Spontaneous Plant Colonization of Newly Established Green Roofs: An Experimental Approach

Urban Heat Island Effect (UHI) increases heat risks in densely developed environments. Ecosystem services provided by green spaces are known to mitigate UHI. Green roofs, designed as novel ecosystems, transform less-utilized spaces like rooftops into functional areas, executing ecosystem functions in densely populated urban environments where traditional green spaces are less common. Research has examined ways to reduce the obstacles of implementing green roofs to maximize accessibility and efficiency of the performed ecosystem services. Research involving plant community dynamics found that maximizing biodiversity on green roofs enhances the ecosystem services provided. Utilizing spontaneous colonizing species, or species that are not intentionally planted, is a low-cost planting strategy found to increase the functional biodiversity of green roofs. This project utilizes twelve 2.4 m raised bed units in the student-constructed Green Roof Experimental Lab (GREL) at The University of Tennessee, Knoxville to assess the occurrences of spontaneous vegetation at two substrate depths (5cm and 15cm) during the Summer of 2023. The unit’s substrate depth was varied by dividing each of the twelve units into equal quadrants (46 x 56 cm) and lifting the substrate to the proper height with foam to assess substrate depth’s role in spontaneous species community composition. The spontaneous plant community included 55 individual plants belonging to 10 different species. Neither depth was found to statistically differ in their final plant communities. However, community development over time differed between the two depths indicating differing growing conditions and interactions with substrate depth driving the more rapid formation of plant communities in shallower 5cm substrate before the 15cm quadrants. The measured difference of Fractional Green Canopy Cover (FGCC) was not found to be significant between the two depth’s communities. These results provide the foundations for further research into the utility of spontaneous green roof vegetation as a planting strategy in shallower extensive level substrate depths in Knoxville, TN. Future work should investigate various physical management strategies as well as abiotic and biotic interactions to further explore the complex interactions influencing spontaneous plant colonization on green roofs.