Increment and yield in mixed and monoculture stands of Pinus sylvestris and Picea abies based on an experiment in southern Sweden

Experimental growth data collected over 20 yrs were used to assess volume increment and yield in mixed and monoculture stands with Scots pine and Norway spruce. Differences in volume increment were generally small between the different types of stand, indicating that there is little to be gained in total volume increment from using mixed stands with Norway spruce and Scots pine instead of monocultures on sites with medium fertility. However, since the volume increment of pine per unit area was higher in the mixture than in the monoculture, the results indicate that retaining some Norway spruce in Scots pine stands would result in an increase in the total volume increment. The diameter of the Scots pine was 1.2 cm greater in the mixture than in the monoculture at the end of the observation period. There was no layering on the plots, but the Scots pine trees were slightly taller and thicker than the spruce. The Scots pine stems tapered more in the mixture than in the monoculture. Stem defects were not consistently different between the stand types. The diameter distribution of the total tree population in the mixture was less tightly clustered round the mean than in the monocultures. Research designed to identify ways to increase the total volume increments using mixed stands should probably focus on mixtures of species that differ more in growth patterns than Scots pine and Norway spruce apparently do.