The effects of functional diversity and identity (acquisitive versus conservative strategies) on soil carbon stocks are dependent on environmental contexts.

• Effects of tree diversity and identity on soil C stocks were highly context-dependent. • Soil C decreased with tree diversity in well-drained and moist temperate forests. • Soil C increased with tree diversity in ecotone between boreal and temperate forests. • Acquisitive tree communities had higher soil C stocks in warm or well-drained sites. Soil carbon (C) plays an important role in mediating global climate change and sustaining environmental condition. Although experimental studies primarily from grasslands revealed the positive effects of plant diversity on soil C storage, there is uncertainty about the directions and magnitudes of tree diversity and soil C relationships in natural forests. Using Canada's National Forest Inventory data from temperate to boreal forests, we examined the relationships between soil C stocks and tree functional diversity and identity, and how these relationships varied along environmental gradients (i.e., biomes, mean annual temperature, climate moisture index, and soil drainage). In contrast to the results from grassland experiments, we found a negative relationship between tree diversity and soil C stocks in temperate forests and a nonsignificant relationship in boreal forests. Moreover, soil C stocks increased with functional diversity in moist and poorly drained sites, but decreased in dry and well-drained sites within the temperate forest biome. Within the boreal biome, soil C stocks increased with functional diversity in warm climates, but decreased in cold climates. Our findings at the macro-ecological scale indicated that the positive effects of plant diversity might be the highest at the boreal-temperate forests' ecotone. In addition, within the temperate biome, tree communities with higher leaf nitrogen, phosphorus content and specific leaf area were associated with greater mineral horizon soil C stocks, especially in warm or well-drained sites. Our finding of the abiotic context-dependent relationships between soil C stocks and tree functional diversity and identity can guide forest management across different environmental conditions. [ABSTRACT FROM AUTHOR]