Soil carbon plays an important role in mediating global climate change and securing food production. Despite rapid declines in plant diversity worldwide, uncertainties remain concerning the relationships between tree diversity and soil carbon stock in natural forests, as well as the climatic factors that drive their directions and magnitudes. Using Canada's National Forest Inventory data, we tested the relationships between soil carbon stocks to tree functional diversity and identity, and how these relationships varied with stand age and climate gradients in the organic horizon, mineral horizon and entire soil profile. We found that the effects of functional diversity on soil carbon stocks were highly climate-dependent, shifting from negative in warm or moist climates to positive or null in cold and dry climates. In addition, tree species with acquisitive traits, such as high specific leaf area, leaf nitrogen content and phosphorus content, increased mineral soil carbon stocks in warmer sites, but decreased those in colder sites. Our results suggest that tree diversity effects on soil carbon are strongly dependent on climate context and promoting high functional diversity is important to increase soil carbon stocks of colder and drier sites in boreal and temperate forests.