Regeneration and early management of birch and Norway spruce mixtures in Southern Sweden

Abstract: Regeneration involving birch and Norway spruce is the most common mixture on clearcuts in southern Sweden. Sometimes the mixture is unintentional, and the naturally regenerated birch is often regarded as a weed-species in planted Norway spruce monocultures. In other cases, the additional seedlings from spontaneous natural regeneration are, perhaps, not planned for, but are still used, as a convenient way to create mixed forests with management for production and other services. The objectives of the research described in this thesis all refer to the establishment and early management of mixtures with planted Norway spruce and naturally regenerated birch. Hypotheses were tested in field experiments in the counties of Kronoberg and Halland. A better knowledge of seed supply, by estimating seed sources and seed dispersal, could be used when planning future stands and in the choice of management. The effect of soil scarification on seed emergence and seedling survival was tested in field experiments and modeled together with distance to seed supply. The combination of spatial information about standing volume and specific site variables produced birch regeneration estimates that could be useful for practical management and planning. Once the seedling population was established, after three to five years, the density, height structure and species composition were tested as variables for further selections in precommercial thinnings. The retained stems, 1000-3000 trees ha-1, responded positively to a reduction in competition even when stand heights were as low as 1-2 meters. The size of neighbors was more important than the species for the individual growth of both birch and Norway spruce. The competition release in the early stand is important if the target is to retain a mixed forest throughout the full stand rotation, otherwise the retained birches will have difficulty competing with the planted Norway spruce in later stages of the rotation. Other common broadleaved species and pine regenerate on the same clearcuts but the current browsing pressure from ungulates reduces the possibility to allow these species to be present in the future stand.