Congruency across taxa and spatial scales: are we asking too much of species data?

Scale dependency is one of the main factors causing the variability of correlations among species diversity parameters. Recent syntheses demonstrated how both extent (the geographical area\ncomprised by the survey or total sample) and grain (the scale of each sampling unit) have a significant effect on cross-taxon congruency. These results, however, do not represent a definitive framework to understand this variation across spatial scales, since this was only indirectly analyzed, by comparing the results of different studies. Our goal is to assess if and how cross-taxon congruency vary across different spatial scales in European temperate forests. Our general hypothesis is that cross-taxon\ncongruency in species richness and composition in European temperate forests increases together with the scale of analysis. We account separately for differences in spatial grain and extent, and evaluate the relationships between all the possible pairs of taxonomic groups. We use a comprehensive set of field data encompassing Italy, France and Hungary (Fig. 1), and including species diversity information for six taxonomic groups: vascular plants, bryophytes, birds, epiphytic lichens, saproxylic beetles, and wood-inhabiting fungi. We calculated effect sizes for species richness and community composition correlation at three different spatial scales of grain and extent: (i) plot grain - site extent, (ii) plot grain - continental extent, and (iii) site grain - continental extent. No pair of taxa had their species richness consistently correlated (either positively or negatively) in all three combinations of grain and extent analyzed (Fig. 2). However, birds and bryophytes, and fungi and lichens had the highest correlation of species richness when sampled at a plot-level grain and site extent. When analyzing congruent patterns of species composition, plants resulted as the best surrogates for other taxa, even across different spatial scales (Fig. 3). We demonstrate that diversity congruency relationships among taxa in European temperate forest are for the most part, scale-dependent. Our results support that increasing database extent and sampling grain masks local patterns of congruency and highlights those due to common response to broad environmental and biogeographic factors that covary across the European continent. We suggest the results presented in this study to be taken into serious consideration when developing indicators of sustainable forest management for the conservation of forest biodiversity.