The competition of a plant community can be positive (net facilitation effect) or negative (net competition effect) (Balandier et al., 2006). Competition between the crop trees and the natural trees or plants is the main process influences survive and growth of the crop trees in newly established plantations (Thompson and Pitt, 2003) and the competition process is unlikely to be constant in time and space (Balandier et al., 2006). Our results indicated that the different silvicultural treatment had different competition effect on the spruce growth and the interaction between the spruce, aspen and the bluejoint grass in different season and year.

Different treatments will create different microclimate which will influence the tree growth conversely. The different treatments have significant seasonal changes in terms of competition level. The competition level of different treatments also changes in different years, the reason might because the temperature, precipitation and other climate factors are different interannually, and also because in different stages, both the crop tree and the neighboring species behave differently, they may have different resources up taking ability and utilize efficiency in different age. That will definitely influence the balance of competition and facilitation among the crop tree and the neighboring species.

The competitive and facilitative mechanisms are operating simultaneously. The life stages may affect the results of interactions between plant species (Callaway and Walker, 1997). Further studies are needed to quantify the competition in different season and in different years to provide more information on the interactions of the spruce and the neighboring species in different life stages (e.g., seedling, juvenile, pre-reproductive adult, reproductive adult, senescent adult).