Volume Yield to Mid-rotation in Pure and Mixed Sown Stands of Pinus Sylvestris and Picea Abies in Sweden

Volume yield to mid-rotation in pure and mixed sown stands of Scots pine and Norway spruce was investigated in an experiment in central Sweden. The 43-year-old stands were situated on a 7-ha site and were treated and inventoried at the time of establishment, then at intervals in the life of the stands, to give results at half-rotation age. The dynamics of the mixed stands implied a favourable ecological mixed-stand effect on the height growth of spruce in early development and before crown closure. Even at this stage, pines were taller than spruces, but height was not influenced by admixture with spruce. Pines continued to grow faster than spruces in both height and diameter, resulting in dense mixed stands with dominant pines and co-dominant or suppressed spruces. Diameter of pines was increased by weaker competition for nutrients, while weaker competition for light led to a lower height of dominant pines in mixed stands than in pure pine stands. The results indicate a slight etiolation effect of competition for light in the crown layer. Total volume yield was higher in mixed stands than the mean yield in pure stands of pine and spruce, mainly owing to the dominance of pine in mixed stands. However, it was lower in mixed stands than in pure pine stands. The growth dynamics to the present time indicates that, after a slow start, volume growth of spruce increases remarkably in pure stands and increases slightly in mixed stands. while volume growth of pine began early and is culminating.