Simulating the influence of initial stand structure on the development of young mixtures of Norway spruce and birch.

Abstract: The development of different stand structures, including Norway spruce (Picea abies (L.) Karst.) and silver birch (Betula pendula Roth), were simulated from the time of precommercial thinning until the dominant height of spruce reached 13m. The aim was to find a combination of Norway spruce and silver birch that ensured a satisfactory development of spruce combined with a long-term admixture with birch. Empirical models were used to create initial stands and to estimate the growth of the individual trees. The initial number of stems was 2500ha⁻¹ and the proportion of birch was 0, 20, 40 and 60%. The initial average height of Norway spruce was set to 3m and for birch the height was varied between 1–6m. Conditions describing a medium fertility in southern Sweden was used in the simulations. In mixtures with the greatest initial birch height, the total volume after 15 years was similar to or greater than in a monoculture of Norway spruce with 2500 stems ha⁻¹. The total volume was greater in mixtures compare to Norway spruce monocultures of the same density as the spruce component in the mixtures. The volume of Norway spruce decreased with increasing initial birch height if the average height of birch exceeded 2m. The status of competitive birches at the time of first commercial thinning was identified by investigating variables describing vital birches in a survey study. Given the conditions used in this study, it was concluded that leaving birch with an average height slightly greater (0.5–1m) than that of spruce after precommercial thinning will result in a large number of competitive birches at first commercial thinning. It was also indicated that the dimensions of the Norway spruce will be similar to a monoculture of Norway spruce with 2500stemsha⁻¹. When the initial average height of birch exceeded 4m the increase in the number of competitive birches leveled off due to intra-species competition. [Copyright &y& Elsevier]