Productivity of Scots pine and Norway spruce in central Sweden and competitive release in mixtures of the two species

A 53 year old growth and yield study of monocultures and mixture of two species, Scots pine and Norway spruce, was analysed. The replacement design of the experiment (at the time of establishment same density in all treatments and 50/50 proportion in the mixture) in ten blocks enables the comparison of site and species specific growth, as well as the species specific response to competition. In monocultures Scots pine, produced 126% more stem wood biomass than Norway spruce. Scots pine benefitted from the mixture and tended to grow as in a reduced spacing. Norway spruce, the subdominant tree species, suffered from the competition, and produced less than expected, with a lower mean diameter compared to the monoculture. Hence, no facilitative or complementary effects was possible to prove when growing the species in mixture. However, the experiment demonstrates that if the uncertainties in choice of species at the time of regeneration is high, then the mixture of two species could be an option. The lower density of the highest yielding species in the mixture compared to the monoculture, could be compensated in growth by the competition release.