LAUSR

The crucial role of forests in terrestrial carbon (C) balance is well acknowledged, but nationwide C assessments still show some uncertainties. We estimated the effect of forest site type on various ecosystem C pools in premature- and mature-aged forests of hemiboreal Estonia. Furthermore, the effects of soil physico-chemical properties and the tree species on soil organic carbon (SOC) stocks were analysed. The weighted mean SOC stock of Estonian forests was 77 Mg C ha−1 in humipedon layer and 118 Mg C ha−1 in solum. The mean ecosystem C stock of Estonian forests was 174 (confidence range: 141 to 214) Mg C ha−1, from which 111 Mg C ha−1 was distributed in the solum layer, 61 Mg C ha−1 in the overstorey tree layer, 0.5 Mg C ha−1 in the understorey tree layer and 1.7 Mg C ha−1 in the ground vegetation. Forest site type had a significant (p < 0.001) effect on all analysed C stocks. The variation of SOC stocks in the humipedon (R2 = 0.71) and in the solum (R2 = 0.79) was explained by the thickness of the layer, the total nitrogen stock, base saturation, hydrolytic acidity, soil trophic and moisture conditions. The higher quantity of deciduous tree species in a stand composition was in a positive correlation with the SOC stock in the humipedon of fertile site types. The accuracy of nationwide assessments of forest C reporting can be improved by using a site-specific approach, considering deeper soil layers and incorporating other structural forest layers.