One taxon does not fit all: Herb-layer diversity and stand structural complexity are weak predictors of biodiversity in Fagus sylvatica forests.

Abstract

Since adequate information on the distribution of biodiversity is hardly achievable, biodiversity indicatorsare necessary to support the management of ecosystems. These surrogates assume that either somehabitat features, or the biodiversity patterns observed in a well-known taxon, can be used as a proxyof the diversity of one or more target taxa. Nevertheless, at least for certain taxa, the validity of thisassumption has not yet been sufficiently demonstrated.We investigated the effectiveness of both a habitat- and a taxa-based surrogate in six European beechforests in the Apennines. Particularly, we tested: (1) whether the stand structural complexity and theherb-layer species richness were good predictors of the fine-scale patterns of species richness of fivegroups of forest-dwelling organisms (beetles, saproxylic and epigeous fungi, birds and epiphytic lichens);and (2) the cross-taxon congruence in species complementarity and composition between herb-layerplants and the target taxa.We used Generalized Linear Mixed Models (GLMMs), accumulation curves and Procrustes analysis toevaluate the effectiveness of these surrogates when species richness, complementarity and compositionwere considered, respectively.Our results provided a limited support to the hypothesis that the herb-layer plants and the standstructural complexity were good surrogates of the target taxa. Although the richness of the herb-layerplants received a stronger support from the data than structural complexity as a predictor for the generalpatterns of species richness, the overall magnitude of this effect was weak and distinct taxa respondeddifferently. For instance, for increasing levels of herb-layer richness, the richness of lichens showed amarked increase, while the richness of saproxylic fungi decreased. We also found significantly similarcomplementarity patterns between the herb-layer plants and beetles, as well as a significant congruencein species composition between herb-layer plants and saproxylic fungi. Finally, when different standstructural attributes were considered singularly, only the total amount of deadwood received supportfrom the data as a predictor of the overall species richness.At the fine scale of this study, herb-layer plants and stand structural complexity did not prove tobe effective surrogates of multi-taxon biodiversity in well-preserved southern European beech forests.Rather than on weak surrogates, these results suggest that sound conservation decisions should besupported by the information provided by comprehensive multi-taxonomic assessments of forestbiodiversity.