Characteristics of tree bark as an indicator in high-immission areas : II. Contents of heavy metals.

1. Studies on indicator characteristics of bark of deciduous trees have been continued by measuring accumulation of Pb, Cd, Ni and Mn in the bark of Fraxinus excelsior at 34 sampling sites over an area of 150 km2 in the immission area of Frankfurt am Main. 2. Iso-lines of equal Pb-contamination have been found. They include zones of graduated immissions. The pattern of these zones shows an eccentrical orientation and is the result of the distribution of the main traffic roads. A significant correlation of Pb in the bark and traffic congestion has been found. 3. The zones of Cd-contamination stretch from SW to NE. The orientation of these zones is determined by the distribution of the big emittenrs and the centers of heating power in the area and the predominating direction of wind. 4. A correlation between the accumulation of Pb and Cd has been detected. It is less significant than that between pH-value and sulfur content in the bark of trees in former investigations. 5. The distribution of Pb and Mn is antagonistic in the bark of Fraxinus excelsior and parallel in the needles of Taxus baccata. The content of Mn in the bark of Fraxinus is low. It must be accepted that this is partly an effect of elution by sulfur acid-aerosol formation. 6. The strongest accumulation of heavy metal immissions is found in mosses on the ground, mainly in Bryum argenteum. Nevertheless the bark of Fraxinus excelsior has shown consequently its accumulation characteristics as a good immission indicator for heavy metal contaminations by traffic and big emittents.

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 SO2 in the air have been found. The sulfur values of bark range from 50 to 270 µg/cm2, the SO2 values from 0.08 to 0.13 mg/m3. 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.