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 between vascular plants, bryophytes and lichens. Community data for these taxa were collected using a restricted random sampling applied in the Bosco di S. Agnese Nature Reserve (Tuscany, Italy). Co-Correspondence analysis (Co-CA) was performed on two different response communities (e.g. bryophytes and lichens presence/absence data) considering three predictor communities (presence/absence of vascular plant, abundance of vascular plants and a structure-based dataset) for three grain sizes (1m2, 100m2, 10000m2). 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; ii) the sign of observed relationships mainly depends on the analyzed taxon and on the choice of predictor variables. Our results highlight the importance of plot dimension, choice of predictor variable and taxon identity in evaluating cross-taxon relationships and provide further insights on the limitations of cross-taxon estimates among vascular plant and cryptogams for biodiversity assessment and conservation planning.