Assessment of ornamental tree species ability to remove atmospheric pollutants and to fight climate change in urban environments

Ornamental woody species in urban environment provide several ecosystem services such as improving air quality and atmospheric carbon dioxide (CO2) sequestration. However, not all tree species show similar capabilities to remove air pollutants. Species with higher stomatal conductance (gs) can remove more gases from the atmosphere, while several species can even worsen air quality by emitting biogenic Volatile Organic Compounds (bVOCs) precursors of ozone (O3). The first aim of this doctoral thesis was to expand knowledge about eco-physiological parameters (i.e., bVOCs and gs) for 14 ornamental species with direct measurements in nursery focusing on the evaluation of their Net O3 uptake. In detail, the highest maximum gs was parameterized for Catalpa bignonioides Walter and Gleditsia triacanthos L. (0.657 and 0.597 mol H2O m–2 s–1, respectively). High isoprene (16.75 μg gdw–1 h–1) and monoterpene (13.12 μg gdw–1 h–1) emission rates for Rhamnus alaternus L. and Cornus mas L. were found. In contrast, no bVOCs emissions for Camellia sasanqua Thunb. and Paulownia tomentosa Steud. were detected. Afterwards, FlorTree, which is an innovative single- tree model to estimate the flux of air pollutants (O3, NO2, and PM10) and select the best species for urban greening, was developed. FlorTree considers species-specific parameters such as tree morphology (height and crown leaf area), leaf/shoot structure, leaf habit (deciduous or evergreen) and physiological responses (bVOCs emissions and gs) to environmental factors. According to the survey for potential candidate ornamental species, FlorTree model suggests that 24 species offered optimal performances for air pollutant removal in Florence. Among them, hardwoods with large crowns at maturity such as Tilia, Acer, Fraxinus, and Ulmus are generally better for the removal of gaseous pollutants (O3 and NO2), while conifers (i.e., Cedrus and Taxus) are to be preferred if we have high levels of PM10 in the air. Conversely, Quercus, Populus, and Eucalyptus should be avoided in areas with high concentrations of O3 because of their high bVOCs emissions. Finally, the last aim of the PhD activities was to examine whether and when a new urban forest becomes a real CO2 sink in a Mediterranean climate. To address this experimental question, 170 urban trees belonging to four genera (Tilia, Acer, Ulmus, and Cupressus) were planted in a new public park of Florence, and species-specific leaf-level net photosynthetic CO2 uptake was modeled considering the response to environmental factors (i.e., light, temperature, relative humidity, and atmospheric CO2 concentration). Life Cycle Analysis was accomplished to calculate the Carbon Footprint (CF) linked to the nursery cultivation, tree planting, and park maintenance over time (life span 50 years). In addition, seasonal CO2 soil respiration fluxes were measured. Results show that CF linked to the setting-up of the new urban forest and its maintenance for 50 years is equal to 14.7 t CO2 equivalent. Maintenance over time is the most impactful phase (62 % of all the emissions), followed by the nursery phase (20 %) and tree planting (18 %). When also the annual soil respiration rate is considered, results indicate that the new urban forest in Mediterranean area is CO2 source for the first 12 years. Starting from the 13th year, the park is able to offset all the emissions and become a real CO2 sink.