Factors influencing overyielding in young boreal mixedwood stands in western Canada

Mixtures of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) are a prominent component of the boreal forests of western Canada. Overyielding, indicating higher productivity in mixtures than in monocultures, has been observed in mature stands but has not been examined in young stands (<30 years old) in this region. We used data collected between 2006 and 2015 at 7 locations in Alberta and Saskatchewan for the Western Boreal Growth and Yield Association (WESBOGY) Long Term Study to examine whether overyielding occurs in young mixtures of these species and to identify factors (species composition, stand density, tree size, species proportion, site quality, and climate) that influence overyielding. Our results show that overyielding is occurring in these young boreal mixedwood stands in western Canada. Relative productivity total (RPT), indicating mixing effect (RPT > 1 indicates overyielding), varies from 0.921 to 1.537 among mixedwood treatments, and thinned aspen stands show higher production than unthinned aspen stands. Initial stand density (basal area) and initial aspen size (QMD at the start of the measurement interval) positively influence mixing effect while initial spruce size (QMD) negatively influences mixing effect. The magnitude of overyielding declines with increasing growing season length (DD5) and the relative productivity of aspen decreases with increasing site quality. Species mixtures support higher stocking than pure stands due to differences in growth rate and shade tolerance of the two species. Tree size is also important since productivity will decline when two species compete with each other for space and resources. Consequently, a mixed species stand that has space partitioning and size inequality between species, which reduces competition and favours expression of the functional traits (e.g., shade tolerance) of each species, tends to have better productivity.