Root anatomy predicts ecological optima in Carex (Cyperaceae) in terms of Ellenberg indicator values

[Display omitted] •Ellenberg indicator values categorize species according to their ecological optima.•The biological causes for these multinomial EIV are still largely unknown.•It is hypothesized that root anatomy explains the EIV and the niche differentiation.•Root characters predict optima for soil moisture, light and soil nitrogen content. The multinomial ecological indicator values (EIV) categorize species according to their ecological optima. They are a widely used expert system in ecology and vegetation science, their biological causes are largely unknown. The root anatomy of Carex shows considerable quantitative and qualitative differences. It is hypothesized that root traits may be responsible for the niche differentiation of the species, which means they predict the ecological optima of Carex species in terms of the EIV. This relationship was tested for the 107 German Carex species over ecological gradients of soil moisture, nutrients, soil reaction, temperature and light. Generalized estimating equations were used to model the multinomial indicator values by six anatomical traits: radius of root and central cylinder (stele), sclerenchyma thickness, aerenchyma width, largest vessel diameter and vessel number. Sclerenchyma thickness and endoderm radius predict soil moisture optima; the endoderm radius also light optima. Species of dry soils have thicker sclerenchyma and a wider stele. Species of shady growth conditions have wider steles. The diameter of the largest vessel was positively related to soil nitrogen, species of nitrogen-rich soils possess wider vessels. The width of the aerenchyma was positively but marginally significant related to increasing soil moisture. Root anatomy predicts the ecological optima of species along several ecological gradients and may thus explain the ecological differentiation of the species. This hitherto unknown pattern could probably only be observed due to the extraordinary diversity of Carex, which occupy widely different habitats within a region.