Vegetation response to high concentrations of heavy metals in the Harz Mountains, Germany
Becker, Thomas; Dierschke, Hartmut
Heavy metal content is assumed to be the most important edaphic factor determining vegetation composition on contaminated soil. We compared the relationships between vegetation composition and heavy metal content at 23 mining sites in the Harz Mountains in Germany with those of other soil environmental factors. 120 relevés were assigned to the Armerietum halleri which was subdivided into three subassociations, A. cladonietosum, A. typicum and A. achilletosum. Within each of the latter subassociations a Typical variant and a Cardaminopsis halleri variant were classified. The first axis of a DCA was positively correlated with Ellenberg's indicator values for soil reaction, nitrogen and moisture, and the concentration of calcium, and negatively with the concentration of copper and the proportion of stones, indicating that these variables were most important for vegetation differentiation. Soil concentrations of lead and exchangeable zinc did not differ significantly between the communities, while concentrations of copper and water-soluble zinc were highest in the A. cladonietosum and lowest in the A. achilletosum. Ellenberg's indicator values for nitrogen indicate poorest nutrient conditions in the A. cladonietosum where soil depth was especially low and richest conditions in the A. achilletosum where soil depth was higher. Logistic regression showed that the presence of the metallicolous Minuartia verna subsp. hercynica increased with the concentration of soluble zinc in the soil, while the presence of Armeria maritima subsp. halleri and Cardaminopsis halleri decreased with increasing concentration of copper. Armeria was furthermore strongly negatively affected by altitude, while Cardaminopsis was positively affected by soil depth and moisture. Silene vulgaris var. humilis was neither influenced by heavy metals nor by other environmental factors. A comparison of the recent number of slag heaps with those listed in a 75 year old study demonstrates a strong decline of these habitats and their specific vegetation due to both human destruction and natural succession. We conclude that heavy metals are by far not the only factor controlling vegetation on metalliferous soils. At the mining sites investigated vegetation is also strongly controlled by low soil fertility.