Hemiboreal Stand Development and Living Biomass Carbon Stocks: An Ontogenetic and Typology-Based Assessment Using National Forest Inventory Data

Carbon stocks in hemiboreal forests are frequently assessed using dominant-species or growing stock metrics, which may obscure ontogenetic and demographic processes governing biomass turnover. This study applied a process-based framework integrating succession theory, forest typology, and ontogenetic strategies to Lithuanian National Forest Inventory data reorganized by forest type-series. Three research expectations were evaluated: whether typology-based classification reveals patterns not visible under dominant-species reporting (H0), whether stand composition converges toward phase-typical configurations (H1), and whether fertile four-phase forests exhibit higher productivity and earlier CO2 flux reversals than three-phase forests (H2). H0 was rejected: forest type-series reorganization revealed contrasts in species assembly, stand structure, and carbon dynamics not detectable under dominant-species classification. H1 was supported: three-phase forests converged toward colonizer-dominated assemblies, while four-phase forests sustained multi-species, multi-cohort canopies with persistent broadleaved contributions. H2 was supported: four-phase forests exhibited higher gross volume increment and transitioned to negative CO2 accumulation 40 years earlier (61–80 years) than three-phase forests (101–120 years). Across both pathways, growing stock volume increased while CO2 accumulation turned negative in older stands, demonstrating divergence between cumulative biomass stock and net annual carbon flux. Forest Inventory data reorganized by forest type-series functioned as a process-based diagnostic tool supporting climate- and biodiversity-oriented management.