Ecological function of urban rooftop garden as a green roof in New York City

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Elizabeth Scott, Natural Sciences, Marymount Manhattan College, New York, NY and Matthew Lundquist, Department of Natural Sciences, Marymount Manhattan College, New York, NY

Background/Question/Methods
Urban green roofs play an important role in combating the effects of climate change and improving urban ecosystem function. Green roofs provide cooling through shading and evapotranspiration, reducing the urban heat-island effect, as well as provide a habitat and stopovers for insects, birds, and other organisms. However, green roofs are expensive to retrofit onto buildings and therefore are not evenly distributed throughout cities. Already existing, small rooftop or terrace gardens, not ostensibly designed as green roofs, may provide some of these ecosystem services and could help connect and expand the growing network of green roofs in cities. The aim of this study was to determine the current status of the Lowerre Family Terrace at Marymount Manhattan College as a small intensive green roof in New York City. Plant richness and abundance on the terrace were assessed and soil temperature, air temperature, and light intensity were measured using HOBO data loggers.
Results/Conclusions
We identified a total of 196 plants consisting of 24 different species in fall 2020. Soil temperatures were measured September 9-21 and averaged 18.91 ± 1.02 oC. Terrace soil temperatures were significantly lower than soil temperatures reported in NYC during the same period, which averaged 21.45 ± 0.97 oC (Paired t-test, t = -13.79, df = 12, P < 0.001). Air temperature measured October 7-21 averaged 21.06 ± 0.20 oC. This was marginally, but significantly higher than the average daily max of 19.93 ± 0.75 oC measured for NYC during that same period (Paired t-test, t = 2.17, df = 14, P = 0.048). Light intensity was also measured from October 7-21 and averaged 73.26 ± 21.36 lux. The high level of variation in average daily light intensity may have been due to the various locations and heights of surrounding buildings. The biodiversity of plants on the Lowerre Family Terrace may provide habitat space for urban organisms, and it has potential for cooling. Small rooftop gardens, with some targeted improvements, may be able to provide similar benefits to green roofs and expand the important network of green roofs and green spaces in cities.