Assessing Urban Green Roofs for Heat Mitigation and Biodiversity Gains

Thesis Abstract

Urban heat islands and biodiversity loss in dense cityscapes necessitate scalable ecological interventions; green roofs offer a multifunctional approach to mitigate warming while supporting urban biodiversity, yet empirical evidence on their effectiveness across urban contexts remains uneven. This study aims to quantify heat mitigation and biodiversity gains provided by urban green roofs and to identify the environmental and design factors that maximize these benefits. Specific objectives are (1) to quantify surface temperature reductions attributable to green roof installations relative to conventional roofs across 20 buildings in a mid-sized metropolitan area; (2) to assess changes in microclimate (air temperature, relative humidity, and wind patterns) within street canyons adjacent to green roofs; (3) to evaluate biodiversity outcomes by surveying plant, insect, and bird communities on green roofs using standardized transects and pitfall traps; (4) to analyze how roof substrate depth, plant functional groups, irrigation regimes, and roof age influence thermal performance and biotic richness; (5) to synthesize findings within a theoretical framework to inform urban planning guidelines. The study adopts a mixed-methods, explanatory sequential design. The population comprises all publicly and privately owned green roofs in the selected city, with a stratified sample of 20 sites representing a range of substrate depths (60–150 mm), plant assemblages (native vs. non-native species), irrigation regimes (seasonal vs. supplemental), and roof ages (2–15 years). Data collection instruments include high-resolution thermal imaging cameras and infrared sensors for surface temperature (daily scans over 12 months), automated weather stations for microclimate variables, standardized biodiversity sampling protocols (vegetation surveys, pan and pitfall traps for invertebrates, and point counts for birds) conducted quarterly, and a roof design audit checklist capturing substrate depth, drainage, and vegetation type. Validity and reliability are ensured through pilot testing of environmental sensors, inter-observer calibration for biodiversity surveys, and the use of established indices such as the Biotic Integrity Index and Shannon-Wiener diversity index. Data analysis involves (i) repeated-measures ANOVA and mixed-effects models to assess thermal performance across roofs and over seasons; (ii) regression analyses to link substrate depth, irrigation, and plant functional traits to temperature reductions; (iii) generalized linear models to model species richness and abundance of plants, insects, and birds as functions of roof characteristics; (iv) multivariate ordination (non-metric multidimensional scaling) to explore community composition shifts; and (v) thematic analysis of qualitative design-notes to contextualize quantitative results within policy-relevant framing. The theoretical framework integrates the Urban Ecology Theory and the Resource-Availability and Niche Theory to explain how structural design and resource provision influence thermal dynamics and biotic assemblages. Expected findings indicate statistically significant surface temperature reductions on green roofs compared with conventional roofs, with greater cooling observed on deeper substrates and with native, drought-tolerant plant assemblages. Adjacency effects on microclimate are anticipated to show cooler street-canopy temperatures in proximate blocks, particularly during summer heatwaves. Biodiversity outcomes are expected to reveal higher plant diversity and increased colonization by pollinators and aerial insectivores on roofs with greater heterogeneity in plant structure and larger, contiguous green patches, though invasion by non-native species may occur in some contexts. The study aims to disentangle the relative contributions of substrate depth, irrigation, and plant composition to both thermal performance and biotic outcomes, offering evidence-based thresholds for design decisions. The contribution to knowledge lies in providing a robust, context-specific evaluation of green roofs as dual-function mitigation and biodiversity infrastructure within urban landscapes, bridging a gap between engineering performance metrics and ecological responses. The research will yield practical guidelines for policymakers, planners, and developers on optimal substrate depths, irrigation strategies, and vegetation schemes to maximize cooling effects and biodiversity benefits, as well as a methodological framework for future comparative urban studies. The main conclusion is expected to underscore the importance of integrated design, where structural and ecological considerations are co-optimized to deliver consistent heat mitigation and habitat provision across urban typologies. Recommendations include adopting standardized performance indicators for green roof certification, promoting native-dominated planting palettes, and incentivizing retrofits for older rooftops to enhance resilience against rising urban temperatures and biodiversity pressures.