Diverging drivers of alpha and beta diversity across management types in Swedish boreal forests

Managing multifunctional forest landscapes requires a better understanding of how biodiversity dimensions respond to habitat structures and management regimes. While species richness is commonly used to assess conservation value, it may not capture compositional differences critical for maintaining regional biodiversity.

We examined how local species richness (alpha diversity) and community turnover (beta diversity) relates to forest structure and management in Swedish boreal forests. We surveyed lichens, bryophytes, and polypore fungi, representing three sessile taxo-ecological groups across 120 sites.. The sites were distributed between three management types: young production forests, retention patches (trees left during harvest to support mature forest species in production forests), and unmanaged set-asides. We used generalised linear models and generalised dissimilarity modelling to assess how habitat structures (deadwood and living tree metrics) and spatial distance explained patterns of richness and turnover across taxa and management types.

For all taxa, species richness increase with lying deadwood volume. Lichen species richness also increased with living tree volume, and decreased with tree density in retention patches. Lichens had higher species richness in retention patches and set-asides, whereas only set-asides had statistically higher bryophyte richness, and there were no differences for polypore richness. In contrast, beta diversity was more often associated with geographic distance, with only limited influence of deadwood volume. The relationships between lichen beta diversity and habitat structures were not consistent between retention patches and set-asides, suggesting that retention patches may have limitations in conserving mature forest species despite supporting similar lichen species richness.

Synthesis and applications. Our results demonstrate that alpha and beta diversity can be driven by different ecological processes and management contexts, and that while tree retention can support higher biodiversity than young production forests in some taxa, it cannot replace the value of conserving mature forests. Effective conservation therefore needs to combine habitat quality and spatial complementarity when planning conservation initiatives to maintain diversity patterns across managed landscapes.