Methodological issues in exploring cross-taxon congruence across vascular plants, bryophytes and lichens

The effectiveness of surrogate taxa as ecological indicators for biodiversity assessment and monitoring depends on different factors, such as the spatial scale of analysis. In this study, we explored the effects of the grain size and the choice of predictor variables on the strength of the community congruence relationships among vascular plants, bryophytes and lichens. Community data for these taxa were collected using a restricted random sampling applied in the Bosco di Sant’Agnese Nature Reserve (Tuscany, Italy). Co-correspondence analysis (Co-CA) was performed on two different response communities (e.g. bryophytes and lichens), considering three predictors (presence/absence of vascular plant, abundance of vascular plants and data on the vegetation structure in each plot) for three grain sizes (1 m2, 100 m2 and 10,000 m2). The effects of spatial grain and the type of predictor variable on the strength of the congruence among the considered taxonomic groups were twofold: (i) the amount of explained variance depends on the grain size and on the analysed taxon; it increased with increasing grain size for bryophytes while showing, in general, an opposite pattern for lichens; and (ii) the observed relationships mainly depend on the choice of predictor variables. The highest predictive power for bryophytes was shown by the vegetation structure predictor, while for lichens it was shown by the presence/absence of vascular plants. Our results highlight the importance of plot dimension, the choice of type of data used as predictors and taxon identity in evaluating cross-taxon relationships and provide further insights into the limitations of cross-taxon estimates among vascular plants and cryptogams for biodiversity assessment and conservation planning.