Nitrogen-fixing Nepalese alder (Alnus nepalensis D. Don.), a pioneer species and nurse tree species, forms pure stands, and sometimes occurs in mixed stands in areas affected by landslides. The objective of this study was to understand the influence of A. nepalensis on carbon stock in white oak (Quercus leucotrichophora A. Camus) forests. We investigated the differences in vegetation biomass carbon (tree, sapling, seedling, shrub and herbs, and forest floor mass), soil organic carbon stock, and sequestration rates in five naturally occurring oak mixed alder (OMA) forest stands and five naturally occurring oak without alder (OWA) forest stands along the basal area gradient in order to investigate the role of A. nepalensis on ecosystem carbon stock. The total basal area ranged from 61.20 to 89.51 m2 ha-1 in the OMA stands and from 38.02 to 53.54 m2 ha-1 in the OWA stands. The total tree density of the OMA stands (1120 to 1330 trees ha-1) was higher than that of the OWA stands (950 to 1230 trees ha-1). The total ecosystem carbon stock in the OMA stands was significantly (P<0.05) higher than that in the OWA stands, ranging from 485.3 to 635.6 Mg C ha-1 in the former and from 378.8 to 472 Mg C ha-1 in the latter. Soil was the second largest carbon pool in all the studied stands, with the values ranging from 238.1 to 254.1 Mg C ha-1 in the OMA and 185.5 to 215.8 Mg C ha-1 in the OWA stands. The soil organic carbon (SOC) stock was 1.19 to 1.28 times higher in the OMA than in the OWA stands. Of the total ecosystem carbon stock in different OMA stands, A. nepalensis stored 16.2 to 38.8%. Annual carbon sequestration rates (6.6 to 9.5 Mg C ha-1 yr-1) in the OMA stands were significantly (P<0.05) higher than in the OWA (2.5 to 5.4 Mg C ha-1 yr-1) stands. Among all the species and across the stands, the greatest carbon sequestration was exhibited by A. nepalensis (3.4 to 5.3 Mg C ha-1 yr-1). The present results show the role of A. nepalensis in ecosystem carbon stock and sequestration rates. Significantly higher rates of carbon sequestration by oak in OMA stands than OWA stands clearly indicate the facilitative role of co-occurring nitrogen-fixing A. nepalensis. The results imply that Q. leucotrichophora mixed with a A. nepalensis plantation may be a good option for enhancing ecosystem carbon stock, carbon sequestration, and habitat restoration in the central Himalaya.