Ecological Drivers of Forest Carbon Sequestration and Net Ecosystem Production at the Intersection of the Euro-Siberian, Mediterranean and Iraniano-Turanian Regions—Part I: Baseline Stand-Level Hierarchy from a Complete National Inventory

Forests are major terrestrial carbon sinks, but the relative effects of ecological zones, tree species, stand structure, and site quality on carbon sequestration capacity (CSC) and net ecosystem production (NEP) remain insufficiently quantified at national scales. We integrated MODIS MOD17A3HGF net primary production data for 2000–2023 with literature-derived ecozone-level heterotrophic respiration (Rh) values and Türkiye’s national forest inventory comprising 2,446,889 stand polygons. Welch’s ANOVA, Games–Howell tests, Hedges’ g, Spearman correlations, and path analysis were used to quantify the associations of ecological zone, tree species, canopy closure, site index, and stand development class with both metrics. Ecological zone explained the largest share of variance in CSC (η2p = 0.3154) and NEP (η2p = 0.2724), followed by tree species, canopy closure, site index, and stand development class. Euxine–Colchic Broadleaf Forests had the highest mean CSC (9.001 t C ha−1 yr−1) and NEP (4.801 t C ha−1 yr−1), whereas East Anatolian Broadleaf Forests showed negative mean NEP. NEP values are first-order modeled estimates based on fixed ecozone-level Rh and are sensitive to Rh uncertainty. Relative humidity was the strongest continuous correlate of CSC (ρ = 0.732) and NEP (ρ = 0.695), and path analysis indicated that the negative total association between temperature and both metrics arose mainly through a humidity-mediated indirect pathway. These findings support ecozone- and structure-sensitive national carbon accounting, monitoring, management, and MRV frameworks for Türkiye’s heterogeneous forests.