Occurrence of epiphytic bryophytes in a 'tetrad' transect across southern Britain 1. Geographical trends in abundance and evidence of recent change

A survey was undertaken by members of the British Bryological Society of all epiphytic bryophytes encountered in 107 tetrads (2 x 2 km land units) located within a systematic selection (1-in-9) of 10 x 10 km grid squares in a belt transect aligned west-east across southern Britain. Special field cards enabled data to be collected on frequency (number of trees occupied), presence of sporophytes, types of tree colonized and position on the host trees. Totals of 112 species and 4 varieties of mosses and 23 species of liverworts were recorded on 52 species or species aggregates of woody plants. Fraxinus excelsior and Sambucus nigra were the most frequently recorded phorophytes and responsible for 34% of all epiphyte records. Only one (Orthotrichum affine) out of the eleven epiphyte taxa which were frequent throughout the transect could be regarded as an obligate epiphyte, the remainder occurring commonly in other habitats. These common epiphytes are all characterized by abundant spore production. Many other epiphytes showed restriction to a portion of the transect. About ten species that were limited to the western part of the transect probably require more frequent rainfall and are possibly excluded by higher atmospheric pollution in the east. Among several facultative epiphytes occurring mainly in the east were Ceratodon purpureus, Grimmia pulvinata and Tortula muralis which are not so restricted in other habitats. The latter two, normally epilithic, may be benefiting from newly available habitats with low competition as a result of the recent decline in atmospheric SO2 levels and slow recolonization by many epiphytic species. Increased N deposition may also be involved. In comparison with their distributions mapped over the preceding 30 years, about 13 obligate epiphytes now appear to have increased markedly, probably in response to declining SO2. Several other obligate epiphytes characteristic of the circumneutral bark of mature, well-lit trees, e.g. Leucodon sciuroides, have not increased or have declined sharply. Bark acidification by atmospheric pollutants and loss of mature Ulmus spp. may be important causes. Lejeunea ulicina, whose western distribution pattern appears to be static, may be extremely poor at colonizing formerly polluted territory owing to the lack of both spores and gemmae.