This study examines the effects of thinning, age, and site quality on aboveground live tree carbon (ATC) (Mg/ha) stocks in upland hardwood forests of mixed-species composition in the southern Appalachian Mountains. In 1974, 80 plots ranging in size from 0.06 to 0.1 ha were established in even-aged, mixed-hardwood forests throughout the southern Appalachians. All trees 2.54 cm dbh within each plot were tagged and measured. Sixty-two plots received a low thinning to a broad range of residual basal areas (BAs) (m/ha) whereas 18 of the 80 plots remained unthinned and served as controls. Remeasurement of plots occurred every 5 years through 2005. Individual tree volumes were converted to stand-level estimates of ATC stocks for each inventory cycle. The average net annual rate of carbon (C) storage in aboveground live tree biomass was significantly greater in thinned versus unthinned stands. We found that light low thinnings had a neutral to slightly positive effect on net ATC stocks over the long-term relative to unthinned stands of similar age and initial BA. Relative differences, however, varied with age when ingrowth data were excluded from the analysis and with age and BA when ingrowth data were included in the analysis. In general, the gains in net ATC stocks in thinned verses unthinned stands were minor. The increased rate at which thinned stands stored C coupled with substantial mortality in unthinned stands was probably responsible for this “crossover” effect of thinning on ATC stocks. FOR. SCI. 58(5):407–418.