The vegetation on soils rich in zinc has recently been studied by Libbert (1930) in the Harz and by Schwickerath (1931) near Aix-la-Chapelle. In addition to the very specialized Viola calaminaria, the latter lists Arineria elongata and Thlaspi calaminare and varieties of Minuartia verna, Silene vulgaris, and Festuca ovina, as occupying the thin sod (Violetum calaminariae) over extensive areas. The soil here has always considerable zinc amounting to 0.78 per cent in the upper horizons and 0.73 per cent in the lower. One slope at Breininger Berg where the soil has 8.69 per cent of zinc is entirely bare of vegetation, this amount of the metal being apparently quite toxic. (Braun-Blanquet 1932:191)
2. Besides these characteristic relict species there are others whose fidelity is due to a narrowly specialized adaptation to definite physicochemical relations of the habitat, such as associations of rock crevices, dunes, and epiphytes; e.g., the association Violetum calaminariae on zinc soils investigated by Schwickerath (1931) contains several characteristic species narrowly limited to this special substratum. (Braun-Blanquet 1932:63)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.