Green roofs have gained recognition and popularity globally for their potential to help mitigate the negative impacts of urbanization such as habitat loss and disruption of the water cycle caused by increased impervious surfaces. However, there is still a need to enhance our understanding of green roof vegetation dynamics and how they affect plant water use and hydrological function under varying environmental conditions. This doctoral thesis aims to address this knowledge gap by applying a wide range methods, including field surveys on full scale gren roofs, a laboratory scale water use experiment, and rainfall runoff monitoring from pilot scale green roofs.

Vegetation surveys on 41 green roofs of varying ages and designs in northern Sweden's cold climate revealed that substrate depth plays a crucial role in supporting greater plant abundance and more species-rich plant assemblages on these roofs. Of the originally intended speceis, 24% were found at the time of surveys whereas spontaneous unintended plant species frequently comprised a substantial proportion (69%) of the species richness on these roofs. No relationship was found between speceis richness and plant cover on the surveyed roofs.

Analysis of Scandinavian green roof vegetation in nine different locations with varying climates revealed that survival rates and covers of the intended vegetation were negatively influenced by low annual temperature. Contrary to the initial hypothesis, high annual precipitation was also negatively related to the survival and cover of intended vegetation. Conversely, spontaneous plants were favored by high mean annual precipitation, compensating for the loss of intended vegetation. Freeze-thaw cycles and longest dry period did not have any detectable effect on vegetation during the two year time period.

Additionally, the thesis explored the potential of spontaneous vegetation as a functional alternative to purposefully planted roofs. While unpredictable, spontaneous vegetation could significantly contribute to the overall ecological function of green roofs, as the spontaneous species found in a speceis survey had complementary life strategies and traits compared to the intended vegetation. The low abundance of most spontaneous species in plant surveys in northern Sweden however, questions their contribution to the hydrological function in that climate.

Growth, leaf traits, and life strategies related to species-specific water use of 10 green roof species was investigated under well-watered and water-deficit conditions in a controlled laboratory setting. Species classified with more competitive or ruderal life strategies were found to display higher water use as compared to stress-tolerant succulent species, and leaf dry matter content (LDMC) was a good indicator of water use for these species. The water use of typical succulent green roof plants (mostly classified as stress tolerators) was the same or lower than the evaporation from the bare substrate and the findings highlighted the potential of considering how species specific traits, life strategies affect plant water use to better understand plants contribution to green roof hydrological function.

Runoff from 34 pilot roof modules (size 2 m²) was measured from rains under natural weather conditions. The impact of four life strategy-based vegetation mixes on green roof hydrological function was assessed and compared to a standard succulent monoculture, non-vegetated bare substrate green roofs, and conventional roofs. All green roof modules, including bare substrates, showed significantly higher stormwater retention compared to conventional roofs. The effect of vegetation type increased with increasing rain volume, and the stress-tolerant strategy based vegetation generally outperformed bare substrates and succulent monocultures, having higher retention and peak flow attenuation.