Soil Organic Carbon Storage in Temperate Forests: Utilizing of the Forestry Site Classification and the Role of Main Tree Species

Soil organic carbon (SOC) storage in forests is governed by complex interactions between site conditions and vegetation. This study quantifies SOC stocks across a gradient of Target Management Sets (TMS) in the Czech Republic (Central Europe) to evaluate the baseline storage capacity of distinct ecological sites and the modifying effects of dominant tree species, specifically Norway spruce and European beech. Utilizing large-scale spatial data, linear mixed-effects models, and piecewise structural equation modeling (pSEM), we analyzed SOC stratification across middle (≈400–600 m a.s.l.) and higher (≈600–800 m a.s.l.) elevational zones. The results indicate that while overall SOC stocks inherently increase with elevation due to climatic constraints, tree species dictate the vertical carbon distribution within the soil profile. Specifically, conifers (i.e., Norway spruce and Scots pine) accumulate SOC primarily in the organic layer, whereas broadleaves (mainly European beech and oak) translocate and stabilize carbon in deeper mineral horizons. The pSEM analysis revealed that beech functions as a ‘calcium pump’, increasing topsoil pH and driving calcium-mediated SOC stabilization in mineral soils. This mechanism is highly effective at middle elevations but partially overridden by abiotic limits at higher elevations. We conclude that inherent site conditions (TMS) determine total SOC capacity, whereas tree species management controls SOC stability. Although no significant differences were observed in total SOC stocks between conifers and broadleaves at the same sites (medians of total SOC ranged from approx. 5 to 16 kg·m−2, depending on the site), converting purely coniferous stands into broadleaves represents an effective strategy for long-term mineral SOC stabilization, particularly in middle-elevation sites.