Effect of aspen (Populus tremuloides) as a companion species on the growth of black spruce (Picea mariana) in the southwestern boreal forest of Quebec

In the western boreal forest of Quebec, black spruce stand productivity is approximately 1 m3/ha/year. The low productivity of these stands is often attributed to the paludification process, which is sustained by low quality black spruce litter and the influence of black spruce on soil moisture. In contrast, aspen hasten nutrient cycling, suggesting that the presence of aspen in black spruce-dominated stands could offset the effect of black spruce on soil processes and positively affect stand productivity. We hypothesised that aspen in black spruce-dominated stands could: (1) increase black spruce DBH, height and volume per stem, (2) increase black spruce productivity without affecting black spruce volume in the stand, and (3) increase total stand volume. In 2001, twelve 14 m diameter plots were sampled for DBH and height of every stem on three black spruce-dominated sites containing various proportions of aspen. Using stem analysis, the time to grow from a height of 5 m to a height of 10 m was determined on three dominant black spruces in each plot. Statistical analyses revealed that DBH, height and volume per black spruce stem were not affected by aspen. However, total black spruce volume decreased with increasing aspen basal area in sites 2 and 3, suggesting that the presence of aspen reduced black spruce density. In site 1, black spruce volume was not affected by aspen, indicating, for total stand productivity, a net gain in aspen fibre. Along a gradient of increasing aspen basal area, the time to grow 5 m decreased in sites 1 and 3. These results suggest that the presence of aspen influences black spruce productivity, although this influence is site-specific and could be dependent on the proportion of aspen, its hierarchical position in the canopy, and the nutrient status of the site. To some extent, this could explain the absence of a general trend concerning mixed stand productivity.