Green roofs have emerged as effective stormwater management systems, but understanding the contribution of their various components to hydrological performance is crucial for optimizing their design and implementation. More empirically measured data on the hydrological function of green roof vegetation is needed, especially under realistic low-maintenance, non-irrigated scenarios. Further, targeted, evidence-based plant selection based on ecological theories may improve green roof hydrological performance. Previous research has suggested that, in contrast to monocultures, mixtures of species with complementary traits could optimize provisioning of various ecosystem services. Thus, species mixtures based on their adaptive life strategy using the CSR theory (Competitor, Stress tolerator, and Ruderal) were hypothesized to have better hydrological performance than a Sedum monoculture or bare substrate under natural conditions over multiple seasons. To test this hypothesis, the runoff from thirty 2 m2 green roof modules was measured. The retention and detention performance of different green roof treatments were evaluated for 84 precipitation events of varying rain depth and intensity during snow-free periods. Differences in retention as well as detention between the vegetation treatments varied, but generally increased with increasing rain event volume and the Stress-tolerant treatment generally performed better than bare substrate. On a mean event basis, the mixture of stress-tolerator species demonstrated a 74 % retention rate, while the Bare substrate retained 72 % of the rainfalls. Overall, the green roofs, including bare substrate and vegetated treatments, effectively retained >50 % of the cumulative precipitation depth. In line with previous studies, the Sedum monoculture generally showed worse hydrological performance than other non-succulent vegetation mixtures, despite its relatively high cover and survival. The vegetated treatment with the highest species richness and diversity in life strategies (Mix) did not provide the best vegetation cover, or hydrological performance. Instead, the Stress-tolerant treatment, characterized by the high survival rate of a single graminoid species, consistently demonstrated superior event-based stormwater retention and peak attenuation capabilities.

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.

The vegetation layer contributes to multiple functions of green roofs including their hydrological function as plants remove water from substrates between rainfall events through evapotranspiration, restoring the green roofs storage capacity for rainfall retention. While individual traits have been related to water use strategies of green roof plants, these traits are inconsistent, suggesting the importance of trait combinations which may be reflected in CSR (competitor, stress tolerator, ruderal) strategies. Therefore, relating plant water use to leaf traits and CSR strategies could help facilitate green roof plant selection into new geographical regions where green roof technology is developing. For example, in high latitude northern European regions with long daylight during the growing season. Growth (shoot biomass, relative growth rate and leaf area), leaf traits (leaf dry matter content, specific leaf area and succulence) and CSR strategies were determined of 10 common European green roof plants and related to their water use under well-watered (WW) and water-deficit (WD) conditions. All three succulent species included in the experiment showed mostly stress tolerant traits and their water loss was less than the bare unplanted substrate, likely due to mulching of the substrate surface. Plants with greater water use under WW conditions had more ruderal and competitive strategies, and greater leaf area and shoot biomass, than species with lower WW water use. However, the four species with the highest water use under WW conditions were able to downregulate their water use under WD, indicating that they could both retain rainfall and survive periods of water limitations. This study indicates that, for optimal stormwater retention, green roof plant selection in high latitude regions like northern Europe, should focus on selecting non-succulent plants with predominantly competitive or ruderal strategies to make the most of the long daylight during the short growing season.

Green roofs have the potential to provide socio-ecological services in urban settings that lack vegetation and open space. However, implementation of green roofs is limited by high construction and maintenance costs. Consequently, green roof projects often disproportionately benefit wealthy communities and can further marginalise disadvantaged communities by increasing property values and housing costs. Vegetation cover on green roofs is crucial to their provisioning of socio-ecological services. Evidence suggests that green roof plantings change over time, especially with limited maintenance, and are replaced with spontaneous “weedy” species. This is often perceived as a failure of the original green roof design intent and spontaneous species are usually removed. However, where good coverage is achieved, spontaneous vegetation could provide beneficial services such as stormwater mitigation, habitat provision, and climate regulation. While social norms about “weediness” may limit the desirability of some spontaneous species, research suggests that their acceptability on green roofs increases with coverage. As spontaneous species can establish on green roofs without irrigation and fertiliser, reduced input costs could help facilitate adoption particularly in markets without an established green roof industry. Construction costs may also be reduced in hot and dry climates where deeper substrates are necessary to ensure plant survival, as many spontaneous species are able to colonise shallow substrates and can regenerate from seed. If implemented based on socio-ecological need, green roofs with spontaneous vegetation coverage may apply less pressure to property values and housing costs than conventionally planted green roofs, increasing the resilience of urban communities while limiting gentrification.

