Too Many Trees Planted In Central Strip Of Roads Can Be Worse Than None; Can Trap Vehicle Exhaust Fumes

Christof Gromke now of the WSL Institute for Snow and Avalanche Research SLF in Davos, Switzerland, and Bodo Ruck of the Institute for Hydromechanics at the University of Karlsruhe, Germany, have investigated the effects of trees on ventilation and pollution levels along city streets lined with densely packed tall buildings, so called urban canyons.

The team's wind tunnel study investigated how air flows along canyon-like city streets and how this is affected by the street having a central line of trees. They looked at how air flows along and upward from the urban canyons and the effect of different spacing for trees and how stationary or moving traffic affects air flow.

Their results suggest that streets with too many trees planted close together along a central strip, as is common in many major European cities could lead to more vehicle exhaust fumes being trapped in the urban canyon than would occur if there were no trees. Specifically, the leafy canopies of a high density tree line hinder the upward flow of pollutants. They also damp down the swirling eddies of air that would otherwise help exhaust gases escape the street.

The study does not advocate removing trees from city streets, of course. Further investigation revealed that a wider spacing of trees, with trees separated by at least the width of their crowns enables pollution-carrying eddies to form and allows the air at street level to clear much more quickly especially when traffic is not at a standstill.

The research could help town planners improve city environments by choosing the optimum spacing of trees when planting new streets or to pollard more appropriately to improve airflow in streets with established tree lines.