Recovery Strategy for the Common Hoptree (Ptelea trifoliata) in Canada
1. COSEWIC Species Assessment Information
Date of Assessment: November 2002
Common Name: Common Hoptree
Scientific Name: Ptelea trifoliata
COSEWIC Status:
Threatened
Reason for Designation: A species of restricted range in Canada with a small population size occurring primarily along sandy shoreline habitats. It has experienced substantial losses at some sites from cottage land development, damage to habitats by increasing numbers of nesting cormorants and other unknown factors. A newly recognized potential threat of unknown impact is posed by a recently discovered twig-boring beetle, which is causing damage to flowers and large portions of the tree crown.
Canadian Occurrence: Ontario
COSEWIC Status History: Designated Special Concern in April 1984. Status re-examined and up-listed to Threatened in November 2002. Last assessment based on an update status report.
* COSEWIC = Committee on the Status of Endangered Wildlife in Canada.
2. Species Status Information
The Common Hoptree, which has likely always been rare in Canada due to the restricted range of its habitat, is listed as Threatened on both Schedule 1 of the Species at Risk Act (SARA) and the Species at Risk in Ontario List (Ontario Ministry of Natural Resources [OMNR] 2010) under the Endangered Species Act, 2007. It is considered Vulnerable in Ontario (S3) and Canada (N3), although Secure in the United States (N5) and around the globe (G5) (NatureServe 2011). The species is introduced in Québec (Rousseau 1974). Less than 0.2% of the species' range is found within Canada (Little 1976).
3. Species Information
3.1 Species Description
The Common Hoptree (Ptelea trifoliata) is a short-lived[1], deciduous[2], typically dioecious[3] tree that grows up to 10 m high and 24 cm in diameter (Waldron 2003), with a reddish-brown, often branched trunk. Alternate, sharp-pointed, nearly stalkless, compound leaves of three leaflets have smooth to very shallow, toothed margins and a wedge-shaped base (Farrar 1995). Their pungent, citrus odour accounts for its other common name, "stinking-ash". Fragrant, cream-coloured flowers are produced in early summer. The fruit, or samara, containing one or two centrally located seeds, surrounded by a flat, veined wing, is dispersed in late autumn and winter (Waldron 2003). Ambrose (2002) provides additional details.
3.2 Population and Distribution
Key characteristics of the distribution of Common Hoptree in Canada are:
- The species reaches the northern edge of its range in southern Ontario (Figure 1).
- Most populations currently occur in seven, well-separated, core areas: Middle Island, Pelee Island, Essex County (including Point Pelee National Park), Walpole Island First Nation, Rondeau Provincial Park/Erieau, Port Burwell Provincial Park, and the Regional Municipality of Niagara (Figure 2).
- The overall Canadian extent of occurrence is approximately 10 174 km2, while the area of occupancy is estimated at 7.5 km2 (Ambrose 2002).
Figure 1: North American distribution of all Common Hoptree subspecies (Ambrose and Aboud 1984).
Figure 2: Canadian distribution of the Common Hoptree (Note: Not all points represent GPS-level accuracy).
- The species' distribution in Canada is believed to be naturally fragmented, with the two largest core areas occurring at the west (Essex County) and east (Regional Municipality of Niagara) ends of Lake Erie. This pattern of occupation, with only a few intervening populations, likely represents colonization from opposite ends of the lake following retreat of the glaciers 8 000 years ago (Ambrose et al. 1985).
- The species is found mainly in sandy shoreline habitats, which are harsh, dynamic and naturally limited in their availability.
- Thirty-five of 39 known populations are extant (see Table 1). This is a slight increase from the 34 extant locations reported in Ambrose (2002). Of those remaining, one has been transplanted (Long Point National Wildlife Area), one has been extirpated[4] (Linden Beach), and two are considered historic[5] (Niagara Glen/Niagara Gorge and four miles north of Queenston). Many others are believed to be of cultivated origin (see Appendix C). Two populations, believed to have been extirpated (Seacliff Park and Erie Beach), have been rediscovered and new populations continue to be located (Ambrose 2002, OMNR unpub. data).
