Quantification of Urban Green Roofs Biodiversity under Microclimate Variability

Thesis Abstract

Urban green roofs are increasingly deployed to enhance city-scale biodiversity and ecosystem services, yet their biodiversity outcomes are highly contingent on microclimatic variability within and between roofscapes, which remains poorly quantified. This study addresses the gap by empirically evaluating how microclimate factors influence plant and invertebrate biodiversity on residential and commercial green roofs across a metropolitan region characterized by a gradient of building heights, exposure, and substrate depths. The aim is to quantify biodiversity responses to microclimate drivers and to identify threshold conditions under which green roofs support robust species richness and community composition. Specific objectives include (1) to characterize microclimatic regimes (temperature, humidity, solar radiation, wind exposure) across 40 monitored green roofs over two growing seasons; (2) to quantify plant, arthropod, and soil microbial diversity using standardized plot-based inventories and metabarcoding approaches; (3) to model biodiversity responses to microclimate variables and roof structural attributes using generalized linear mixed models (GLMMs) and structural equation modeling (SEM) to disentangle direct and indirect effects; (4) to assess the influence of substrate depth, roof age, and plantings on biodiversity under varying microclimates; and (5) to derive practical design thresholds for maximizing biodiversity in urban green roofs. The methodology integrates a stratified purposive sampling design targeting roofs with diverse substrate depths (5–25 cm), irrigation regimes, and plant assemblages within a single metropolitan region. Data collection will involve continuous microclimate monitoring (HOBO data loggers for temperature and humidity; pyranometers for photosynthetically active radiation; anemometers for wind speed) and seasonal biotic surveys. Plant biodiversity will be assessed through species richness, Shannon diversity, and functional trait analyses from annual vegetation surveys (n=2000 quadrats across roofs). Invertebrate communities will be sampled using standardized suction and pan-trap methods, while soil microbial diversity will be characterized by 16S rRNA and ITS metabarcoding of soil samples (n=200 samples). Analytical techniques include GLMMs to relate alpha diversity to microclimate metrics and roof covariates, multivariate ordination (non-m metric multidimensional scaling) for community composition, SEM to test pathways from microclimate to biodiversity via substrate depth and vegetation structure, and regression-based species-area considerations to evaluate spatial scaling. Model validation will employ cross-validation and information criteria (AIC/BIC). The study will also apply variance partitioning to apportion the relative contribution of microclimate, roof design, and management practices to observed biodiversity patterns. Theoretical framing will draw on the Niche Theory and the Stress Gradient Hypothesis, with a conceptual model linking microclimate heterogeneity to species coexistence and functional diversity. Expected findings include (i) a positive association between moderated microclimatic variability (moderate temperatures, stable humidity) and higher plant and invertebrate species richness, with hotspots on roofs exhibiting moderate substrate depth and structural complexity; (ii) distinct microclimate-driven community assemblages, evidenced by significant differences in beta diversity across roofs with contrasting microclimatic regimes; (iii) substrate depth and plant functional diversity mediating the strength of microclimate effects on biodiversity, as demonstrated by SEM pathways; (iv) microbial community composition correlating with substrate conditions and plant host diversity, indicating soil-plant-microbe co-structure under urban microclimates. The study contributes to knowledge by providing robust, scalable empirical estimates of how microclimate variability shapes rooftop biodiversity, integrating above- and below-ground communities, and delivering design guidance for urban planners and green roof practitioners. Anticipated recommendations include (a) prioritizing roof installations with diversified microclimates and substrate heterogeneity to maximize biodiversity; (b) guidelines for substrate depth ranges and plantings that optimize habitat complexity under expected urban microclimate scenarios; and (c) monitoring protocols incorporating microclimate metrics and biodiversity indicators for adaptive roof management. The findings are expected to inform policy frameworks promoting biodiversity-responsive urban greening and to advance theory on biodiversity responses to microclimatic stress in built environments.