The influence of stand age and site conditions on the structure of coniferous stands was studied in the boreal forest of Québec's Côte-Nord, a region with low fire recurrence. Stand diameter diversity was measured in 2202 forest inventory plots in black spruce (Picea mariana), balsam fir (Abies balsamea), and mixed stands using the Shannon-Wiener diversity index. A relative productivity index was developed based on the relationship between height and age of dominant trees. A stepwise regression analysis indicated that this productivity index best explains stand structure variation in all composition types, while stand age seems to influence structure more at the beginning of stand development. The results suggest that productive stands become uneven-sized earlier than unproductive stands and also maintain a greater diameter diversity. These contrasting structural dynamics may be explained by (i) a higher growth rate in richer stands that likely induces earlier senescence and thus an earlier passage to an uneven-sized structure and (ii) a restricted maximum tree diameter in poor stands caused by a scarcity of resources, which in turn reduces the diameter diversity of these stands, even after their break-up time.