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Urban Outfitters—Scientists Are Searching for Native Plants Adapted to the Concrete Jungle
Plants & Gardens News | Volume 20, Number 3 | Fall 2005/Winter 2006
by Niall Dunne
An intriguing experiment is under way at the Fresh Kills landfill, on Staten Island, New York. In summer 2005, researchers from Brooklyn Botanic Garden, Rutgers University, and the NYC Department of Parks and Recreation planted a small garden plot here with wildflowers collected from various sites in the New York metropolitan region. They are hoping to identify plant populations that have evolved special adaptations to city life. If successful, they plan to make these plants available to restoration ecologists and gardeners in the Big Apple to help them maximize their chances for planting success.
A seed-collection site for the urban ecotype study in Bayonne, New Jersey. Photo by Tim Chambers.
Anyone who gardens in a city knows that the urban environment is a particularly stressful one, especially for native plants that evolved for thousands of years under quite different conditions. Generally speaking, soils are degraded and polluted, hydrology is highly modified by extensive impervious surface, the climate is warmer, and weeds run riot. And yet some wild native populations manage to survive, hanging on—or even doing well—in parks, brownfields, and abandoned lots and along roadsides, train tracks, and rights-of-way. It was from such highly disturbed sites as these that plant seeds were gathered for the current study.
"One site where we collected was the side of railroad tracks in New Jersey," says Steven Handel, professor of ecology at Rutgers and one of the lead investigators in the study. "Plants there grow in very hot, very dry gravel. There is little in the way of nutrients or water, and no care is given to the plants at all, yet we found wildflowers that were large and full of seed. These plants must have special traits to tolerate a site that would frighten any garden plant."
Urban Versus Rural Ecotypes
Five native meadow species were chosen for the study: Canada goldenrod (Solidago canadensis), white snakeroot (Eupatorium rugosum), switch grass (Panicum virgatum), common milkweed (Asclepias syriaca), and flat-top golden top (Euthamia graminifolia). Seeds from these plants were collected and propagated in the spring of 2005 at the Parks Department's Greenbelt Native Plant Center, on Staten Island, New York.
There were several criteria for seed collection.
A garden plot site at Dreier Offerman Park, near Coney Island, New York. Photo by Tim Chambers.
"We went to the worst sites we could find," says botanist Tim Chambers, seed collector at Greenbelt. "And we collected seeds from plants that showed the most promise for early colonization of disturbed areas. There also had to be at least 10 to 30 individuals of a particular species so we could get enough seed and capture a good cross-section of the population's genetic makeup."
Seed for the five natives was also collected from a rural site in New Jersey and plants propagated from it installed in the test plot at Fresh Kills. For the next three years, Handel and company will compare the performance of the urban-collected plants with that of their country cousins. In their "common garden," all these plants share a soil that's poor in quality and laced with urban debris. If the city slickers thrive and the country bumpkins don't, the researchers will know that they have urban-adapted plant populations—or, more technically, urban ecotypes—on their hands.
Fresh Kills is not the only site hosting the ecotype experiment. Similar plots have been established in a 110-acre urban park near Coney Island, a half-acre roadside site in Brooklyn, and a former industrial brownfield in Edison, New Jersey. Though these locations were chosen to match the urban sites where seed was collected, each presents its own suite of challenges to any prospective colonizer. For example, plants at the roadside site will have to tackle such stresses as intensive runoff, high salinity from deicing salt, and automobile pollution. If, say, a city-born common milkweed has evolved a trait such as salt tolerance, it will have a distinct survival advantage over a common milkweed from the countryside.
There's a good deal of scientific evidence supporting the theory that under extreme environmental conditions, natural selection can take place in plants over relatively short distances and periods of time. New York City is pretty extreme, very big, and has been around for a while, so the researchers are optimistic about finding native plants with urban adaptations.
"People say that New Yorkers have to be tough," quips Handel. "These plants may be the ultimate New Yorkers."
Evolution and the Common Garden
Though the search for urban ecotypes is a recent phenomenon, experiments using common gardens to explore genetic variation within species have a venerable tradition. They were made famous by the Swedish botanist Göte Turesson. In the 1920s, Turesson conducted a number of pioneering studies on the narrowleaf hawkweed (Hieracium umbellatum), a composite plant common in southern Sweden.
In its principal habitats—woodlands, sandy fields, dunes, and clifftops—this hawkweed exhibits distinct differences in morphology (form and structure). For example, plants growing in sandy fields have an extremely prostrate growth habit, whereas those growing in wooded areas are stout and erect.
Turesson wanted to know if these differences were physiological (due merely to variation in nutrient levels, availability of water, and so on) or genetic. So he planted variants of the hawkweed in uniform garden plots to see if the distinctiveness was lost or retained. It was more or less retained, and Turesson interpreted this as evidence that natural selection had worked on local populations of the plant, preserving mutations in their genes that enabled them to adapt to their specific environments. (A low growth habit, for instance, may help dune plants withstand daily battering from high winds.)
Turesson called these locally adapted populations "ecotypes" and identified five of them for Hieracium umbellatum. He went on to describe patterns of habitat-correlated variation in 50 other common European species. His work was very important: Turesson demonstrated that plant species are not uniform entities, but rather groups of individually adapted populations—which is why today's conservation biologists focus just as much on preserving the genetic diversity of individual species as they do species diversity itself.
Stanford botanist Jens Clausen and his associates performed similar ecotype experiments on herbaceous plants in central California in the 1940s. In one study, they transplanted woolly yarrow (Achillea millefolium) from alpine habitat in the Sierra Nevada (where it had low growth form) to the coast to see if it would grow big and lush like the coastal populations did. However, it fared very poorly, demonstrating that for an alpine ecotype, the apparently benign campus of Stanford represents a tough, stressful environment.
In addition to growth form, ecotypic differences within species can include water and nutrient requirements, fertility, flowering time, and toxic metal tolerance. (In the 1960s—before the days of lead-free gasoline—researchers in Durham, North Carolina, found that narrowleaf plaintains, Plantago lanceolata, next to the road were more lead tolerant than ones growing just a few meters away!) Indeed, any trait that's coded by DNA is fair game for natural selection and can become the basis for a new ecotype.
Stock Options
Handel and his associates hope to find many useful traits in their urban ecotypes. Some, such as tolerance for high soil pH (due, say, to cement-leached lime), may not be immediately obvious, and further study will be required to nose them out. Other traits, such as drought tolerance, may have telltale morphological signifiers—like thick leaves—visible to the eye.
"After isolating these ecotypes, we plan to set up an urban plant-material center at Greenbelt to support the restoration of New York's degraded urban ecosystems and enhance its existing native habitats," says Steve Clemants, vice-president of Science at BBG and another lead investigator in the team. Currently, the native plant stock for these kinds of projects is of rural origin (with the exception of Greenbelt, New York's major nurseries are all based in the countryside) and may not be genetically equipped for the job.
Large-scale testing and selection of the plants may also lead to the development of urban ecotypes for New York City gardeners. "This will help them expand the kinds of native plants they can use," says Handel, "and make care of the urban garden a whole lot easier."
Niall Dunne is the editor of Plants & Gardens News.