Scaling a- and b-diversity : bryophytes along an elevational gradient on a subtropical oceanic Island (La Palma, Canary Islands)

QUESTIONS: We addressed three questions: (1) what is the variation of a- and b-diversity of bryophyte communities across spatial scales in an elevational gradient; (2) is spatial variation characteristic of different phylogenetic (mosses/ liverworts) and/or ecological groups (terrestrials/epiphytes); and (3) what is the CONTRIBUTION of species richness and species replacement (true turnover) to b-diversity along the elevational gradient? LOCATION: La Palma, Canary Islands, Spain. METHODS: An elevational transect spanning 0–2200 m a.s.l was sampled at altitudinal intervals of ca. 200 m, two 10 m 9 10 m plots were established at each elevation and inside each plot three 2 m 9 2 m quadrats were randomly selected. Within each quadrat, bryophytes were sampled in three replicate microplots of 10 cm 9 5 cm, for each substrate: soil, rocks, leaves, humus, decaying wood and tree trunks at three heights. We evaluated a- and b-diversity at plot and elevational level for all bryophytes and for phylogenetic and ecological Groups. Values of b-diversity were further decomposed into replacement and richness differences. We tested the relationship between a- and b-diversity and elevation using linear models with and without a quadratic term (elevation2) in an AICc-based framework. RESULTS: A total of 121 species were recorded along the transect. Alpha-diversity showed a hump-shaped pattern with elevation for all bryophytes, liverworts and epiphytic species, however, terrestrial species and mosses displayed no clear pattern. Species replacement was the main driver of b-diversity for all bryophytes and also for mosses and terrestrial species, and at different scales (elevational belt and quadrat). The contribution of species replacement increased with elevation, except for epiphytes and terrestrials. CONCLUSIONS: Our study contributes towards an understanding of the spatial organization of bryophyte diversity at different scales along an insular elevational gradient. We showed that bryophyte diversity was mostly influenced by species replacement at the largest scale. The observed differences in b-diversity decomposition between bryophyte phylogenetic and ecological groups could be dependent on climatic conditions and substrate availability. The strong differences observed along the elevational gradient related to true turnover are especially important in the current global change scenario.