The most common type of young stands in Nordic countries is mixed birch and Norway spruce, Fig. 5. Many reports describe how to manage birch and Norway spruce. In Finland, Norway and Sweden the management of mixed stands is common (Mielikanen, 1985; Braathe, 1988; Tham, 1988; Mard, 1997; Klang and Eko, 1999). Frivold and Groven (1996) discussed the importance of managing mixed stands for future high timber quality. The competition between the taller birches and Norway spruce may adversely affect spruce growth. Therefore the birches must be carefully managed with respect to both numbers of stems removed and controlling competition. A common recommendation is to leave 500­1000 stems ha-1 when the birches are 10-15 years old. A Finnish study of a mixed stand of birches (downy and silver) and Norway spruce examined the influence of competition (Valkonen and Valsta, 2001). A reduction of 7-15 % by volume production was reduced by 7-15 % in mixed stands with 1000 birches ha-1 compared to pure spruce stands.

Below an experiment in mixed stands of birch and Norway spruce is described (Johansson, 2000b). The experiment was started in 1983 and was based on trials established at eight localities in central and southern Sweden. The experimental stands were 20-30 years old. They were dense, 1520-20,280 stems ha-1, and self regenerated.

The experiment included three thinning regimes:

• Thinning of the birch overstory to create a shelter of 500 stems ha-1.

• Total removal of the birch trees

• Only Norway spruces

At the first cutting, to create the shelter and the pure Norway stands, 1520 to 20,280 birch stems ha-1 with a mean diameter of 5.2 cm were removed. After 5 years, 373 to 507 birch stems ha-1 with a mean diameter of 15.7 cm were recorded.

Data collected five years after the experiment started are presented in Table 2. The competition by the birch shelter did not influence the growth of Norway spruce. As shown in the table, the mean diameter of the Norway spruce trees was almost the same in the shelter as in the pure stands, 7.6 and 7.0 cm respectively.

Norwegian experiment on farmland that involved planted European aspen and Norway spruce. Aspens and Norway spruces were planted each at a density of 2000 stems ha-1. The aspens were thinned 30 years later and 580 stems ha-1 were retained. Recommendations based on the study stated that planting densities of 2000 Norway spruce and 1000 aspen ha-1 would avoid strong competition by the aspens.

5.1.5 Mixed stands of alder and Norway spruce

Naturally established mixed stands of alder are common on wet or moist sites, (Fig. 7). Few studies have examined mixed stands of alder and Norway spruce; those which do exist are based on stands that were not managed correctly during the first ten years after establishment (Lines, 1982; Johansson, 1999d).