Standard succulent vegetation mixes developed mostly in temperate climates are being increasingly used on green roofs in different climate zones with uncertain outcome regarding vegetation survival and cover. We investigated vegetation on green roofs at nine temperate, cold, and/or wet locations in Norway and Sweden covering wide ranges of latitude, mean annual temperature, annual precipitation, frequencies of freeze-thaw cycles, and longest annual dry period. The vegetation on the roofs were surveyed in two consecutive years, and weather data were compiled from meteorological databases. At all sites we detected a significant decline in species compared to originally intended (planted/sown) species. Both the survival rate and cover of the intended vegetation were positively related to the mean annual temperature. Contrary to a hypothesis, we found that intended vegetation cover was negatively rather than positively related to mean annual precipitation. Conversely, the unintended (spontaneous) vegetation was favoured by high mean annual precipitation and low mean annual temperature, possibly by enabling it to colonize bare patches and outcompete the intended vegetation. When there is high mortality and variation in cover of the intended vegetation, predicting the strength of ecosystem services the vegetation provides on green roofs is difficult. The results highlight the needs for further investigation on species traits and the local factors driving extinction and colonizations in order to improve survivability and ensure a dense vegetation throughout the successional stages of a green roof.

Green roofs are complex systems, with a vegetation layer covering the outermost surface of the building shell. An effective design may confer environmental and energy benefits. Most field studies evaluating green roof performance have been conducted in warmer climates with few studies of full-scale green roofs in cold regions. No study has so far evaluated the energy performance of a green roof in a sub-arctic climate. This study demonstrates the heat flow and thermal effect of an extensive green roof versus a black bare roof area on a highly insulated building in the sub-arctic town of Kiruna, Sweden, for the period from November 2016 to February 2018. Measured temperature and heat flux values were consistently higher and more variable for the black roof than the green roof, except during the snow-covered winter months when the responses were similar. The cumulative heat flux showed that the net heat loss was greater through the black than the green roof, but the values remained low. Overall, the study confirms that the energy benefit of a green roof on a highly insulated building in a subarctic climate is low.

Both vegetation abundance and community composition play important roles in functions of green roofs (e.g. stormwater retention, habitat provision, aesthetic appearance). However, green roofs’ vegetation, and hence their functions, can change significantly over time. More understanding of these changes is required, particularly in cold climates. Therefore, this study investigated vascular plant covers and species compositions on 41 roof sections located in Sweden’s subarctic and continental climate zones. For the roof sections with a known originally intended vascular plant composition (n = 32), on average 24 ± 9% of the intended species were detected in surveys, and unintended species accounted for 69 ± 3% of the species found. However, most colonizing species formed sparse cover on the roofs. Thus, they may make less contributions to green roofs’ potential functionalities related to vegetation density (e.g. social perception, effectiveness in stormwater management and thermal performance) than the intended vegetation. The intended species dominated plant cover (93 ± 3%) and Sedum acre (58 ± 36% cover) was the most commonly detected species and as found in previous studies, substrate depth was positively related to both plant cover and species richness. Contrary to a hypothesis, the roofs’ vascular plant cover was not related to species richness but was significantly and negatively correlated with moss cover. The results highlight the importance of substrate depth for both plant abundance and species diversity and show that even in a cold climate, colonizing unintended species can strongly contribute to green roofs’ species richness.

A dense vegetation cover is usually desired to fulfil aesthetical expectations of green roofs, and therefore stress tolerant Sedum vegetation has traditionally been favoured for extensive green roofs. However, Sedum species’ low water use and low root biomass could prove suboptimal for the hydrological function of green roofs compared to vegetation’s with different resource use. This study looks at the hydrological performance of four different vegetation mixtures grouped based on Grime’s C-S-R life strategies a Sedum monoculture and a non-vegetated control. Runoff from seven rainfall events (3.4–8.4 mm) was recorded during one autumn season when temperatures were getting lower (6–13˚C) The results showed no relationship between vegetation cover and retention, and the roofs planted with a stress tolerant mixtures of species showed the greatest overall retention. Roofs planted with Sedum monoculture had the greatest vegetation cover but the lowest mean retention.

Une couverture végétale dense est habituellement recherchée pour répondre aux attentes esthétiques associées aux toitures végétalisées. En conséquence, les plantes tolérantes au stress de genre Sedum ont été traditionnellement préférées pour les toitures végétales extensives. Cependant, la faible consommation en eau et la faible biomasse racinaire des Sedums pourraient s’avérer sous-optimales pour la fonction hydrologique de ces toitures en comparaison à une végétation qui aurait des besoins différents. Cette étude s’intéresse aux performances hydrauliques de quatre mélanges de plantes groupés suivant la théorie des stratégies CSR de Grime, d’une monoculture de Sedum et d’un toit de contrôle non végétalisé. Les débits de ruissellement provenant des différentes toitures ont été mesurés et enregistrés durant 7 pluies (3.4–8.4mm) d’automne (températures entre 6 et 13 ˚C). Les résultats n’ont pas mis en évidence de relations entre la couverture végétale et la rétention hydraulique des toits. Les toits végétalisés avec des mélanges de plantes tolérants au stress avaient, dans l’ensemble, la plus forte rétention hydraulique. Les toits végétalisés avec la monoculture de Sedum avaient la plus haute couverture végétale mais la plus faible rétention hydraulique.