1. Bannink, J. F., Leys, H. N., and Zonneveld, I. S.: 1973, ?Vegetation, Habitat and Site Class in Dutch Conifer Forests?, Versl. Landbouwk. Onderz. 800, Bodemk. Studies 9, Stiboka, Wageningen, 188 pp. (in Dutch).
2. Bannink, J., Leys, H. N., and Zonneveld, I. S.: 1974, ?Weeds as Environmental Indicators Especially for Soil Conditions?, Versl. Landbouwk. Onderz. 807, Bodemk. Studies 11, Stiboka, Wageningen, 88 pp. (in Dutch).
3. Boer, Th. A. de: 1983, ?Vegetation as an Indicator of Environmental Changes?, Environmental Monitoring and Assessment 3, 375?380 (this issue).
4. Ellenberg, H.: 1974, ?Zeigerwerte der Gefäszpflanzen Mitteleuropas?, Scripta Geobotanica, 9, Göttingen, Verlag Erich Goltze. Z. Auflage 122 p.
5. Hengeveld, R. and Haeck, J.: 1982, ?The Distribution of Abundance, I. Measurements?, J. Biogeography 9, 303?316.