Effects of management abandonment and tree health on castanea sativa mill. stands across a latitudinal gradient in Italy.

Italian chestnut (Castanea sativa Mill.) forests have undergone major shifts, moving from traditional management to widespread abandonment, a process exacerbated by declining tree health under climate change pressure. This study investigated whether understorey plant diversity follows geographical patterns or is driven by local factors: management history, abiotic stress, and biotic stress. Data from 229 10 x 10 m plots surveyed in 2025 across the Italian peninsula and major islands were analysed. To focus on natural understorey dynamics, C. sativa was excluded from diversity calculations. Drivers of understorey diversity were evaluated by comparing the predictive power of latitude, management types (managed orchards and coppices, and their abandoned counterparts), and a tree health index (DBHweighted average of crown transparency). These factors were tested using linear mixed-effects and generalised least squares models. Non-metric multidimensional scaling (NMDS) and indicator species analysis (ISA) evaluated changes in community composition and habitat integrity regarding European Habitat 9260. Model comparisons showed that latitude was not a significant predictor of biodiversity variation (p = 0.36), performing worse than the null model. In contrast, management and tree health were primary drivers (p < 0.001). Managed orchards were the most diverse, significantly outperforming abandoned stands in all comparisons (p < 0.05). However, NMDS ordination revealed strong regional clustering, indicating that while alpha diversity is driven by management, regional biogeography shapes specific species composition. ISA results showed a decline in floristic diversity following abandonment: managed orchards supported 112 significant indicator species, including key diagnostic plants for Habitat 9260 (e.g., Luzula pedemontana, Lathyrus linifolius, and Platanthera chlorantha). In abandoned sites, indicator species dropped drastically as specialised flora was replaced by common forest species (e.g., Fagus sylvatica, Fraxinus ornus, and Hedera helix). The future of biodiversity in Italian chestnut forests depends on management choices rather than geographical location. Active management, particularly in orchards, is essential to maintain local plant community integrity. Protecting chestnut biodiversity requires recognising geographical distinctions while promoting traditional management practices and mitigating emerging plant health threats.