The greening of the cities is one of the most encouraged mitigation strategy to face the urban heat island phenomenon, as vegetation increases the evapotranspiration and the shading. Moreover, trees improve the air quality of cities, as they enhance the pollutant deposition rate thanks to their large surface area. However, few studies on the impact of tree planting on the natural ventilation and dispersion mechanisms inside urban street canyons are available. In the present thesis, we report the results of an experimental campaign performed in the wind tunnel facility of \'Ecole Centrale de Lyon, aimed at understanding how the presence and the density of trees affect the air exchange between the urban street canyon and the external atmosphere, and if it hinders the pollutant removal mechanisms.