Lead tolerance of Betula and Salix in the mining area of Mechernich/Germany

Natural populations of woody perennials on lead-mining sites in the Mechernich area of the Eifel Mountains were investigated with respect to soil factors determining the degree and type of heavy metal tolerance. Salix caprea L. (Goat Willow) grew on soils with up to 17000 mg kg⁻¹ total lead (ca. 4000 mg kg⁻¹ ammonium acetate-exchangeable Pb). Betula péndula Roth (Silver Birch) was found on soils containing as much as 29000 mg kg⁻¹ total lead (7000 mg kg⁻¹ ammonium acetate-exchangeable Pb). Other woody perennials, with the exception of the dwarf shrub Calluna vulgaris, were not found in the contaminated area even though they did occur in the immediate vicinity. The two lead-tolerant tree species did not form mixed populations. Because of a significantly lower Pb/Ca ratio in Salix soils (2.2) compared with Betula soils (7.4), a calcium-dependent mechanism of lead tolerance is suggested for Salix, but not for Betula. The Betula population could be divided into two groups, each showing a highly significant correlation between root-lead content and exchangeable lead amounts in the soil, but with different levels of lead uptake. The only soil factor distinguishing the two groups was found to be the level of soluble phosphate. A distinctly low level of soil phosphate correlated with a high lead concentration in roots of the one group (30000 mg Pb kg⁻¹ DW), whereas high phosphate amounts corresponded with a much lower lead concentration in roots of the other (12000 mg Pb kg⁻¹ DW⁻¹). Since the correlation between lead in the soil and in plants was similar for the two groups, it is concluded that the type of lead tolerance in Betula is determined by the status of plant phosphate nutrition, rather than by simple phosphate precipitation in the soil. A comparison of growth between different populations of Betula seedlings on homogenized soils from the mining area revealed the Mechernich population to be a distinct ecotype with respect to lead tolerance. The control population obtained from a non-contaminated area exhibited a lower degree of lead tolerance coupled with a two-step strategy of adaptation to lead.