About the influence of alleyway trees on conditions of meteorological exchange in streets

Alleyway trees are commonly regarded to be esthetical, have a psychological as well as urban-climatological significance; thus, they are associated with enhancing human health. Regarding their influence on air-hygienic conditions in streets their effect is doubtful. In areas of the inner city where streets are heavily used by traffic, buildings on either side, and older trees (with closed canopies), reduced levels of air exchange and thus, remarkably higher concentrations of pollutants have to be assumed. These conditions will have a very negative effect especially on pedestrians (in the lower layer) and flats (in the upper layers). The assumption mentioned above could be verified by measurements of pollutants in Mülheim an der Ruhr in Germany.

In order to examine changes of air exchange conditions in streets by alleyway trees, measurements were made in the Leineweber Street in Mülheim an der Ruhr.

On one side of Leineweber Street trees with an age of about 45 years were growing until 1997; sidewalks, one side of the street and the fronts of buildings were thus completely protected up to a height of 25 m whereas the opposite side of the street remained treeless. This treeless side has hosted a climatic station since 1995 where wind in 6 m above ground and air temperature and relative humidity are measured in 2.5 m above ground, respectively. In 1997 trees were planted on the opposite side as well; these trees were only 3 to 4 m high and thus remained below the measurement device for wind direction and speed for some time. During the last years the tree canopy grew higher and the measurement station has been topped by more than one meter in the meanwhile. Continuous records of these changes in air flow enable us to analyse the proposed effect. Measurement results were then related to further locations where conditions have been stable over the period of time observed; thus, we were able to detect distinct changes.

In addition to long- term measurements short- term measurement campaigns (of up to a week) using 3- dimensional sonic- anemometers were made in order to examine exchange conditions in 10 m above ground directly in between the older trees (canopy) and on the treeless side; big differences could be observed here as well.

The results and knowledge derived from these are used in further town planning processes concerning restoration and maintenance of alleyways in the inner city of Mülheim an der Ruhr.