Occurrence of epiphytic bryophytes in a 'tetrad' transect across southern Britain. 2. Analysis and modelling of epiphyte–environment relationships

Abstract

Frequency data for epiphytic bryophytes in systematically sampled 2 x 2 km grid squares within a belt transect aligned S. W. to N. E. across southern Britain were analysed by canonical correspondence analysis (CCA). The environmental data used in the analysis included variables representing climate, atmospheric pollutants, forest cover, geology, altitude and presence of water courses. The two most important canonical gradients obtained expressed the effects of moisture availability and atmospheric pollution/geology on the epiphytic flora. Frullania tamarisci, Neckera pumila, Metzgeria temperata, Microlejeunea ulicina and Hypnum andoi were restricted to tetrads with high moisture availability, whereas Syntrichia ruralis, Grimmia pulvinata, Tortula muralis and Aulacomnium androgynum only occurred as epiphytes in tetrads with low moisture status. At one end of the second CCA axis were species characteristic of acid woodland, including Dicranum tauricum and D. montanum, which may be indicative of elevated deposition of sulphur and nitrogen from atmospheric pollution. Calcicole bryophytes and taxa that avoid acid substrata (e.g. Syntrichia laevipila, S. papillosa and Ulota phyllantha) were positioned on the other end of this axis. The CCA axes were used to generate log-linear regression models for individual epiphyte species. For 12 species, fitted distributions for the transect tetrads were compared with the observed distributions and used to predict epiphyte distributions for a wide area of southern Britain based on an independent set of environmental data. A separate analysis was made of the associations between particular epiphytes and their phorophytes by detrended correspondence analysis (DCA). The primary DCA axis represented a sequence of epiphytes from species more commonly associated with calcareous masonry (e.g. Syntrichia ruralis and Grimmia pulvinata) to calcifuges such as Tetraphis pellucida, Hypnum jutlandicum, Dicranum montanum and Orthodontium lineare. The corresponding ranking of host trees was from Sambucus nigra, Malus sp., Ulmus spp., Salix spp. and Acer campestre, all species characterized by nearly neutral or nutrient-rich bark, to those with strongly acidic bark including Carpinus betulus, Betula spp. and Castanea sativa.