Plant assemblages respond sensitively to aluminium solubility in acid soils

Aluminium as a growth limiting factor has been recognized for many years. At high concentrations, aluminium (Al) ions reduce nutrient availability in soils, harm plant cells and thus inhibit plant growth. In addition, Al concentration may be a major factor filtering species composition on acid soils in favour of Al-resistant plants. In this study we analyse species responses and turnover along soil pH and Al gradients and we attempt to interpret the results with respect to the recognised aluminium solubility patterns. Plant community and soil data collected from mesophilous and acidophilous submontane broad-leaved forests of Western Slovakia were used for this purpose. Topsoil horizons were analysed for soil reaction (pH), organic carbon and extractable total aluminium. Species responses to the Al measurements were analysed and tested using CCA and the Huisman-Olff-Fresco (HOF) model. We calculated species turnover by accumulating the first derivatives of all HOF response curves, and interpreted them with respect to the Al solubility pattern observed in the soil dataset. We also performed a bioindication experiment to test how a species assemblage indicates the aluminium gradient. In total, 81% of species shows a significant response to the soil Al gradient. We identified that a rapid retreat of many species and, in consequence, high compositional turnover (ecotone) corresponded with a discontinuity in Al solubility observed at 130 mg Al kg−1 (pH 3.8). Here, the exchangeable Al became increasingly under-saturated with respect to the equilibrium attained at higher pH. This discontinuity was also visible in the bioindication experiment, where the prediction algorithm operated better at the acidic end of the gradient. The results indicate that the studied plant assemblages respond sensitively to soil Al solubility. Changes in aluminium solubility in soils correspond with ecotone between adjacent types of vegetation