Low cross-taxon congruence and weak stand-age effects on biodiversity in Swedish oak forests

Abstract Assessing cross-taxon congruence is vital for effective forest conservation, because different taxonomic groups may respond inconsistently to key habitat variables such as stand age. We examined six taxonomic groups—insects, arachnids, springtails, epiphytic lichens, bryophytes, and vascular plants—across 25 Swedish oak stands ranging from 19 to 165 years old to determine whether species richness correlated among groups (cross-taxon congruence) and how it related to stand age. In total, we identified 22,276 unique taxa (with on average 4,128 per stand) using COI metabarcoding for arthropods and field surveys for lichens, bryophytes, vascular plants. Associations of species richness in each taxonomic group with richness in the others were weak, indicating low cross-taxon congruence. Only lichens showed a significant, positive relationship of species richness with stand age, while springtails exhibited a unimodal pattern, and the other four groups were unaffected by stand age. Although species composition in four groups changed with stand age, the explanatory power was generally low. Overall, the heterogeneous responses of different groups indicated by our findings caution against the use of single taxonomic groups or environmental variables as indicators and keys to successful protection of biodiversity. Instead, forest management strategies should adopt multi-taxon assessments and recognize the value of both younger and older stands to safeguard biodiversity in oak-dominated landscapes.