Tree composition and carbon turnover dynamics across 22 years in a Costa Rican premontane moist forest fragment

Aim of study: Tropical forests are increasingly threatened by deforestation and fragmentation. In Costa Rica, extensive land-use change has left forest fragments whose long-term ecological dynamics remain poorly understood. This study assessed changes in tree diversity, composition, structure, and aboveground carbon stocks during a 22-year period (2000-2022) in a premontane moist forest fragment in Costa Rica. Area of study: Municipal Forest of Atenas (26.4-ha), Alajuela, Costa Rica. Material and methods: Tree inventories were conducted in 32 plots (10 m radius) in 2000 and 33 plots in 2022. We analysed diversity and composition using Bray-Curtis dissimilarity, Hill numbers, and Importance Value Index, while tree structure and carbon stocks were assessed using tree size distributions, PERMANOVA, and allometric models. Main results: We identified 71 species and 69 genera in 2000 (N = 535), and 62 species and 60 genera in 2022 (N = 534). We found high compositional stability, with a 76% similarity in genus abundance between years, and consistent dominance by the early successional species Inga densiflora Benth and the invasive Syzygium jambos (L.) Alston. A gradual shift toward intermediate and late-successional genera suggests ongoing successional processes. Carbon storage more than doubled, from 1,847.2 to 3,903.7 tC (70.0 tC/ha in 2000 and 147.9 tC/ha in 2022), approaching values reported for old-growth tropical forests. These changes may be driven by the presence of large individuals of Ficus and Anacardium excelsum (Bertero & Balb.) Skeels, supporting the mass-ratio hypothesis. Research highlights: These findings underscore the ecological importance of conserving secondary forest fragments in human-modified tropical landscapes, which can support long-term forest recovery and contribute to ecological resilience, carbon storage and climate change mitigation.