The 54 megalithic monuments in the Netherlands together represent a large (proportion of the exposed siliceous non-littoral rock surface in this mostly alluvial country. They are rich in lichens and currently harbour 128 different species, 27 of which are (classified on the Red List. In 1988, 1993 and 2000 the lichen flora of all granite monuments has been examined and the lichen flora shows an unexpected, strong change in response to environmental factors. The most important change is the increase of the number of lichens over the years, especially those who are usually found corticolous, as well as nitrophilous (species and species growing on humid, acid substrates. This partly reflects changes in air pollution, but also a change in surrounding habitat: the monuments are now often somewhat shaded, whereas they were originally all fully exposed. Removing trees at shaded sites can help to restore the original lichen flora