A Lichen Flora of Berkshire, Buckinghamshire and Oxfordshire

Abstract

Thames and its tributaries. In this paper names refer to the Watsonian vicecounties 22 (Berks), 23 (Oxon) and 24 (Bucks), and recent boundary changes are neglected. The altitudinal range is from 17 to 292 m, and the region includes both scarp and dip slopes of the Berkshire Downs and Chiltern hills, part of the dip slope of the Cotswold hills, the Vale of the White Horse, and the valleys of the Cherwell and Kennet. While most of the region is covered by calcareous soils, there are extensive acid heaths in south Berkshire. Detailed geological and geographical information about the three counties have been fully covered in other floras (Druce, 1897, 1926, 1927; Jones, 1952a; Bowen, 1968) and will not be repeated here. The region contains no lichen sites of national importance, but has been kept under botanical observation since the late seventeenth century so that its flora is relatively well known. The region has a mean rainfall ranging from 640 to 830 mm per year, uniformly distributed through the year and strongly correlated with altitude. The highest rainfall occurs at Inkpen in south-west Berkshire and along the summit ridge of the Chilterns. The mean temperature varies between 5 °C in January and 17 °C in July, and is somewhat warmer within the larger cities such as Oxford and Reading, but distinctly colder at higher altitudes. The mean relative humidity is 70-90%, but the area is one of the least humid parts of Britain, and consistently high humidities are confined to wooded valleys and fen carr. The present distribution of lichens in the region is influenced by air pollution more than by either rainfall or temperature. The iso-concentration lines for sulphur dioxide lie approximately north-north-west to south-south-east, and the west part of the region is notably less polluted and also richer in lichens than is the east. A major source of this sulphur dioxide is Greater London to the south east, including the industrial estate and power station at Slough, Buckinghamshire. Reading, with a population of 139,000 and its own power station, has moderate levels of sulphur dioxide pollution which closely follow its suburban limits (Marsh and Foster, 1967). Didcot power station has too high a stack to have much effect on local lichens, while Oxford, with a population of 128,000 and no heavy industry or power station, is not heavily polluted and Parmelia species can be found on bark and wood within its suburbs. Most of north Buckinghamshire now has a poor bark lichen flora, and has probably been affected by the large brick kilns at Calvert and