Growth performance and multiple ecosystem services of different oak plantation types in Central Italy

Abstract The afforestation of marginal agricultural land has been promoted across Europe to reduce agricultural overproduction, produce high-quality timber, and enhance ecosystem services. This study investigates the long-term development of mixed oak plantations ( Quercus robur L. and Q. petraea [Matt.] Liebl.) established in 1999 on former agricultural land in Central Italy using the triplet design. The system combines valuable broadleaved species with different proportions (from 0 to 33%) of the N-fixing shrub Elaeagnus umbellata Thunb. The main objective was to assess, after 25 years, stand growth, candidate tree development, and ecosystem services under different intercropping treatments. Complete inventories of diameter and total height were conducted before and after thinning in 2014 and 2021. Since 2020, transmittance measurements were taken annually inside and outside the triplets. All candidate trees were monitored annually for growth, tree stability, crown development, stem quality and epicormic branches. Among the ecosystem services, natural regeneration, biodiversity, carbon storage (total volume, biomass, and carbon stocks), and truffle production (assessed separately in 2019, 2023 and 2024 in June–July and October–November) were analyzed. Long-term monitoring showed that site conditions, species composition and silvicultural interventions influenced stand development and candidate tree performance. Early and sufficiently intensive thinning improved candidate tree stability, growth, and structural attributes. Nevertheless, site fertility and climatic constraints remain key factors influencing the overall success of plantation management. Differences among the three stands indicated that higher proportions of N-fixing shrubs were associated with greater regeneration density and plant diversity. Regeneration ranged from 1.5 to 3.6 seedlings m⁻ 2 and the Shannon diversity index from 0.58 to 1.57. Candidate tree growth and stem quality also differed among stands, with mean DBH ranging from 15.3 to 18.2 cm and the proportion of trees in the highest stem quality classes from 35 to 64%. Carbon stocks were primarily related to stand structure and site fertility, with oaks contributing the largest share of biomass. Truffle production was negligible and represented only a minor component of system multifunctionality. Overall, the observed differences among the studied stands suggest that site fertility may influence plantation development, while stands containing higher proportions of N-fixing species were associated with greater regeneration, biodiversity, and candidate tree quality, but not consistently with higher timber production. Although the lack of stand-level replication prevents the separation of treatment effects from site-specific conditions, the results highlight the potential of mixed-species plantations to promote ecosystem multifunctionality under Mediterranean conditions.