THE LICHEN FLORA OF MEGALITHIC MONUMENTS IN THE NETHERLANDS

A recent thorough examination of the lichens on the Dutch megalithic m onuments, called 'hunebedden', has been carried out. 'Hunebedden' prov ide unique environments for saxicolous lichens in the Netherlands beca use natural siliceous outcrops are absent otherwise. Several Red List species occur only or almost exclusively on these boulders, including the nearly endemic Lecidea promixta. Three species are reported new to the Netherlands, viz. Endococcus propinquus (Korber) D. Hawksw., Fusc idea praeruptorum (Du Rietz & Magn.) Wirth & Vezda and Thelocarpon coc cosporum Lettau. Many species are threatened by increased shading by t rees and tourism. Some previously recorded species were found to be ex tinct. Unexpected was the discovery on granite of many usually cortico lous species like Buellia griseovirens, Gyalideopsis anastomosans and brown Parmeliae (Melaneliae), some of them reflecting the eutrophiatio n by ammonia from manure, for example Xanthoria polycarpa. The most im portant abiotic factors correlating with the lichen vegetation appear to be the exposure to sunlight, the pressure of recreation and the exp osure to wind. Sufficient measures should be taken to minimize the neg ative effects of the increased treegrowth, and to protect some vulnera ble monuments against intensive recreation.