Dramatic diversity losses in epiphytic lichens in temperate broad-leaved forests during the last 150 years

Comparing data of epiphytic lichen diversity in semi-natural broad-leaved forests in north-western Germany from the 19th to early 20th centuries with recent inventories revealed strong changes, even though forest structure and tree species composition had changed only little. In three study areas, between 55% and 70% of the species became rarer during the 100–150-year long observation period. In the spatially extended study areas Weser-Ems Lowlands and Solling Mountains, 36% or 39% of the species, respectively, could not be rediscovered in the recent survey. Considering that species might have been overlooked during revisitation, the extinction rate was estimated to be 28% in the Weser-Ems Lowlands and 30% in the Solling Mountains based on a estimated probability for recovering the species of 75% in crustose lichens and 90% in foliose and fruticose lichens. The main causes of the species decline are thought to be forest management (especially the reduction of overmature and decaying trees), the reduction of soil moisture and, with it, air humidity due to drainage as well as the deposition of acidifying and fertilizing substances from the atmosphere. Lichens specialized on rain-sheltered bark furrows and cavities of old trees or smooth, shady bark or moist thick-stemmed deadwood in the forest interior have suffered the strongest declines, including the epiphyte flora of Fagus sylvatica, Central Europe’s most abundant native forest tree species. Only few lichens which benefit from nitrogen deposition, global warming or the acidification of bark due to sulphur dioxide pollution have spread.