Since the 2002 COSEWIC assessment, new information has become available. Key characteristics of the population sizes and trends of Common Hoptree in Canada are:
- The estimated 920 to 1 025 mature individuals in Canada (Ambrose 2002) is now believed to be an underestimate.
- A recent, targeted survey in Point Pelee National Park found nearly 16 900 stems of all age classes within a 1.75 km2 area of occupancy (Jalava 2008). Additional trees have since been found along the eastern shoreline (Parks Canada unpub. data). This represents the majority, possibly 96%, of Canadian trees based on previous population estimates.
- The Lighthouse Point population on Pelee Island is believed to be underestimated in size, perhaps by a factor of ten (Woodliffe pers. comm. 2009).
- Available population sizes (Ambrose 2002) for many other populations require updating.
- At least eighteen new locations have been discovered since 1984. While some do represent previously undiscovered populations, none are believed to signify species recovery since the 2002 COSEWIC report. Many expand the known area covered by existing populations.
- Population trends, based on available information, can be summarized as follows:
- In 2002, six populations were shown to have an overall decline of 12%, with reproductive individuals having dropped by 43% across 17 populations (where data were available for comparison), and 60% at Fish Point Provincial Nature Reserve alone over the previous 17 years. When all populations were considered, the decline in the number of mature individuals was suspected to be lower and less drastic than this 43% decline (Ambrose 2002), although this figure was used by COSEWIC to designate the species as Threatened.
- Current information suggests that the Point Pelee National Park mainland Common Hoptree population is likely relatively stable (Jalava 2008), while the Middle Island population increased slightly from 322 to 342 individuals of all age classes between 2000 and 2008 (Ambrose 2002, Jalava et al. 2008). However, substantial Double-crested Cormorant (Phalacrocorax auritis) impacts were noted on Middle Island that are likely to have long term, population level effects (Jalava et al. 2008).
CA=Conservation Authority, NCC=Nature Conservancy of Canada; PP=Provincial Park; RM=Regional Municipality
3.3 Needs of the Common Hoptree
In Canada, the Common Hoptree is limited to extreme southwestern Ontario by climate, growing degree days, and its specialized habitat requirements. The small Canadian population has a restricted area of occupancy (only 7.5 km2), focussed almost entirely along the sandy, well-drained, often xeric[6], disturbed shorelines of Lake Erie (Ambrose 2002). There, it occurs along the leading edge of woody shoreline vegetation, most often in thickets intervening between the beach grass and/or savanna communities and the dry woodland edges, where it is found less frequently (Ambrose et al. 1985, Ambrose 2002, Jalava et al. 2008). On Pelee and Middle Islands, it also grows on limestone-based substrates, including alvars[7] on the former, as well as in the lake-bottom clays and clay-loams of Pelee Island drainage ditches (Ambrose 2002). The hoptree is shade intolerant, showing a significant reduction or absence of flowering and fruiting even in partial shade (Ambrose et al. 1985). Established seedlings can and need to withstand high soil temperatures, high evaporation, drought, low soil nutrients and sand instability (McLeod and Murphy 1983). Flowers require insect-pollination (Ambrose et al. 1985). The samaras or winged fruit may be dispersed by wind, water, or ice rafting. However, seeds tend to establish under or near existing trees and, as such, much apparently suitable habitat remains unoccupied (Ambrose et al. 1985).
1 Ambrose (2002) reported a scarcity of very large individuals and surmised a high turnover within populations.
2Deciduous trees shed their leaves each year.
3 In dioecious plants, male and female flowers are found on separate individuals.
4 An extirpated designation means that the species has been confirmed to no longer exist at a site.
5 An historic designation means the species is previously known from the site, but has not been verified within the last 20 years.
6 Xeric plants require only a small amount of moisture.
7 In the Great Lakes basin, "alvar" refers to naturally open areas with shallow soils over relatively flat, limestone bedrock, with trees absent or at least not forming a continuous canopy (Reschke et al. 1999, Brownell and Riley 2000).