Extending Ellenberg's indicator values to a new area: an algorithmic approach.
Ellenberg's indicator values scale the flora of a region along gradients reflecting light, temperature, continentality, moisture, soil pH, fertility and salinity. These can be used to monitor environmental change. Ellenberg values can be extended from central Europe, for which they were defined, to adjacent parts of Europe. Given a database of quadrat samples, they can be repredicted by a simple algorithm consisting of two-way weighted averaging, followed by local regression. A database of British samples was assembled from two large surveys of a full range of habitats (grasslands, wetlands and forests). Ellenberg values were repredicted. Except for the indicator of continentality, the correlation of repredicted and original values was in the range 0.72 (light) to 0.91 (moisture). The continentality indicator could not be adequately repredicted by the algorithm, and was unusable in Britain. Discrepancies between original and repredicted values were attributed to various causes, including wrong original values, differing ecological requirements in Britain and central Europe, biased sampling of the British range of habitats, and the occurrence of small plants in shaded or basic microhabitats within well illuminated or predominantly acid quadrats. The repredicted values were generally reliable, but a small proportion was clearly wrong. Wrong values were due to either inadequate sampling of species' realized niches in Britain or sampling with quadrats that were too large and included species that were not close associates.