Between a block and a yard place: studying urban plant communities

Street tree plantings in New York City.
Image from NYC Dept of Parks and Recreation

Urban ecology applies ecological principles that biologists typically conceptualize “out there” (in restored or protected natural areas) to cities and urban areas. In green spaces like gardens, lawns, flower beds, green roofs, and curbsides, native as well as nonnative plant and animal species can thrive and coexist with human development. Limited research has been done in urban ecology (Lepczyk et al., 2017), so foundational studies characterizing what sorts of organisms are living in urban environments are important in establishing our baseline understanding of urban plant and animal communities.

One such descriptive study of an urban green space was conducted over five years by researchers at St. John’s University and Lehman College in New York (Stalter & Rachlin, 2018). The researchers identified all the herbaceous plants in 1,000 sidewalk “plots” (green areas between sidewalks, see right image) that experience road salting and are regularly visited by dog walkers. Each plot was originally planted with one tree and no herbaceous plants, so all of the studied vegetation was introduced by birds, wind, humans, or vehicles--not planted by the city or neighborhood. The authors conducted multivariate statistical analysis to describe differing community structure of plots in Brooklyn compared to plots in Queens. They identified a total of 121 plant species, 69 percent of which were non-native (including three plants associated with humans: marijuana (Cannabis sativa), petunia (Petunia X atkinsonia), and watermelon (Citrullus lanatus)). 

Summary of the vascular flora present at the two sites studied over all five years
(from Stalter & Rachlin, 2018)

The researchers observed divisions between the Brooklyn and Queens plant communities, including that species richness was greater in Brooklyn (84 taxa) than Queens (71 taxa), possibly because two plots in Brooklyn were mulched, watered, and fertilized by residents (creating a more favorable growing environment) or because Queens plots were treated with Roundup twice each summer for four years of the study (creating a less favorable growing environment). 50 percent of the species they identified were only observed 1-3 times in 5 years—the structure of these communities is ever-changing and diverse. The authors state that the plants “occupy an ephemeral foothold surrounded by a sea of concrete and asphalt” (Stalter & Rachlin, 268). The findings of this study suggest that even tiny green spaces in cities can support a large diversity of non-cultivated plant species and that human actions like fertilizing, applying herbicides, and watering these areas can affect the plant communities there.

Polygonum arenastrum. Image from Queensland Government.

One of the most common species in this study was Polygonum  aviculare, which is closely related to Polygonum arenastrum (see right image), an invasive species that a fellow Grinnell College student and I have been studying on our campus in Grinnell, Iowa. We are investigating the distribution of P. arenastrum in relation to sidewalks after noticing that the plant grows along the border of nearly every sidewalk on campus. We suspect that the higher level of disturbance from foot traffic, snow plowing, and de-icing directly next to the sidewalk, compared to the grass a few meters away from the sidewalk is the mechanism behind P. arenastrum’s distribution. We think the water runoff from the sidewalk may also be creating greater access to water for the plants, so we are measuring the incline right next to the sidewalk (assuming that more steep inclines facilitate greater runoff). We are intrigued to see if human-caused disturbance will alter the urban, non-cultivated plant community on our campus in the same way it seems to have in New York.

References

Lepczyk, C. A., Aronson, M. F. J., Evans, K. L., Goddard, M. A., Lerman, 

S. B., & Macivor, J. S.(2017). Biodiversity in the City: Fundamental Questions for Understanding the Ecology of Urban Green Spaces for Biodiversity Conservation. Bioscience, 67(9): 799-807.

Stalter, R., & Rachlin, J. (2018). The vascular plant species of sidewalk plots in Brooklyn and Queens: New York City's overlooked “island” flora. Journal of the Torrey Botanical Society, 145(3), 263-270.