Vascular plants as indicators of species diversity in European temperate forests: are they effective for multiple taxa at multiple scales

Synthesizing the complexity of biodiversity to the lowest possible number of relatively simple indicators is a common goal in conservation actions (e.g. LIFE+ projects). Cross-taxon congruency could represent a widely applicable tool for such initiatives. Up to now this was hampered by the low degree of consistency of cross-taxon relationships across studies, with recent synthesis indirectly demonstrating that scale-dependency is among the major factors that determine these inconsistencies. We used a multi-taxonomical dataset partly deriving from the FAGUS LIFE project to assess whether vascular plant species richness and composition are effective surrogates for those of five other taxa (bryophytes, birds, epiphytic lichens, saproxylic beetles, and wood-inhabiting fungi). By analyzing the same dataset at different spatial grains (i.e. resolution of data) and extents (i.e. size of the study area) we were also able to directly observe if and how cross-taxon relationships vary across different spatial scales. The study area encompassed 23 forest sites of approximately 200-400 km2 located in Italy, France and Hungary. We aggregated data on more than 350 plots that include all the European deciduous forest types. We calculated effect sizes and confidence intervals based on correlations between the species richness and composition of different taxa in a meta-analytic framework. This allowed us to take into account the differences in sampling methodologies and designs. Plant species richness was significantly correlated to that of some taxonomic groups (bryophytes, fungi and beetles), however the magnitude and the sign these relationships were not consistent across spatial scales. Contrastingly, the spatial patterns of species composition of all taxonomic groups but birds were positively correlated with plant species composition, and most of these relationships held true when changing the spatial extent and grain. Being the major structural and functional component of forest ecosystems, plant species composition strongly drives the spatial patterns of several taxonomic groups at different scales. Relying on plant species richness only, however, may not be particularly useful in informing conservation decisions. Overall, the efficiency of conservation-oriented forest management actions, as those performed in the FAGUS LIFE project could be soundly assessed only by sampling multiple taxonomic groups.