Influence of chemical and physical factors in Quercus and Fraxinus epiphytes at Loch Sunart, western Scotland: a multivariate analysis

1. Epiphytes on Quercus petraea and Fraxinus excelsior were studied in the native woodlands around Loch Sunart, western Scotland, to obtain information on community variation and habitat requirements of the Lobarion pulmonariae alliance at an unpolluted site. 2. Percentage cover data for 119 species were obtained from 207 randomly located 20-cm x 20-cm quadrats on Quercus and Fraxinus and from a further 18 quadrats subjectively sited in Lobarion communities on Quercus. Bark pH and cation levels and direct and diffuse radiation inputs (hemispherical photography) were measured for each quadrat. 3. The bark chemical factors and radiation estimates, respectively, showed many strong inter-correlations. Mean bark pH was 0.7 units higher and mean bark Mg and K were, respectively, 1.8 and 3.7 times higher in Fraxinus than in Quercus. Bark pH was positively correlated with bark Ca in Quercus, whereas in Fraxinus, bark pH increased with girth but was unrelated to cation levels. In Quercus, bark Mg content declined with increasing altitude probably due to increasing insect defoliation of the canopy at higher altitudes. Bark pH or cation concentrations were positively correlated with soil exchangeable cation levels. 4. Correlations between the epiphytic flora and environmental variables were explored using numerical classification (two-way indicator species analysis), indirect ordination (detrended correspondence analysis) and direct ordination (canonical correspondence analysis). 5. Height on the bole and bark angle were identified as the most important factors influencing epiphytes. They probably determine the degree to which water is available on, or evaporated from, the bark surface. Bark pH correlated strongly with the second ordination axes but radiation was a relatively unimportant predictor of epiphyte community composition. 6. Classification of the floristic data resulted in the recognition of eight quadrat groups. Species of the phytosociological Lobarion pulmonariae alliance were distributed between two groups, one predominantly on Quercus, the other, with richer representation of cyanobacterial lichens, on Fraxinus. Both groups were characteristic of bark with high pH and high cation levels, and were best developed in low-altitude sites, possibly indicating that acidic inputs may restrict their colonization at higher altitudes. Neither group was strongly associated with high light levels which were previously thought to be important for Lobarion lichens. Key-words: correspondence analysis, light, Lobarion pulmonariae alliance, TWINSPAN Journal of Ecology 1992, 80, 163-179