Forest composition is critical for its functioning. While global environmental changes have modified forest composition, it remains unclear how temporal shifts in functional composition differ across regions and biomes. Using extensive spatial and long-term forest inventory data (17,107 plots monitored 1951–2016) across Canada, we found that functional composition shifted toward fast-growing deciduous broadleaved trees and higher drought tolerance over time; notably, this functional shift was more rapid in colder regions (boreal biome). Further, the functional composition of colder plots shifted toward drought tolerance more rapidly with rising CO2 than warmer plots (temperate biome), suggesting the vulnerability of boreal forests against global environmental changes. Future ecosystem management practices should consider spatial differences in functional responses to global environmental change, with attention to boreal forests experiencing higher rates of warming and compositional changes.

1.      Soil carbon plays an important role in mediating global climate change and sustaining environmental quality. Although experimental studies have established that higher biodiversity can lead to greater soil C storage, uncertainties remain concerning the relationships between tree diversity and soil carbon stocks in natural forests, as well as the climatic factors that drive their directions and magnitudes. 2.      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 climate gradients in the organic horizon and mineral horizon. 3.      We found that climatic factors were the most important model predictors with the largest and constant impacts. Compared with climatic factors, tree diversity and composition were relatively poor predictors of soil carbon stocks. Importantly, the effects of functional diversity on soil carbon stocks were climate-dependent, shifting from negative in warm or moist climates to positive in cold and dry climates. In addition, forest communities with acquisitive resource utilization strategies had higher, while those with conservative resource utilization had lower mineral horizon soil C stocks in warmer sites than in colder sites. 4.      Synthesis. Our results provide large scale evidence that tree diversity and identity effects on soil carbon are dependent on climatic context and suggest that converting high functionally diverse natural forests to monoculture plantations can decrease soil carbon stocks of colder and drier sites in boreal and temperate forests.