Assessing multi-taxon biodiversity is crucial to understand forests’ response to environmental changes and to
inform management strategies. In Europe, forest biodiversity monitoring is still scattered and heterogeneous,
although a long-term monitoring network has long been advocated. Given the monitoring aims reported in
various EU policies, this network should be accurately designed also through the estimation of its sampling effort,
here intended as the number of sampling plots and sites. We used a novel database of forest multi-taxon biodiversity for a pilot study to: estimate the minimum sampling effort needed to: assess variation in species richness and composition; compare these estimates with the fungiefforts invested in the pilot database; discuss estimates’ differences across taxonomic groups and forest categories. We focused on six taxonomic groups (vascular plants, birds, epiphytic lichens and bryophytes, wood- inhabiting fungi and saproxylic beetles) across six forest categories. Based on 6,165 plots at 2,084 different locations across Europe, we benchmarked the effort to achieve: a complete species richness estimate through interpolation/extrapolation curves, and a precise evaluation of species composition variation through multivariate standard error. Keywords: Birds; Epiphytic lichens; Epiphytic bryophytes; Forest biodiversity; Monitoring network; Multivariate standard error; Rarefaction curves; Saproxylic beetles
