Biodiversity in primary vs. managed forests: Biological legacies of old living and large dead trees drive lichen diversity

Anthropogenic activities have significantly contributed to the loss and fragmentation of primary forests across the globe, which has accelerated biodiversity decline, particularly among highly specialised species dependent on unique forest structures. Nevertheless, comparative studies between primary and managed forests are scarce, despite their importance for effective monitoring and conservation planning. To address this knowledge gap, we conducted a comparative study using a unique dataset of permanent study plots established across some of the best-preserved, mixed-beech primary forests and their adjacent managed counterparts in the Western Carpathian Mountains. We assessed the effects of forest structure and tree age—determined through extensive dendrochronological reconstructions—on contemporary lichen communities. Lichen species richness and the richness of red-listed species were 26% and 50% higher in primary forests than in managed forests, respectively, highlighting the outstanding conservation importance of primary forests. Generalised least squares (GLS) modelling demonstrated that in managed forests, lichen species richness was strongly associated with structural attributes: It increased with maximum tree age and the diameter of standing deadwood, and decreased with higher basal area (BA) of living trees, likely due to reduced understory light. In contrast, no structural variables significantly explained richness in primary forests, likely due to structural saturation and widespread microhabitat availability. Elevation emerged as the sole variable with significant explanatory strength.These findings underscore the critical role of structural complexity in supporting lichen diversity under different management regimes and provide a robust evidence base for promoting elements such as old trees, deadwood—especially large standing deadwood—and reduced canopy density. At the same time, they reaffirm the irreplaceable value of primary forests as biodiversity refuges and highlight the need for landscape-level conservation strategies that integrate both intact primary and structurally enriched managed forests.