Understory herbs outperform trees in driving the accumulation of stable soil organic carbon in temperate forests, especially in fertile soils

Plot Information: Plot ID, location (Harbin, Northeast China), 720 contiguous 10×10 m quadrats.Tree Layer Traits: Stand density (SD), mean DBH, tree height (TH), under‑branch height (UBH), proportions of large‑diameter (LT%) and small‑diameter (ST%) trees, species diversity indices (S′, H′, D′, J′), spatial structure indices (mingling index M, uniform angle index W, neighborhood comparison index U).Herbaceous Layer Traits: Mean herb height (GH), total cover (TC), relative coverage (RC), species richness (SR), cover of C₃ and C₄ plants, annual/perennial cover, deep‑/shallow‑rooted cover, root weight density (RWD).Soil Physicochemical Properties: Bulk density (BD), pH, electrical conductivity (EC), water content (WC).Soil Nutrients: Total nitrogen (TN), alkaline hydrolyzable nitrogen (AN), nitrate‑N (NO₃⁻‑N), ammonium‑N (NH₄⁺‑N), available phosphorus (A‑P), easily extractable and total glomalin‑related soil protein (EEG, TG).Water‑Holding Capacity: Litter maximum/effective water‑holding capacity (Maxwcl, Ewcl); soil maximum, capillary, field, and non‑capillary water‑holding capacity (Maxwc, Cpwc, Fcwc, Ncpwc); canopy interception (Canopy‑w).Soil Carbon Fractions: Light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), heavy fraction percentage (Heavy%).