.
COSEWIC: Endangered( 2007)
Ontario (NHIC): S2
Status and Reasons for Designation
Reasons for Designation: A small understory or forest-edge tree present only as small populations within the fragmented woodlots of southern Ontario’s Carolinian forest. The spread of dogwood anthracnose disease has caused dramatic declines in the Canadian populations that reflect similar declines throughout the species’ range in eastern North America. This assessment of risk applies only to wild populations and not to cultivated plants in nurseries, parks, and gardens.
Applicability of Criteria
Criterion A (Declining Total Population):
Meets Endangered A3e + A4ae since a population decline of >50% is projected over the next 3 generations (30-90 yrs) based on the impacts of a pathogen and as a consequence of having observed and projected a decline of >50% over 3 generations that includes the past and future.
Criterion B (Small Distribution, and Decline or Fluctuation):
Not met. Although the area of occupancy is under the threshold for endangered, the population is not considered to be severely fragmented in view of the ease of dispersal of fruit and seeds by birds and there are no extreme fluctuations in such factors as numbers of populations.
Criterion C (Small Total Population Size and Decline):
Meets Endangered C1 + C2a(i) based on population size estimated as <2500 trees and a significant continuing decline is projected due to the dogwood anthracnose disease with no population having >250 trees.
Criterion D (Very Small Population or Restricted Distribution):
Not met since numbers of trees exceed limits for Endangered and Threatened and the area of occupancy of the numerous sites exceeds 20 km2.
Criterion E (Quantitative Analysis):
Acknowledgements and Authorities Consulted
Mike Oldham provided records from NHIC, including herbarium records from the major Ontario herbaria; Gerry Waldron provided information on sites he was familiar with in Essex and Chatham-Kent and assisted in field observations there; Paul O’Hara provided information on sites he knew of in Halton and Hamilton, and assisted in field observations there and in Niagara; George Meyers provided information on recent observations in Niagara; Richard Wilson (provincial Forest Pathologist) visited several sites with the author and provided confirmations of dogwood anthracnose from collected specimens, as did Chuck Davis; Allen Woodliffe (MNR District Ecologist-Alymer) provided detailed field sightings for Skunks’ Misery; Jane Bowles (University of Western Ontario Herbarium) provided field sightings from her recent field projects; Rob Guthrie provided Ontario Tree Atlas information and generated the map from the provided data points; Henry Kock (University of Guelph Arboretum) provided information on a population he knew of in the Ausable Valley; Mary Gartshore provided observations of early dogwood anthracnose symptoms in Norfolk County; Larry Lamb (University of Waterloo ecologist) provided confirmation that he had searched for the historic Brant County sites and not found them.
Ambrose, J.D. & G. Waldron, 2004. Botanical Inventory and Natural Heritage Assessment of 24 Norfolk County Forests. Bird Studies Canada on behalf of the County of Norfolk.
Anderson, R.L., J.L. Knighten, M. Windham, K. Langdon, F. Hendrix & R. Roncadori, 1994. Dogwood Anthracnose and Its Spread in the South. USDA Forest Service report R8-PR26.
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John Ambrose came to the University of Guelph Arboretum in 1974, after receiving a PhD in Botany from Cornell University. At the Arboretum, in addition to being the Curator, he developed a program based on the rare woody plants of the Carolinian Zone of southern Ontario, including field surveys, status reports and detailed studies of their population and reproductive biology. After 17 years there, he moved to the Toronto Zoo as Curator of Botany/ Manager of Horticulture. There he developed new natural habitat exhibits and a naturalization program for peripheral lands of the site, in addition to his exhibit responsibilities. These reflect his growing interest in restoration ecology. In 1999 he left the Zoo to teach a new course in restoration ecology at the University of Guelph. He currently is self-employed and continues to work with endangered species recovery planning, serving on three recovery teams for Carolinian trees and drafting two ecosystem-based recovery strategies.
Current records from the Natural Heritage Information Centre (Peterborough, Ontario) and files of herbarium records from CAN and DAO were sent by Mike Oldham, and original field records compiled by the author while at the University of Guelph Arboretum were consulted.
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