Dynamics of competition in boreal mixedwood stands

This dissertation investigated selected aspects of competition dynamics in boreal mixedwood stands of trembling aspen and white spruce. Results indicated that competition indices are effective for predicting the availability of light in the understory of young boreal mixedwood stands of variable density. However, these indices appear less suited to predict understory light in natural stands aged 20-60 years with a more narrow density gradient. Simple indices such as Lorimer's and basal area provided similar predictions compared to indices based on crown measurements. Relationships between aspen basal area and light transmittance differed significantly between geographical locations. Models suggested that incorporating the distance between subject trees and competitors did not improve predictions of spruce growth as a function of competition indices. The addition of initial spruce size significantly increased the predictive power of growth models.I investigated whether morphological attributes of spruce can be substituted to competition indices to predict spruce growth. Results showed some promise in using simple attributes such as height to diameter ratio for evaluating the vigour and growth of spruce seedlings.Finally, I evaluated the impact of density reduction on frost incidence in young boreal mixedwood stands. It appears that aspen cover provided protection against frost, and more importantly these effects are site specific and depend on local site characteristics. The differential frost regime contributed to the variation in growth relationships.I examined whether a single relationship can be applied across a broad range of locations and across stands of different ages. Results indicated that models of spruce growth differ with geographical location and stand age. It appears that the balance between competition and facilitation may shift from location to location and with stand age. This implies that changes in the occurrence and relative importance of each contributing factor add to the variation in relationships between growth and competition indices.