Mixedwood sites are the most productive and common forest types in Alberta (Comeau et al. 2006).Trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca [Moench] Voss) mixtures are the most common forest type in the boreal forests of western Canada (Rowe, 1972; Beckingham and Archibald, 1996). However, growing mixedwood stands faces several challenges compared with monocultures (Comeau et al.1999a) and establishment of white spruce following harvesting is problematic throughout the central boreal forest (Groot, 1999) due to vigorous regeneration and faster growth of aspen ( Comeau et al., 2006). Underplanting has been suggected as a regeneration method for white spruce (Liffers et al. 1996, Comeau et al. 2004), but underplanted seedlings will hardly grow fast (Sutton, 1986). Studies and operational experience showed that when sufficient advance spruce regeneration is present in the understory of an aspen dominated stand, protection of this spruce understory can lead to an aspen dominated stand or a pure stand. However, this approach may be unsuccessful due to the poor quality of understory spruce regeneration (Comeau et al. 2006). Besides, dense aspen cover can also seriously reduce survival and growth of white spruce.

Calamagrostis canadensis (Michx.) Beauv.(bluejoint or marsh reed grass) is a common species native to the mixedwood associations of the boreal forest (Beckingham and Archibald, 1996) and one of the most competitive herbaceous species in the forest understory in North America (Hitchcock, 1971). Calamagrostis competition can cause delay or failure in stand regeneration (Eis, 1981; Lieffers et al., 1993. Matsushima and Chang (2006) suggested that competition from an understory dominated by Canadensis had negative effects on the growth of the studied 13-year-old white spruce stand. The extremely high N uptake ability of Calamagrostis can cause strong competition for the limited N in the soil.

Trembling aspen and Calamagrostis are the two major competitors of white spruce in boreal mixedwood in western Canada. They may have different influence on microclimate factors such as light availability, soil moisture and temperature, soil nutrient, air temperature and humidity, thus they could have different competition impact on spruce growth, it will be meaningful to know the impact of trembling aspen and Calamagrostis on spruce growth separately.

Vegetation management decisions are being driven by the need to maximize productivity at minimum treatment cost (Richardson et al., 2006). The two most common forest vegetation management objectives are minimize resource competition and develop methods for managing specific weed species (Richardson et al., 2006). The key factor in managing mixed wood stands is to find a balance between the competitive effects of deciduous and the understory species on conifers and the facilitative effects of those species (Comeau et al., 1999a). Therefore, practical regeneration strategies are needed to reproduce the mixedwood forests with the best composition, density and structure (Pitt et al. 2005). Results showed that early growth of white spruce responded strongly to overstory and vegetation-control treatments (Groot, 1999). The planted conifers benefit from vegetation control treatments applied earlier than typical operational treatments, which are normally carried out several years after planting (Wood and von Althen 1993; Wanger et al.1996). Use herbicide control all the understory competition will not be a wise method since the operation costs and environmental concerns. But if we don’t control aspen and other understory competitors at all, spruce will not grow well. So we need to control the aspen and Calamagrostis grass competition to levels which will allow spruce to grow well while at the same time keeping silviculture costs low.

Resource availability such as light availability, soil moisture and temperature, soil nutrient, air temperature and humidity in forest stand vary intrannually and interannually, thus tree growth and competition relationships vary with the variations of the resource availability. In different seasons of the year, the main factors which influence the tree growth and the competition relationships may not be the same since different species themselves react differently under different microclimate conditions. Kimberley and Richardson (2004) suggested that incorporating the seasonal term into the time scale worked better than either of the size scale methods when they modeling interactions between radiate pine and weed species. In different years, the microclimate in forest stand may change vigorously due to drought or precipitation. Understand the intrannual and interannual dynamics of competition in mixedwood stand may help us understand the trend of the competition relationships, it will also help us make silvicultural operations to increase productivity and predict future growth pattern and competition.