Aluminium toxic effects on seedling root survival affect plant composition along soil reaction gradients – a case study in dry sandy grasslands

Question Aluminium (Al) toxicity is one of the most important factors restricting plant establishment on acidic soils, but its ecological significance for the occurrence of species along natural pH gradients is still under investigation. Are species occurring on acidic sandy soils less susceptible to A l toxic effects on germination and seedling root growth rate than species from calcareous sandy soils? How strong is the explanatory power of species' Al susceptibility for their occurrence along a pH gradient, as represented by their Ellenberg indicator value (EIV) for soil reaction (R)? Can Al tolerance of species be used as an independent trait to support Ellenberg's empirically‐derived reaction indicator values? Location Dry sandy grasslands in Southern Germany, with soil reactions ranging from acidic to calcareous. Methods We tested early seedling responses to different Al concentrations in 15 species from dry sandy grasslands. A filter paper‐based system was used to germinate seeds under Al concentrations of up to 10 mM. Germination, absolute root growth and the length of the root hair zone were recorded 7 and 14 d after first germination. Al concentrations that reduced root growth by 50 or 95% (ED50 and ED95, respectively) were correlated to Ellenberg indicator values (EIV) for soil reaction. Results EIV explained 66% of the variance in species' Al sensitivity. Tolerated Al concentrations resemble those concentrations the individual species are exposed to in their natural habitats. Conclusions Among all soil factors varying with soil pH , Al is one of the strongest restrictions to species' occurrence in acidic soils. Al acts as an environmental filter by allowing only Al‐tolerant seedlings to grow roots and establish. Al sensitivity is a measurable objective trait that could form a crucial physiological component in defining R indicator values.