THE EFFECTS OF LEAD AND ZINC ON THE DISTRIBUTION OF PLANT-SPECIES AT FORMER MINING AREAS OF WESTERN-EUROPE

The distribution of plant species in respect of heavy metal levels (Pb, Zn) and heavy metal toxicity of soils was the subject of a detailed investigation at two metalliferous regions of the Eifel Mountains, Germany, and the adjacent areas of Belgium. Heavy metal toxicity, defined as the ratio of heavy metal to calcium in the ammonium acetate-soluble soil extract, gives a better indication of the sensitivity of plants to the toxic effects of lead and zinc than do the actual metal concentrations present in the soil, particularly in the case of lead. Highly lead-tolerant plants, such as the metallophytes Armeria maritima and Silene vulgaris, occur on soils in which the molar ratio of lead to calcium is greater than 1. Highly sensitive species are only found on such soils in exceptional cases. Certain species, for instance Agrostis capillaris and Festuca ovina(o), occupy a broad spectrum of Pb/Ca values. An analogous situation exists in the case of zinc, although it is not as distinctive. The molar ratio of zinc to calcium separating highly sensitive species from more tolerant ones lies between 0.1 and 0.2.