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    Trees as habitat islands: temporal variation in alpha and beta diversity in epiphytic laurel forest bryophyte communities

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    Aim: Trees represent striking examples of habitat islands, with various degrees of spatial isolation and evolving properties during their life cycle. Here, we investigate whether changes in habitat conditions, dispersal limitations or competition cause variation in patterns of epiphytic species richness and turnover. Location: Madeira island. Taxon: Bryophytes. Methods: Using linear mixed effect models, we test whether species richness exhi bits a monotonic or hump-shaped relationship with time. Two groups of host-tree species, late- and early-successional, were considered. We further identify the mechanisms explaining the observed variation in species composition by dividing beta diversity into its nestedness (bsne) and turnover (bsim) components and correlat ing them with tree age and geographical distance among trees. Results: The best-fit models all included tree age (T), but its quadratic term (T2 ) and tree height (H, here a surrogate of area) were not systematically included. bsim, but not bsne, correlated with host-tree age, and both bsim and bsne correlated with geo graphical distance. Main conclusions: Tree age was consistently included in all of the best-fit models, reflecting the progressive increase in epiphyte bryophyte species richness through time. The limited contribution of T2 and H to the best models suggests that compe tition for space is not a key factor on mature trees. The correlation of bsim, but not bsne, with host-tree age, and of bsim and bsne with distance among trees, suggests that variation in species composition is caused by (1) temporal community shifts due to allogenic drivers and (2) dispersal limitations, which are reflected by the higher similarity of the epiphyte communities on clustered trees rather than by an increasing probability of colonization with tree age. Since actual ancient laurel for ests may no longer exist in Madeira, the conservation of clusters of late-succes sional trees, enhancing connectivity at small spatial scales, is of utmost importance for the conservation and recovery of the unique laurel forest epiphytic flora.info:eu-repo/semantics/publishedVersio
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