How green should green roofs be?

This project focuses on extensive (Sedum) green roof nitrogen cycling and the effects of fertilisation on roof ecosystems. Previous studies have established that these ecosystems change significantly with age. In older roofs, mosses and lichens establish and often begin to dominate over the initial plant community, substrate nitrogen concentrations increase, and the amount of nutrients lost in runoff decreases. However, many details about these processes are still unclear. For example, there is very little information on the role of mosses and lichens in roof nutrient cycling, or on what forms of nitrogen actually accumulate in the substrate and whether they are usable by plants. There is also very little information on how fertilisation affects green roofs in the longer term, since most such studies have only used roof boxes over a single growing season.

In order to fill these knowledge gaps, this PhD project will follow roofs of different ages as they respond to fertilisation over several years. I will also keep collecting yearly data on aging roofs and analysing it as a continuation of the green roof project’s previous studies. Finally, I will try to untangle the role of vascular plants (Sedum and Phedimus species), mosses, and lichens in nitrogen processing and fixation, in the hopes that this clarifies how and why nitrogen accumulates in older roofs.