Changes in species density along the soil pH gradient - Evidence from German plant communities

The relationship between small-scale species richness and soil pH in plant communities was reviewed using 40 (mainly phytosociological) studies primarily from the northern parts of Germany. Four formations were distinguished (forest, grassland, mire and sand). To examine the above relationship, two approaches were applied: (a) regression analysis using linear and quadratic models, and (b) calculation of the mean pH of the five plots with the highest species density. Despite methodological problems (lack of single plot data in some studies, varying plot sizes, different ways of measuring pH) some general patterns could be identified. In forests, the regressions in most cases indicated a linear or curved increase in species density with increasing pH, whereas maximum species density was, except in one case, found at an intermediate pH of around 5. In contrast, in grasslands and mires, most studies showed hump-backed relationships between species density and soil acidity, and, accordingly, maximum species density was found at moderately high pH between 5 and 6. For sand vegetation, the few studies available revealed an increase in species density with increasing pH. The findings imply that the predominance of calciphilous species vs. acidophilous species in the Central European flora does not always translate into positive relationships between plant species density and pH on a local plot scale. The hump-backed species density-pH pattern may be explained by confounding effects of other environmental factors and of productivity, but also by the large overlap of species tolerances in the middle of the acidity gradient.