Lichen species co-occurrence patterns along an edge-interior Mediterranean forest gradient

The co-occurrence pattern of epiphytic lichen communities was evaluated relative to microenvironmental heterogeneity along an edge-interior forest gradient. We collected data on the occurrence of 57 epiphytic lichen species from 452 plots in a Mediterranean forest remnant. We used two realistic null models to test for non-randomness in the structure of epiphytic lichen communities and generalized linear models to test the influence of micro-environmental variables on co-occurrence indices along the edge-interior gradient. We found that epiphytic lichen communities co-occurred less often than expected by chance, although the segregation pattern was not highly structured or recurrent. Less species co-occurrence was detected in the assemblage structure in the forest interior and with a southerly exposure. However, there were no significant relationships between co-occurrence and the number of coexisting species, the dominant species coverage or bryophyte coverage. In summary, our cooccurrence analyses suggested that epiphytic lichens were competitively structured, while we found no evidence of facilitation in more stressful conditions, as predicted by the stress-gradient hypothesis. Nevertheless, these results should be treated with caution because co-occurrence patterns may involve specific unknown modes of facilitation and competition, which relate to the life history of these