THE LICHEN VEGETATION OF CHALK AND LIMESTONE STREAMS IN BRITAIN

The results of a nationwide survey of lichens associated with streams on chalk and limestone are reported. The lichen communities are presen t as a series of overlapping bands related to length of submergence. T hese have been named the submerged, fluvial mesic, fluvial xeric and f luvial terrestrial zones. The flora involves around 50 lichens, many o f which are rarely recorded specialists restricted to this habitat. Th e richest sites for submerged species are headwaters, especially aroun d springs. Active tufa deposition and disturbance of the stream bed by farm stock are detrimental to aquatic lichens. Diversity further down stream is related to the amount of exposed limestone available for col onization, with topographical complexity also important. An investigat ion of the influence of water quality suggests that suspended solids ( silting) is a significant determinant of lichen distribution together with eutrophication. (C) 1996 The British Lichen Society