Assessing above- and belowground competitive effects in pure and mixed stands of jack pine (Pinus banksiana) and black spruce (Picea mariana)

Ecological principles of niche, competition and competitive reduction suggest that competition in mixed stands composed of pine and spruce may experience positive interactions through (1) vertical stratification of canopy and (2) vertical separation of roots.Competition indices showed that spruce generally experienced a wider variety of competitive conditions over pine. Values computed from a species-specific index indicated that there is a strong intraspecific effect within both the target tree species. However, while jack pine showed to have a strong interspecific competitive effect on black spruce, black spruce had a weak interspecific competitive on jack pine. The weak relationship between radial growth and competition index suggests that competition may not be strong in these stands, which may reflect the complementary growth patterns and resource requirements of the component species or the stage of stand development.Jack pine preferred to put its roots in the upper portion of the mineral soil regardless of stand type. However, there was a significantly higher percentage of jack pine roots in the lower mineral soil layers of the mixedwood stand compared to spruce, which indicates jack pine are exploiting more of the soil profile in the mixtures. It was found that black spruce prefer to root in the organic and upper mineral soil layers regardless of site or stand type. In the mixedwood, the spruce had a significantly smaller amount of biomass because it is sharing soil space with pine, causing the spruce to put their roots in the available space.These results suggest that pine-spruce mixedwood have the potential to be more productive than pure stands of the component species as competition is likely reduced in these stands due to differing growth patterns. Investigation into the productivity of these stands is necessary so that silvicultural prescriptions can be developed for these naturally occurring stands commonly found throughout northwestern Ontario.In order to evaluate above- and belowground competitive interactions of pure and mixed stands composed of jack pine (Pinus banksiana (Lamb.) and black spruce (Picea mariana (Mill.) B.S.P.) I: (1) reviewed mechanisms of competition and competitive reduction, (2) located even-aged, unmanaged single- and mixed-species stands of pine and spruce on intermediate sites within the boreal forest of Northwestern Ontario, (3) calculated three competition indices to quantify competition in pure and mixed stands of the component species and compared the indices to identify which best describes competition in a mixed stand, and (4) determined if rooting patterns of pine and spruce are affected by a mixture.Keywords: aboveground competition, belowground competition, conifer mixedwood, competition index, competitive reduction, radial growth.