Ecology and Physiology of Green Roof Plant Communities.
Abstract: Green roofs mitigate many negative environmental effects of
urbanization, especially stormwater runoff and the urban heat island effect. There is also
potential for green roofs to function as islands of biodiversity within urban and suburban
environments. Historically most roofs have been planted with
Sedum, a very stress-tolerant plant, but many people are
promoting the planting of... read morea more diverse set of plants, especially native plants. The
performance of other species has been mixed and this necessitates greater focus on both
patterns and mechanisms of plant growth and survival. In this dissertation, I began by
reviewing and analyzing rationales for preferring native plants on green roofs. I
identified 113 green roofs planted with native plants and 89 scholarly papers that promoted
this practice. Scientific arguments were commonly used, but rarely tested experimentally. I
then conducted a rooftop experiment to assess suitability of 19 native and non-native plant
species. Summer water deficit resulted in high mortality of all but the most popular green
roof species: Sedum (Crassulaceae). To determine if
Sedum's high performance was due to photosynthetic plasticity,
I grew Sedum under wet and dry conditions in a greenhouse.
There was variation in photosynthetic pathway among the eight species tested, including
examples of C3, CAM-cycling, and CAM-idling. Furthermore, several species exhibited rapid
switching in photosynthetic pathway in response to short-term changes in water
availability. Finally, I tested the hypothesis that Sedum
species would reduce peak soil temperature and increase performance of neighboring plants
during summer water deficit. During a three-year experiment on the Tisch Library Green
Roof, Sedum species decreased peak soil temperature by 5 - 7
°C. Overall, Sedum reduced neighbor growth during wet periods,
but increased neighbor performance during summer water deficit. The results of this
dissertation suggest that plant diversity on green roofs is constrained by abiotic stress,
especially summertime water deficit and heat. Many Sedum
species used on green roofs have high photosynthetic plasticity, which may explain their
success as green roof plants. The palette of green roof plants can be expanded by using
Sedum species as nurse plants.
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Biology.
Advisor: Colin Orians.
Committee: Frances Chew, George Ellmore, and J. Michael Reed.
Keywords: Ecology, Biology, and Environmental science.read less