Characteristics of tree bark as an indicator in high-immission areas.

In the immission area of Frankfurt a.M. measurements of pH value, sulfur content and concentrations of Ca, K and Na of tree bark have been made. In the case of the deciduous trees Fraxinus excelsior, Acer platanoides and Tilia spp. all parameters can be used as an indicator for the degree of immissions. Even in the "desert of lichens" a division into four different zones of load is possible. Because in the Rhein-Main area winds from the southwest predominate, the size of zones of greatest air pollution depends on the distribution of winds. Correlations between the sulfur content of bark and the arithmetical mean value of SO in the air have been found. The sulfur values of bark range from 50 to 270 μg/cm, the SO values from 0.08 to 0.13 mg/m. A correlation between electric conductivity and Ca values in the bark suspension was also found. With distance from the immission center the Ca content of bark is decreasing. In the immission area of Frankfurt a.M. pH value and sulfur content of bark of deciduous trees must be regarded as indicators for air quality and the Ca values as indicators for dust immissions.