An international team of plant explorers track down rare hawthorns thriving in the suburban wild.
A small natural area surrounded by restaurants and hospitals; behind a row of backyards in a suburb; next to hole five on a golf course—these are not locations generally considered when looking for rare trees. However, this is where our group found itself looking for a hawthorn in London, Ontario, Canada this past September.
Our species of focus was possibly one of the more cryptic taxa on the target list of the Arnold Arboretum’s Campaign for Living Collections—a decade-long initiative to add nearly 400 taxa of high conservation, research, and horticultural value to the arboretum’s living collections. True to a taxonomically complicated genus, Crataegus perjucunda (pearthorn or Middlesex frosted hawthorn) presents something of a mystery. Originally collected by D.W. Beadle in London, Ontario in 1903, and first described by Charles Sargent in a 1908 treatment, “Crataegus in Southern Ontario,” it is presumed to be endemic, and known only from a few voucher locations in Middlesex County, Ontario. The species received reinvigorated interest in the 1970s from James Phipps, an authority on the genus. Working with graduate student Quinn Sinnott, Phipps identified and collected voucher material in Ontario in 1977 as part of an analysis of Crataegus series Pruinosae in the region. Their research identified C. perjucunda as a distinctive entity, which, despite growing alongside other Crataegus species and thus theoretically subject to hybridization, reproduces “true to form.” Currently, Flora of North America treats C. perjucunda within its closest congener, C. cognata, but as a “local species” chiefly differentiated by shorter, finer thorns and wider leaves. That it exists alongside multiple congeners but has remained readily distinctive over the decades suggests that it is a local apomict, reproducing from seed without fertilization. Its listing as “Critically Endangered” by NatureServe, and the need for germplasm to be included in collections for ex-situ conservation, prompted its inclusion in the Campaign for Living Collections list, and our subsequent quest to re-survey it.
In the summer of 2023, the Arboretum’s assistant curator, Miles Schwartz Sax, spearheaded an effort to re-survey Phipps and Sinnott’s 1977 herbarium voucher locations. An ambitious undertaking like the Campaign for Living Collections encourages people and institutions to pool expertise in the interest of plant conservation. Reflecting shared institutional interest in documenting the botanical biodiversity of Ontario as well as the genus Crataegus, our group came together from the Arnold Arboretum, the University of Toronto, the University of Guelph Arboretum, and Royal Botanical Gardens, Canada. We spent weeks communicating by email as we consulted herbarium specimens, acquired permits, set dates, and coordinated field supplies.
Armed with herbarium presses and pole pruners, we began our short trek away from the sounds of the city.
On a sunny Saturday morning at the end of September, the four of us met in a parking lot near the most promising set of coordinates to re-survey, in the Westminster Ponds Conservation Area, a roughly 200-hectare reserve established by the Upper Thames Conservation Authority and the City of London in the 1970s. Armed with herbarium presses and pole pruners, we began our short trek away from the sounds of the city and toward the spot where the species was last seen in 1977. We took in the unique ecology of the area, walking past kettle ponds—created from remnant ice blocks left by retreating glaciers thousands of years ago—and through low forests of red maple and bordering swamps. Nearing our first point west of Spettigues Pond, the upland forest composition offered a beautiful example of secondary succession, where thinning was creating gaps in an overstory rich with oaks, maples, beeches, and hop-hornbeams, providing resources for a burgeoning understory.
When we were nearly on top of our first coordinates, we found a Crataegus immediately off the trail, tucked decidedly between canopy and understory. While this individual fit the distinguishing traits for the species, we decided to mark it and continue to scout for other congeners in the area. Hiking past a kettle pond and up a hill, we chose paths near the forest edge, looking for ideal hawthorn habitat. Sure enough, we soon saw another Crataegus, followed by another, and then three more. In true Crataegus form, all looked slightly distinct but not distinct enough to avoid confusion. While this presents an absolute dream for some botanists, it is a nightmare for others. After much measuring of thorns and leaves, consulting of keys, and discussing the details of Crataegus series Pruinosae, we returned to our original coordinates with greater confidence in our identification.
