Studying　the　effects　of　trees　on　airflow　and　pollutant　dispersion　in　urban　street　canyons　is　of　considerable　significance　to　clarify　the　laws　of　urban　micro-scale　airflow　and　pollutant　dispersion.　To　characterize　the　trees　in　street　canyons,　different　from　the　traditional　vegetation　resistance　source　method,　in　this　paper,　a　fine　tree　model　is　used　for　numerical　simulations　of　the　airflow　and　pollutant　dispersion　in　street　canyons　at　varying　inflow　wind　velocities.　It　is　found　during　the　study　that　the　presence　of　trees　physically　blocks　the　airflow　in　street　canyons.　The　airflow　is　sheared　by　the　trunk,　canopy,　or　branches,　and　then　circumvents　them,　especially　at　a　high　inflow　wind　velocity.　The　low-velocity　area　in　the　street　canyon　distributes　to　the　leeward　side　of　the　tree　trunk　as　well　as　in　the　canopy　area,　and　a　discrete　low-velocity　distribution　exists　mainly　in　the　canopy　area.　The　average　wind　velocity　in　a　street　canyon　with　trees　is　approximately　39.5%　lower　at　an　inflow　wind　velocity　of　1.7　m/s　than　that　in　a　canyon　without　trees.　In　the　presence　of　trees,　the　pollutant　concentration　in　street　canyons　increases　significantly,　and　the　pollutants　significantly　accumulate　between　the　tree　trunk　and　the　leeward　side.　With　increasing　inflow　velocity,　the　pollutant　concentration　in　a　street　canyon　constantly　changes　but　is　much　higher　than　that　in　the　absence　of　trees.　At　a　wind　velocity　of　5.7　m/s,　the　average　pollutant　concentration　is　18.6%　higher　in　street　canyons　with　trees　than　in　those　without　trees.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.