Neighbor identity and competition influence tree growth in Scots pine, Siberian larch, and silver birch

Previous studies on competitive interactions among silver birch, Scots pine, and Siberian larch have not addressed the direct importance of the species identity of nearby competitors. We examined the joint importance of competition and species identity, using subject trees with a high local abundance of a single dominant neighboring species. Interspecific neighbors influenced annual height increment, shoot length, and branch number per unit crown length, especially in Scots pine. Silver birch and Siberian larch were predominantly affected by the level of competition alone, as estimated with competition indices. In Scots pine, the effects may have been a direct consequence of the species identity of neighbors or they may have acted as a substitute for the effect of some non-measured variable associated with species-specific characteristics. These functionally equivalent alternatives suggest that simple indices are not adequate measures of the neighbor effect for Scots pine. A simple extrapolation of individual tree growth to the stand level suggested that Scots pine and silver birch may grow faster in mixed than in pure stands. Siberian larch showed negative growth responses to interspecific neighbors, but the effects may be counterbalanced at the stand level by a corresponding increase in pine or birch growth.