the activity or alkalinity of the substrate can act on a lichen thallus in numerous ways. Various minerals and organic substrate are in different chemical states under different pH regimes, some more available than other; diffusion rates may change at different pH's; some substances are toxic under acidic condition and harmless when deacidified. Whatever the mechanism of influence, it is evident from numerous studies that the distribution of some lichens and lichen communities is strongly correlated with substrate acidity