Summary 1.Although fine roots (< 2 mm in diameter) account for a major share of the production of terrestrial ecosystems, diversity effects on fine root productivity and their mechanisms remain unclear. 2. We hypothesized that: (i) fine root productivity increases with tree species diversity, (ii) higher fine root productivity is a result of greater soil volume filling due to species-specific patterns of root placement and proliferation, and (iii) differences in fine root productivity and soil volume filling associated with tree species diversity are more pronounced in summer when plants are physiologically active and demand for water and nutrients is at its greatest. 3. We investigated the effects of tree species diversity on fine root productivity and soil volume filling of boreal forest stands that have grown naturally for 85 years on similar sites. 4. Annual fine root production was 19-83% higher in evenly mixed-than single-species-dominated stands, and increased with tree species evenness, but not tree species richness. Fine root biomass was higher in evenly mixed-than single-species-dominated stands in summer months, but not in spring or fall. Higher fine root productivity in evenly mixed-than single-species-dominated stands was realized by filling more soil volume horizontally and vertically in the forest floor in the mixtures of deep-and shallow-rooted species vs. the deeper mineral soil in the mixtures of deeprooted species. 5. Synthesis. Our results provide some of the first direct evidence for below-ground species complementarity in heterogeneous natural forests, by demonstrating that tree species evenness increases fine root productivity by filling/exploiting the soil environment more completely in space and time, driven by differences in the inherent rooting traits of the component species and variations of root growth within species.

J é r ô me Lagani è re and Brian Brassard X. Cavard (xavier.cavard@uqat.ca) and Y. Bergeron, NSERC-UQAT-UQAM Industrial Chair in Sustainable Forest Management, Univ. du Qu é bec en Abitibi-T é miscamingue, 445 boulevard de l ' Universit é , Processes governing tree interspecifi c interactions, such as facilitation and competition, may vary in strength over time. Th is study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana , trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not diff er with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecifi c interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecifi c interactions may diff er with soil nutrient availability.