Baardmossen (Usnea spp.) in de Biesbosch, (vooral) vroeger en nu

New and (mostly) old records of species of Usnea in the Biesbosch A chronological overview is presented of the Usnea-species that have been found in the National Park the Biesbosch. The Biesbosch is a freshwater tidal area in the basin of the rivers Meuse and Rhine. Due to the closure of the Haringvlietdam in 1970 the tidal influence in the parts called ‘Dordtse’ and ‘Brabantse’ Biesbosch was greatly reduced. In the ‘Sliedrechtse’ Biesbosch a direct connection with the sea has remained and here still an amplitude of ca. 80 cm exists. Since 1955 and especially after 1970 on a large scale cultured willow coppices were abandoned here and developed into more natural willow forests. These woods have been surveyed for bryophytes and lichens by the authors since 1983 and this yielded six species of Usnea. With two additional species found by A.M. Brand (*) the total comes out at eight: U. fulvoreagens, U. subfloridana, U. glabrata (*), U. cornuta, U. ceratina, U. hirta (*), U. esperantiana and U. flavocardia. In 1969 U. fulvoreagens was the first species that appeared. It was found several times until 1990 by A.M. Brand, mainly in the forest complex called ‘Grienden van de Dood’. Several Usnea’s were only found incidentally, and sometimes even on a single willow tree: U. glabrata (1981), U. cornuta (1984), U. ceratina (1984 2× and 1987) and U. hirta (1990). In the beginning of the eighties of the last century U. subfloridana was rather common in young and old willow forests and formed vital thalli on living and dead branches. Soon after, however, it rapidly decreased and practically vanished after 1991. Since then for many years Usnea’s were absent in the Biesbosch. In 1999 (leg. and det. A. Aptroot) and 2012 U. esperantiana was found in wet, low lying willow thickets. Also in this habitat in 2016 a local population of Usnea flavocardia was discovered. The increased air pollution with ammonia may have caused the disappearance of Usnea species in the late eighties. We think that, additionally, lower levels of air humidity may have been harmful. This is indicated by the warmer climate and the considerable decrease of the average number of days with fog and mist in the last decades. This may also explain why relative newcomers were species like U. esperantiana and U. flavocardia, since they both have their main distribution in SW-Europe.