Impact of Urban Pollution on the Growth and Photosynthesis of Common City Trees

Thesis Abstract

Urban environments are increasingly characterized by elevated levels of pollution, which pose significant challenges to the health and sustainability of city green spaces, particularly common street and park trees. This study investigates the impact of various urban pollutants—including particulate matter, nitrogen oxides, sulfur dioxide, and heavy metals—on the growth parameters and photosynthetic efficiency of selected common city tree species. The primary aim is to elucidate how air and soil pollution influence physiological and morphological aspects of urban trees, thereby contributing to urban ecosystem resilience and environmental planning. The specific objectives are to (1) quantify pollution levels at multiple urban sites, (2) assess the growth indicators of selected tree species across pollution gradients, (3) evaluate the photosynthetic pigments, rates, and stomatal conductance in relation to pollution intensity, and (4) explore the relationship between pollution exposure and tree vitality. The study adopts a cross-sectional comparative design, integrating both quantitative field measurements and laboratory analyses. The population comprises street and park trees of species such as Ficus benjamina, Mangifera indica, and Delonix regia within a metropolitan area with variable pollution zones. A stratified random sampling technique was employed to select 150 individual trees equally distributed among the different pollution zones. Data collection instruments include portable air quality monitors for real-time assessment of pollutant concentrations, standard increment borer tools for measuring radial growth, and a portable photosynthesis system for recording gas exchange parameters. Additional data on soil pollutants and moisture content are obtained through laboratory analysis of soil samples. Validity and reliability of measurements are ensured through calibration of instruments, standardized procedures, and repeated measures. Data analysis utilizes descriptive statistics to profile pollutant levels, and multiple regression analysis, including stepwise regression, is conducted to determine the relationships between pollution variables and tree growth and photosynthetic parameters. Analysis of variance (ANOVA) tests are employed to compare means across pollution zones, while structural equation modeling (SEM) explores causal pathways. The theoretical framework underpinning the study includes the Stress Physiology Theory, which explains plant responses to environmental stressors, and the Urban Ecology Theory, which contextualizes tree adaptation in city landscapes. Expected findings suggest a negative correlation between pollution levels and growth metrics such as diameter at breast height (DBH), height, and canopy size, along with diminished photosynthetic activity evidenced by reduced chlorophyll content, lower net photosynthesis rates, and decreased stomatal conductance. The study anticipates that different tree species vary in sensitivity to pollutants, with some exhibiting adaptive physiological responses. These findings will fill notable gaps in existing research by providing detailed, species-specific data on the physiological impacts of pollution in urban trees, and by identifying key environmental thresholds associated with decline in tree health. The contribution to knowledge lies in enhancing understanding of the mechanistic effects of urban pollution on tree vitality, informing species selection for urban forestry, and guiding pollution mitigation policies. The main conclusion emphasizes the necessity for integrated urban planning that incorporates pollution control and tree conservation strategies. Recommendations include developing pollution-resistant tree planting schemes, implementing regular monitoring of urban air quality and tree health, and promoting green infrastructure that buffers pollution exposure. The study also advocates for further longitudinal research to examine the long-term effects of pollution stressors on urban biotic components and ecosystem services, thereby supporting sustainable urban development practices.