A study of the epiphytic communities of Atlantic oak woods along an atmospheric nitrogen deposition gradient

1. Atlantic oak woods are of high conservation value and contain many rare lichens and bryophytes. The effects of nitrogen pollution on these epiphytic communities have not been previously studied. We investigated the composition of Atlantic oak wood epiphytic communities in relation to atmospheric N deposition in order to identify N indicator species and propose a critical load for such communities. 2. The epiphytic communities of seven Atlantic oak woods receiving estimated total nitrogen deposition in the range 10-53 kg N ha-1 year-1 were surveyed. Exposure of epiphytes to atmospheric N over 7 months was measured in terms of stemflow flux and concentration and airborne NH3. 3. Redundancy analysis (RDA) related the species to stemflow chemistry and bark pH. Different groups of species were found at (i) a coastal low N deposition site (Graphina ruiziana, Lecania cyrtella, Lobaria pulmonaria, Opegrapha atra, Orthotrichum affine, Melanelia fuliginosa ssp. glabratula and Pertusaria hymenea), (ii) inland low N deposition sites (I. myosuroides, F. tamarisci, Plagiochila atlantica, Cladonia chlorophaea, C. squamosa, Hypotrachyna laevigata and Thelotrema lepadinum) and (iii) high N deposition sites (Hypnun andoi, Hypnum cupressiforme. Calicium viride, Chrysothrix candelaris, C. coniocraea, Hypogymnia physodes, Parmelia saxatilis, Phyllospora rosei and Usnea subfloridana). 4. RDA analysis showed that bark pH and NH 4+ concentration in the stemflow explained the greatest amount of variation in the species composition among the sites. The sites separated into two groups (low and high NH4+ concentration) leading to an estimate for the critical load for N deposition for epiphytes in Atlantic oak woods of 11-18 kg N ha-1 year -1. 5. Analysis of the occurrence of species against N levels suggested that I. myosuroides and F. tamarisci are indicators of small N inputs, whereas H. andoi, C. coniocraea, P. saxatilis and H. physodes are tolerant of increased N loads. 6. Different forms of N (NO3-, NH 4+, NH3) were found to significantly affect the occurrence of different species. Some species were sensitive to total flux while others were sensitive to concentration.