Cation ratios in Cladonia portentosa as indices of precipitation acidity in the British Isles

The relationship between rainfall chemistry and the concentrations of potassium, calcium and magnesium in the apices (top 5 mm) of the cushion-forming lichen Cladonia portentosa (Dufour) Coem. was investigated. Lichen samples, together with underlying topsoil, were collected from heathlands in close proximity to rain gauges in the UK Acid Deposition Monitoring Network, located in rural areas of the British Isles, which provide wet deposition data based on weekly bulk samples. The ratios K+:Mg2+and extracellular Mg2+:intracellular Mg2+in the lichen apices were strongly correlated with H+concentration in precipitation. It is suggested that shifts in these ratios occur owing to enhanced displacement of extracellular Mg2+by elevated H+concentration in acid rain. By contrast, there was no indication of any relationship between total acid deposition and lichen chemistry. The concentration of Mg2+in the lichen was weakly correlated with that in soil, whereas lichen Ca2+content was not correlated with either precipitation or soil chemistry. It is concluded that these ratios describing changes in lichen Mg2+content provide good biomarkers for wet-deposited acidity.