Plant Ecology 135: 113–124, 1998. 113 c 1998 Kluwer Academic Publishers. Printed in Belgium. Calibrating Ellenberg indicator values for moisture, acidity, nutrient availability and salinity in the Netherlands A. C. D. Ertsen 1 , J. R. M. Alkemade 2 & M. J. Wassen 1 1 The Netherlands Centre for Geo-ecological Research, Functioning of Landscape Ecosystems Research Group, Department of Environmental Studies, Utrecht University, P.O. Box 80.115, 3508 TC Utrecht, The Netherlands 2 National Institute of Public Health and the Environment, P.O. Box 1, 3720 BA Bilthoven, The Netherlands Received 17 April 1997; accepted in revised form 4 December 1997 Key words: Impact-assessment modelling, Indicator values, Regression analysis Abstract A general calibration of Ellenberg indicator values for moisture, acidity, nutrient availability and salinity was carried out on a large database of relev´ ees and environmental variables from a variety of ecosystems in the Netherlands. Satisfying relationships with Ellenberg indicator values for moisture, acidity and salinity were found for mean groundwater level in spring time, soil pH and chloride concentration in groundwater. For mean groundwater level in spring and chloride concentration in groundwater subdivision of the database led to clearer relationships with indicator values. For the Ellenberg indicator value for nutrient availability satisfying calibration results were only achieved with data on standing crops and N stock in standing crop. The relationship with soil chemical variables was less clear. Although the correlation between indicator and measured values is obvious, the variation around the regression lines is considerable. However, because of the size and composition of the database, it is unlikely that our calibration results can be much improved by adding more (Dutch) data. The calibration results will be applied in the multi-stress model SMART-MOVE, developed to predict changes in species composition due to acidification, eutrophication and the effects of lowering groundwater. Introduction Several statistical models have been developed in the Netherlands to predict the effects of changes in habitat conditions on vegetation composition. Most of these models are based on field data of vegetation and site factors, collected for this purpose. The occurrence of plant species is related by Gaussian logistic regres- sion analysis to site factors to derive response curves for the species (Hosmer & Lemeshow 1989; McCul- lagh & Nelder 1989; Ter Braak & Looman 1986). Examples of these models are ICHORS, developed for aquatic ecosystems in the provinces of Noord-Holland, Zuid-Holland and Utrecht (Barendregt et al. 1993), ITORS, developed for terrestrial ecosystems in Noord- Holland (Ertsen et al. 1995) and HYVEG, developed for dune slacks (Noest 1994). These models were all developed for specific regions. It would be very costly and time consuming to collect sufficient data to devel- op an adequate statistical model based on field data for the use on a national scale. Developing response models on a national scale demands another approach using an extensive database consisting of 30,000 vegetation relev´ ees from the Neth- erlands. Unfortunately, the corresponding site charac- teristics are not known. However, Ellenberg indicator values offer an alternative for measured site character- istics. Ellenberg et al. (1992) assigned indicator values for moisture, acidity, nutrient availability and salin- ity for about 3000 vascular plants in the western part of Central Europe. These indicator values are used to estimate the value of an environmental factor by aver- aging the indicator values of all species present for this factor (e.g., Ellenberg et al. 1992; Ter Braak & Bar- endregt 1986). Ellenberg indicator values have been demonstrated in numerous publications as being very