Assessing Urban Green Roof Species in Singapore’s Green Ecosystem Initiative

Thesis Abstract

Urban green roofs are increasingly deployed in Singapore to mitigate urban heat, enhance biodiversity, and improve stormwater management amid dense tropical urbanization. However, the performance of green roof systems is contingent on species selection, local climate resilience, and maintenance practices, yet empirical guidance for Singapore’s Green Ecosystem Initiative remains fragmented. This study addresses the problem of suboptimal plant assemblages on municipal and commercial green roofs that underperform in biomass production, retention of stormwater, and habitat provision for pollinators, thereby limiting ecosystem service delivery. The aim is to evaluate species performance, ecological compatibility, and service provisioning of green roof plant assemblages within Singapore’s built environment, with specific objectives (i) to classify and compare by-use functional groups (thermophilic, drought-tolerant, and pollinator-friendly species) across 20 representative green roofs; (ii) to quantify growth metrics, evapotranspiration, soil microbial activity, and substrate moisture retention under parkland-derived versus commercially sourced plantings; (iii) to model relationships between species traits, microclimate variables, and ecosystem services using multivariate regression and structural equation modelling (SEM); (iv) to identify barriers to successful establishment and maintenance through qualitative stakeholder interviews; and (v) to formulate an evidence-based guide for species selection aligned with Singapore’s urban resilience goals. The study adopts a convergent mixed-methods design anchored in trait-based ecology and urban ecology theory. The population comprises green roofs within Singapore’s public housing estates, commercial high-rise developments, and governmental campus facilities (n ? 20 sites). A stratified random sample yields 60 plots (3–4 plots per roof) for quantitative assessment, complemented by 25 semi-structured interviews with facility managers, landscape architects, and maintenance crews. Data collection instruments include a standardized plant performance survey, in situ abiotic measurements (soil moisture, substrate temperature, light intensity), biomass harvests where permissible, soil microbial activity assays (dehydrogenase and phosphatase activities), and pollinator visitation logs. Vegetation traits are extracted from the TRY plant trait database and augmented with in situ measurements (species height, leaf area index, osmotic adjustment indicators). Instrument validity and reliability are established through pilot testing (n = 10 plots) and inter-rater reliability checks for biomass estimates. Analytical techniques comprise descriptive statistics, ANOVA to compare performance across functional groups and roof types, multiple regression to identify drivers of evapotranspiration and biomass, and SEM to elucidate causal pathways among species traits, microclimate, substrate properties, and ecosystem services. Thematic analysis of interview transcripts is guided by grounded theory to triangulate quantitative findings with practitioner experiences. Expected findings indicate that thermophilic and drought-tolerant species with shallow-rooted architectures exhibit superior biomass accumulation, substrate moisture retention, and rapid colonization under Singapore’s hot, humid regime. Pollinator-friendly assemblages are anticipated to enhance in-plot visitation by native pollinators, though may require targeted maintenance to sustain establishment. Substrate properties and microclimatic gradients are predicted to mediate species performance, with higher photosynthetically active radiation and elevated temperatures correlating with greater evapotranspiration but increased water stress for non-tolerant species. The study anticipates identifying a core set of 8–12 species that consistently satisfy biomass production targets (? 1.2 kg per plot per season), evapotranspiration goals, and pollinator habitat value, alongside maintenance practices (fertilization regimes, irrigation schedules, and pruning cycles) that optimize service delivery. Contribution to knowledge encompasses (i) a context-specific, operational framework for tree and herbaceous green roof species selection in tropical urban environments; (ii) empirical relationships between plant functional traits and ecosystem services in Singapore’s built environment; (iii) an integrated decision-support model combining regression and SEM outcomes to guide policy and practice; and (iv) a replicable methodological blueprint for similar tropical city-regions undergoing rapid urban greening. The main conclusion is that coordinated species selection, informed by trait-based ecology and site-specific microclimate data, significantly enhances thermal regulation, water management, and biodiversity on Singapore’s green roofs. Recommendations target municipal procurement standards, maintenance scheduling, and the scaling of a standardized plant palette to maximize ecosystem services, with implications for tropical urban sustainability policies and future research on long-term performance under climate variability.