Retrospective long-term effects of halo thinning: Ground-cover vegetation diversity 20 years after remnant oak release

Integrating biodiversity objectives into production forestry requires management practices that maintain structural and functional diversity over time. In hemiboreal regions, historical favouring of conifers has simplified forest composition, reducing broadleaved components crucial to understorey diversity. Selective canopy release through halo thinning has been proposed as a multifunctional approach to enhance habitat heterogeneity, yet its long-term ecological outcomes remain insufficiently quantified. This study retrospectively assessed canopy light conditions and forest ground-cover vegetation around halo-thinned pedunculate oak ( Quercus robur L.) trees twenty years after the intervention in mixed hemiboreal forests of Latvia. Vegetation surveys and hemispherical canopy photography were conducted in the vicinity of 22 halo-thinned and 21 control oak trees across eight forest stands. Ground-cover vegetation structure beneath remnant oak trees was broadly comparable between managed and control plots, with no persistent differences in cover, richness, or diversity. Species associated with oak-dominated habitats likewise showed limited responses, indicating that targeted oak release alone was insufficient to recreate conditions characteristic of broadleaved forests. Notably, herb-layer richness showed no consistent relationship with stand basal area or gap fraction but was strongly associated with fine-scale canopy architecture, emphasising that foliage distribution, rather than overall stand openness, shaped long-term understorey responses. Overall, the findings suggest that small-scale halo thinning alone is unlikely to induce persistent shifts in understorey composition, but it may help maintain structural heterogeneity around remnant oaks. When integrated with broader or repeated interventions, halo thinning may contribute to multipurpose forestry strategies that reconcile oak conservation with production-oriented forest management. • Ground cover vegetation was characterised 20 years after remnant oak halo thinning in mixed stands. • Stand structure metrics and hemispherical canopy photography were used to describe light conditions. • 20 years after halo thinning, vegetation diversity responses to management were limited. • Shrub layer cover had the most pronounced and ambiguous effect on herbaceous diversity. • Foliage distribution, rather than overall stand openness, shaped long-term vegetation responses.