Effects of Urban Pollution on Native Tree Species' Growth and Reproduction

Thesis Abstract

Urbanization and associated environmental pollution pose significant threats to native tree species, impacting their growth and reproductive success, which are vital for maintaining biodiversity and ecological stability in urban ecosystems. This study aims to assess the effects of various pollutants—specifically airborne particulate matter, nitrogen oxides, sulfur dioxide, and heavy metals—on the growth parameters and reproductive health of selected native tree species within metropolitan environments. The specific objectives include determining the correlation between pollution levels and tree height, diameter at breast height (DBH), leaf area, and seed production; evaluating differences in physiological stress indicators among trees exposed to varying pollution intensities; and identifying the threshold levels of pollutants detrimental to tree vitality. A quantitative, cross-sectional research design underpins this study, integrating geospatial pollution mapping with field-based biological assessments. The study population comprises five native tree species, namely Ficus sycomorus, Balanites aegyptiaca, Ziziphus spina-christi, Acacia nilotica, and Tamarindus indica, located within three urban districts characterized by differing pollution profiles in the metropolitan city. A stratified random sampling approach selected 150 individual trees—30 per species per district—ensuring representation across pollution gradients. Data collection instruments include portable air quality analyzers for pollutant measurement, standardized dendrometers for growth parameters, leaf chlorophyll meters, and reproductive output assessments through seed counts. Physiological stress indicators such as chlorophyll content, leaf surface wax deposition, and antioxidant enzyme activity levels will be measured through laboratory analysis utilizing spectrophotometry and enzyme assays. The validity and reliability of instruments are ensured through calibration with standardized controls and repeated measurements. Data analysis employs multiple regression analysis and Analysis of Variance (ANOVA) to examine relationships between pollution variables and biological responses, with the Structural Equation Modeling (SEM) technique used to explore causal pathways, grounded in the Stress-Physiological Response Theory and the Toxicity Threshold Model. It is anticipated that higher pollution levels will correlate with reduced growth metrics, such as decreased tree height and DBH, along with impaired reproductive output indicated by lower seed production. Physiologically, trees exposed to elevated pollutant concentrations are expected to exhibit significant stress responses, including diminished chlorophyll levels, increased oxidative enzyme activities, and altered leaf surface properties. These findings are expected to fill gaps in existing literature regarding the specific physiological and morphological thresholds at which pollutants adversely influence native trees in urban areas. The study contributes new empirical evidence integrating spatial pollution data with biological responses, supporting the development of pollution tolerance indices for native species. It offers practical recommendations for urban forestry management, including the implementation of pollution mitigation strategies, planting of more resilient species in high-risk zones, and formulation of policies for air quality improvement to safeguard native biodiversity. The research underscores the need for integrating environmental monitoring with urban planning to ensure sustainable management of urban green spaces, ultimately promoting resilience of native tree populations amidst escalating urban pollution challenges.