We took in the individual before us, right next to the edge of the trail. While unremarkable at first glance, it appeared to be quite old, with the slow, gnarled growth typical of Crataegus marking difficult growing conditions in dry Berrian sandy loam soils. The base of the trunk measured only 10.3 cm in diameter, the old gray-brown bark flaking off in irregular patches. It was extraordinary to think how many thousands of people had passed by it during the past half-century, not recognizing the special tree only a few feet away. We began to gather collection notes and herbarium vouchers, occasionally being passed by community members volunteering their Saturday to pick up trash throughout the natural area.
As we collected data and vouchers from our first specimen, we spotted another Crataegus that fit the description of our target—full of pinkish-red fruits, just six meters deeper into the forest. Although both trees looked like they could be over 50 years old, their canopies averaged out to just under four meters tall and wide. Crataegus taxonomy may be brutal, but lucky for us, their shorter height presents less of a challenge when sampling in the field (so long as you mind the thorns). We set to work with our pole pruners, collecting and counting seed carefully so as not to over collect and risk decimating the species we were there to preserve. Three branch samples were collected and field-pressed from each tree, with herbarium vouchers to be dried and frozen by staff at Royal Botanical Gardens, Canada. One voucher from each tree would be retained at the herbarium of Royal Botanical Gardens, Canada, with the other sets sent to the Arnold Arboretum, as well as the Royal Ontario Museum herbarium to reside along with those from the 1977 Phipps collection. Additionally, small lots of fruit were bagged for later processing and division between the Arnold Arboretum, the University of Guelph Arboretum, and Royal Botanical Gardens, Canada, with the goal of including this species in the living collections of these organizations.
Re-surveying and determining a species’ absence provides critical information to calibrate future conservation efforts.
Following our successful collection at Westminster Ponds and still riding the high of fieldwork, Jon Peter mentioned that he had contacted the golf course where one of our remaining coordinates was located—squarely on the green of hole five. While the tree was unlikely to be present at that exact location, hole five bordered on a large expanse of forest leading down to the Thames River. Deciding it was worth a look, we met with an employee at the pro shop, who, intrigued by our story, chaperoned us via club car to our coordinates. After waiting for play to end, we trampled through the overgrown wood’s edge in our dirty field gear, catching a few bemused looks from club members rolling by in carts and starched shirts. In the edge habitat between the course and deep woods we spotted a lone Crataegus—but not our target species. A similar search ten minutes away, at a set of coordinates next to a ravine in a recently built suburban development, also turned up dry.
Although unsuccessful at the golf course and in the depths of suburbia, we had been able to re-survey three of Phipps’ and Sinnott’s original 1977 collection locations, including Westminster Ponds. While one out of three may not seem like a great result, re-surveying and determining a species’ absence provides critical information to calibrate future conservation efforts.
The importance of time spent in the field may be measured most readily in physical results like herbarium vouchers and seeds. Less tangible but equally important is the activation (or reactivation) of threads between plant research, conservation, and horticultural institutions to continue conservation efforts. Beyond sharing vouchers and germplasm, ideas are generated, information is disseminated, and the network of people focused on a species is continually widened.
Trips such as this also bring to our attention the importance of urban conservation areas—even small ones—as critical refugia. London is one of Canada’s fastest growing metropolitan areas and lies within the most species-rich region in the country. A small city of 125,000 in 1950, over the past 70 years the population of London, like many other cities its size across North America, exploded to over 422,000 by 2021.
With ever-expanding urban development, protection of lands like the Westminster Ponds Conservation area is of the utmost importance, not just for the multitude of benefits to humans, but for the many species that rely on these relict refugia in now highly developed spaces. Crataegus, just one of hundreds of plant genera that call southern Ontario home, provides food and shelter for many species of birds and mammals. Hawthorn flowers provide food for nectar-feeding insects, and the trees serve as important host plants for over 130 species of caterpillars, making Crataegus a critical, keystone genus in eastern temperate forests. While its taxonomy may often be cryptic and confounding, this diversity is critical to us and the many other species that rely on it, and it is well worth the challenge to conserve.
Sarah Shank, Arnold Arboretum; Jon Peter, Royal Botanical Gardens, Canada; Melanie Sifton, Institute of Forestry & Conservation, University of Toronto; Sean Fox, University of Guelph Arboretum.